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Development of an integrated wetland microbial fuel cell and sand filtration system for greywater treatment
Journal of Environmental Chemical Engineering 7 (2019) 103249
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Journal of Environmental Chemical Engineering journal homepage: www.elsevier.com/locate/jece
Development of an integrated wetland microbial fuel cell and sand filtration system for greywater treatment Chloe Rose Bolton, Dyllon Garth Randall
T
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Civil Engineering Department, University of Cape Town, 7700 Cape Town, South Africa
A R T I C LE I N FO
A B S T R A C T
Keywords: Bacteria Cape Town Drought Handwashing Water reuse
This research focused on the design and operation of a system combining a constructed wetland microbial fuel cell (CW-MFC) and a biological sand filter (BSF) for the continuous treatment and recycling of handwashing greywater. The technology was assessed both in terms of its ability to treat the greywater and to generate bioelectricity. The performance was evaluated by regularly monitoring the organic material, nutrient and E. coli removal efficiencies as well as the power generation. The system was observed to achieve a complete bacterial removal for a 4-log E. coli influent load and a 99% COD removal was achieved with a 432 mg L−1 organic load. The final effluent quality was found to comply with South African standards for drinking water quality, illustrating the potential for reuse. Moreover, a maximum power density of 4.33 mW m-3 was achieved by the system thus demonstrating the ability of the process to recover power from handwashing greywater. It was recommended that future investigations focus on determining the performance and maintenance requirements of a pilot-scale system operated using real handwashing greywater, appropriate methods for disinfection of the effluent, improved power generation and the expected life-cycle behaviour of the greywater treatment system.
1. Introduction South Africa as a nation is no stranger to threats on water security – Cape Town having experienced the lowest recorded rainfall in the past century in both 2015 and 2017 [1]. As a result, Level 6B water restrictions were implemented in Cape Town as of 1 February 2018 although later relaxed to level 5 restrictions as of 1 October 2018. At its peak, the target was set by the City to reduce water usage to 450 ML day−1 with residents being limited to just 50 litres per person per day whether at home, work or elsewhere [1]. Furthermore, all non-residential properties were instructed to cut down water use by 45% compared to the permissible pre-drought consumption or risked facing strict penalties [2]. The level 6 tariffs for the provision of commercial water and sanitation (USD4.12/kL and USD3.20/kL respectively) saw an increase of more than double the level 4 tariffs of 2017 [2]. This tariff increase had significant financial implications for businesses if they did not take immediate action to manage their water usage. While many people have adopted measures to cut down on freshwater consumption on an individual level, the question remains as to how the commercial sector can contribute effectively to water saving initiatives. The water demand of commercial buildings may be reduced through the incorporation of innovative water efficiency measures. Examples of such measures that have successfully been implemented in commercial ⁎
buildings across the globe include the collection and use of rainwater, on-site collection and treatment of effluent for reuse and integrated building water management plans [3]. The largest portion of non-domestic urban water use is attributed to office and public buildings [4]. Between 20 and 40% of the indoor water used in these commercial buildings is generated as greywater from washbasins and kitchenettes while the rest may be attributed to the flushing of toilets and urinals [4]. By incorporating greywater treatment systems in commercial office buildings, the light greywater generated may be collected, treated and reused. This can reduce the freshwater consumption of office buildings by up to 50% [4] which has positive impacts on the environment and economic benefits for businesses where water and sanitation tariffs are fixed – as is the case for South Africa. In addition, technologies such as waterless fertilizer-producing urinals can also be implemented to reduce water consumption in buildings [5–7]. Greywater is urban indoor wastewater that is generated from several different activities but specifically excludes any wastewater generated from the flushing of toilets. Common contaminants in greywater include substances such as food particles, soap, microorganisms, salt, oil and dirt [8]. Since greywater can be made up of any combination of these wastewater sources and as such concentration of contaminants, the chemical composition of greywater is considered to be highly variable [8]. It is, however, apparent that the greywater produced in an
Corresponding author. E-mail address: [email protected] (D.G. Randall).
https://doi.org/10.1016/j.jece.2019.103249 Received 24 May 2019; Received in revised form 26 June 2019; Accepted 29 June 2019 Available online 02 July 2019 2213-3437/ © 2019 Elsevier Ltd. All rights reserved.
Journal of Environmental Chemical Engineering 7 (2019) 103249
C.R. Bolton and D.G. Randall
technology since they involved the slow treatment of water and required significant land area but recent studies have shown that by combining wetland and innovative microbial fuel cell technology, these challenges can be overcome [14]. Microbial fuel cells (MFCs) are a category of fuel cells which produce electricity from biomass and bacteria and have the potential application as an alternative renewable energy source [15]. Organic matter is degraded by microorganisms inside of the MFC anodic chamber and in the process electrons are released [16]. These electrons are collected and conveyed through an external electric circuit from the anode (negative terminal) to the cathode (positive terminal) and as such useful electrical energy can be produced [15]. Since wastewater contains stored energy in the form of organic matter it stands to reason that MFCs are capable of generating electricity from wastewater sources [15]. CWs and MFCs are considered to be compatible technologies as the redox conditions that are required for the development of MFCs are known to develop naturally in wetlands. Both systems additionally rely on the actions of bacteria to remove contaminants from water [17]. Studies have shown that the combination of the systems can improve the performance of the wetland in treating wastewater by 27–49% while simultaneously allowing for the generation of electrical energy [14]. Srivastava and co-workers [14] found that closed circuit CWMFCs outperform normal constructed wetlands, removing 86% of chemical oxygen demand (COD) in comparison to just 63.8% under an organic load of 0.75 g L−1. This enhanced performance is attributed to the improved microbial electron transfer from the anaerobic zone to the aerobic zone through the external electric circuit [14]. Anaerobic treatment pathways are known to be dominant in wetland systems, however, they are described as being both slower and less efficient processes for the oxidation of organic material due to the scarcity of electron acceptors. The same study showed that the maximum power density and current density that was produced by the CW-MFC system were 321 mW m-3 and 422 mA m-3 respectively. Variations in reported power outputs for CW-MFCs may be attributed to the redox conditions within the cell which are established naturally. These conditions are influenced by the set-up and operational conditions as well as the characteristics of the wastewater used. The primary aim of this work was to design and test a system combining CW-MFC and BSF technology such that handwashing water could be continuously recycled on-site. A secondary focus of the research was to investigate if the system could simultaneously produce bio-electricity. The research aimed to investigate the potential for the advancement of biotechnologies and on-site greywater treatment systems in an effort to determine affordable means of reducing the water demand of commercial buildings.
office environment is likely to be primarily attributed to handwashing activities. It is estimated that soap is the most significant pollutant in handwashing greywater, accounting for 90% of all contamination [9]. The physical and chemical composition of the greywater produced in these buildings is, therefore, believed to be unique in the sense that it can somewhat be controlled through the soap products that are provided for use [9]. Commercial buildings are expected to be suitable for the development and application of on-site greywater treatment systems. There are a number of different treatment technologies that have been developed and tested for the purpose of treating greywater. These systems are classified in terms of the primary mechanism employed for the treatment of the water and include physical, biological, chemical, natural and hybrid designs [10]. The latter refers to systems that make use of a combination of different treatment methods to improve the treatment efficiency as well as the quality of the effluent [10]. Research suggests that the optimal design for greywater treatment systems consists of multiple stages including biological treatment, filtration and disinfection [4,10]. Slow Sand Filtration (SSF), which could be used to treat greywater, is one of the earliest known methods of water treatment and has been used by humans since the development of the first sand filtration systems in the early 19th century [11]. These systems were used extensively over Europe, however, lost popularity in the 20th century with the development of other treatment processes that could operate on smaller footprints and handle greater variations in influent quality [11]. In the past 30 years there has been a renewed interest in the development of SSFs and biological sand filters (BSF) for domestic use in rural contexts [11]. This is attributed to the simplicity of design as well as its low energy and chemical requirements. The main difference between a SSF and BSF is the development of a biofilm layer in the latter which allows for biological treatment processes to remove harmful microorganisms from the wastewater. A typical BSF is comprised of a layer of graded filter sand that is supported by layers of gravel material. The system is designed to allow water to flow vertically downwards on a gravity basis. BSFs mimic naturally occurring physical and biological water purification processes for the removal of contaminants such as pathogens, organic matter and suspended particles [11]. The influent water is introduced at the surface of the filter and allowed to percolate through the sand layers [10]. As the water filters through the layers, pollutants are removed from the water by way of mechanical action (such as absorption, diffusion and sedimentation) as well as biological action (such as metabolic breakdown) [11]. BSF systems have been adapted from traditional SSF designs for the purpose of intermittent domestic use. In addition to the standard filter sand and support layers, designs include a diffusion plate for the homogenous supply of influent water without disrupting the biofilm and an outlet pipe which is fixed at a specific height so as to control the standing water level within the system [12]. Constructed wetlands (CW) are systems that have, similarly, been developed for the purpose of treating wastewater. They are engineered wetlands that are designed to mimic natural processes for water treatment [13]. They consist of vegetation, soils, microorganisms and water and use physical, chemical and biological processes to remove pollutants from a water source. The application of constructed wetlands for water treatment has gained popularity due to the potential it holds to offer a cost effective and low-energy solution to treating wastewater [13]. The major disadvantages presented by the technology as it stands, however, is that wetlands are outdoor systems that are land intensive and may be sensitive to climate change [10]. CW systems have emerged as an option for low cost treatment of wastewater and have been readily tested for their performance in removing contaminants such as heavy metals, dye, nutrients and microbes from a range of domestic, industrial, urban and agricultural wastewater [14]. CWs have in the past shown some limitations as a
2. Material and methods 2.1. System design and set-up In this study a laboratory scale CW-MFC and BSF system was designed and constructed for the purpose of treating synthetic handwashing greywater. The system was designed to comprise of two cylindrical columns, the first a wetland microbial fuel cell and the second a biological sand filter as illustrated in Fig. 1. The design of the CW-MFC was chosen based on the system reported by Srivastava and co-workers [14] to perform optimally both with respect to wastewater treatment and electrical energy production. The BSF was constructed as an adaptation to the traditional slow sand filter as a smaller unit designed for the purpose of intermittent use [12]. The system was later modified to include a granular activated carbon (GAC) filter (diameter 65 mm, height 400 mm) at the final stage of the treatment chain to investigate a means of improving the removal efficiency of pollutants from the wastewater. The modification was made after the first testing period after observing lower than required removal 2
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C.R. Bolton and D.G. Randall
done to maintain a standing water level at a height that would prevent the filter sand from drying out as a means of protecting the biofilm from damage. Finally, a diffuser plate constructed from stainless steel was installed 140 mm above the filter sand layer to ensure even distribution of the influent greywater into the BSF unit. 2.3. Constructed wetland microbial fuel cell assembly The vertical upflow CW-MFC was fabricated in a transparent acrylic plastic column with an internal diameter of 100 mm, a height of 650 mm and a final liquid volume of 2.2 L. The emergent plant species employed in this study was Cyperus papyrus ‘nana’. The plant, which is indigenous to eastern and southern Africa, was chosen based on a study which reported that the plant displays high resistance to applied greywater conditions [18]. The first layer in the cell consisted of washed LECA (lightweight expanded clay aggregate) with a pellet size of 8–16 mm that was placed to a height of 100 mm from the base of the column, acting as both protection for the inlet pipe and supporting layer for the anode. The cell was equipped with an anode constructed from granular activated carbon (GAC) (1.7–3 mm) that was compacted around a cylindrical steel mesh electron collector (diameter 30 mm) in a nylon fabric sleeve for ease of placement. The anodic chamber extended to a height of 230 mm above the supporting layer and was overlain by a 20 mm glass wool separating layer and 300 mm layer of LECA making up the cathodic chamber. The cathode was constructed from a 70 x 70 mm platinum (Pt) coated carbon paper (0.4 mg cm−2) and placed such that the lower end of the cathode was always in contact with the wastewater. Finally, the electrodes were connected across a 1000 Ω resistor via nickel-coated copper wires that were properly sealed by epoxy material.
Fig. 1. Schematic of CW-MFC and BSF system [(1) Cyperus papyrus ‘nana’ plant species, (2) Cathode, (3) Anode, (4) Glass wool, (5) 1 000 Ω Resistor, (6) Multimeter, (7) Sampling port, (8) Diffuser plate, (9) Standing water layer, (10) Outlet, (11) peristaltic pump] used in this study.
efficiencies, since the intention of the system was to return the greywater back to its original quality fit for handwashing purposes. The system design was chosen based on literature [12,14] to maximise treatment efficiency of the unit by allowing for both physical and biological treatment processes to take place. An integrated greywater treatment system such as the one designed in this study is believed never to have been tested for the intended purpose of recycling handwashing greywater on-site.
2.4. Synthetic greywater 2.2. Biological sand filter assembly For the purpose of this study, a synthetic greywater recipe was designed as a modification of the recipes developed and used for testing by various researchers [9,19]. The synthetic recipe was developed to recreate the physical and chemical characteristics of a generic handwashing greywater. The greywater was designed for the purpose of testing the treatment unit under laboratory conditions and was, therefore, required to mimic real handwashing composition, provide a matrix that would allow for the survival of micro-organisms and be reproducible to obtain consistent quality between batches [19]. Generic handwashing greywater was simulated using Earth Sap East India Islands Liquid Soap (which is biodegradable and contains no petrochemicals or synthetic fragrance) and municipal tap water. Three alternative synthetic greywater recipes were tested to investigate the effect of providing different organic loading rates to the CW-MFC and BSF as well as to investigate the performance of the treatment unit in removing microbiological contaminants from the greywater. The composition and parameters of the different greywaters that were used have been provided in Table 1. The greywater was prepared in 25 L batches every three days to correlate with each testing period over the duration of the experiments. To prepare the synthetic handwashing greywater, the soap required for a 5 L solution was weighed and mixed into 500 mL of municipal tap water using a magnetic stirrer (MSH20, Labcon, South Africa) at low speed for one minute or until visibly dissolved. The concentrated soap
The biological sand filter unit was constructed within a cylindrical transparent acrylic plastic column with an internal diameter of 290 mm and a total height of 800 mm and a working volume of approximately 14.8 L. A transparent system was used for the purpose of monitoring the formation of the biofilm and filtration process. The BSF consisted of layers of selected filter media that were sieved and washed in accordance with the CAWST Biosand Filter Manual [12]. The bottom most layer was constructed from 12 mm crushed rock aggregate that was placed to a height of 50 mm above the base of the BSF. This layer was provided as a drainage layer to ensure the free flow of water from within the BSF into the outlet pipe (nominal diameter 10 mm) fixed at the base of the column. The second layer was constructed from 6 mm crushed rock aggregate with a total depth of 50 mm and was placed as a separating layer between the filter sand and drainage layer. Builder’s sand was selectively sieved using standard sieves no. 16, 30, 50, and 100 to allow for the removal of unwanted debris and large particles. The portion of sand passing through the 1.18 mm sieve (no. 16) was used as the filter medium in the present study. The filter sand was placed to a height of 400 mm above the separating gravel layer. A separator of fine woven polyester fabric was placed above the separating layer to prevent the sand from drawing down into the gravel to protect against potential clogging of the outlet pipe. The outlet pipe was fixed at a level 50 mm above the top of the fine filter sand layer. This was Table 1 Composition of the greywaters used for this study. Component
Greywater 1
Greywater 2
Greywater 3
Soap (Earthsap) Tap Water Escherichia coli (E. coli)
1.2 g/handwash 300 mL/handwash –
1.2 g/handwash 600 mL/handwash –
1.2 g/handwash 600 mL/handwash 4 μL/L at 0.674 OD600nm
3
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2.6. Electrochemical characterisation
solution was added to a further 4.5 L of room temperature tap water in a clean container to obtain the required greywater concentration. The greywater was mixed thoroughly by hand before it was added to the 25 L feed tank. This process was repeated five times to obtain the desired quantity for the three-day test period. The different concentrations of soap meant the COD concentration could be fixed for each type of greywater. The handwashing greywater 3 was inoculated with a saturated overnight culture of wild-type E. coli to account for microbial and pathogenic components of real greywater. This was done for the purpose of monitoring the potential for the system to remove microbiological contaminants from the wastewater. For bacterial dosing, 0.1 mL of E. coli inoculum was mixed with every 25 L batch of greywater. Samples were taken, using autoclaved sample cups, from the inlet and outlet flows as well as the intermediary sampling port after 5 days for analysis. The tests were conducted over a period greater than the hydraulic retention time (HRT) of 2.2 days to ensure that the measured concentrations of E. coli in the effluent were not affected by any standing water in the system which would not have contained the bacteria. The samples, once collected, were kept at 4 °C, until such time they could be analysed, as a means of stopping any further bacterial growth. MacConkey’s Agar plates were used to enumerate the colony forming units (CFU) present in the water samples.
The overall electrical performance of the CW-MFC was evaluated based on the power output and coulombic efficiency (CE) of the system. The power output was calculated as shown in Eq. 1. (1)
P = I Ecell
where, P is the power in Watts (W), I is the current in Amperes (A) and Ecell is the cell voltage in Volts (V). The voltage was measured across a fixed external resistance (Rext) of 1000 Ω and recorded every 10 min using a data acquisition device (USB-6000, National Instruments, Hungary). The current across the resistance was thus determined according to Ohm’s law as shown in Eq. 2.
I=
Ecell R ext
(2)
The direct measure of power across a fixed resistance was calculated by expressing the power as a function of the cell voltage and external resistance. The maximum obtainable power, however, was determined by obtaining the polarisation and power curves for the system under each organic loading condition. The polarisation curve represents voltage as a function of current and was obtained by varying the external resistance over the CWMFC from 100 to 1,000,000 Ω and recording the steady state voltage. The internal resistance of the CW-MFC was obtained by determining the slope of the linear potion of the polarisation curve (Rint=-ΔEcell/ΔI). The power curve was obtained by representing the power as a function of the current across the external resistance with the maximum obtainable power occurring at the point where Rext=Rint (Logan et al., 2006). The power density (W m−3) and current density (A m−3) were normalised to the anodic volume for the purpose of comparing the power output of the system to different systems investigated in the literature. This was achieved by dividing the observed power by the volume of the anodic chamber. Furthermore, the coulombic efficiency, which is a measure of the proportion of electrons transferred to the anode from the substrate, was calculated on the basis of current generated at steady state as shown in Eq. 3 (Logan et al., 2006):
2.5. Operation, sampling and monitoring The CW-MFC and BSF unit was operated under continuous mode with the organic loading being the variable that was changed over the duration of the experiment. The study was divided into three distinct testing periods. The first period comprised of the system being fed greywater with an average influent chemical oxygen demand (COD) of 807 ± 26 mg L−1 (Greywater 1), which was seen to represent handwashing greywater during times of restricted water use, for a period of 12 days. During the second period the system was fed greywater with an average influent COD of 432 ± 16 mg L−1 for 12 days (Greywater 2), which represented general handwashing greywater. During the third period the greywater (Greywater 3) was inoculated with wild-type E. coli as a surrogate bacteria for the purpose of monitoring the potential of the system to remove pathogens and viruses from the wastewater. This testing period lasted 5 days. The synthetic greywater was fed into the system at 16 mL/min using a peristaltic pump (MasterFlex L/S, Cole-Parmer, United States) operated between 09h00 and 17h00 daily. This established a hydraulic retention time (HRT) of 2.3 h for the CWMFC and 2.2 days for the integrated treatment system. Furthermore, the experiments were conducted in a 12:12 h light/dark cycle with ambient light provided using a 50 W COB LED grow light to simulate natural growth conditions for the wetland plant. The ability of the CW-MFC and BSF unit to treat handwashing greywater was evaluated in terms of its removal efficiencies of total COD, ammonia (NH3-N), nitrite (NO2−-N), nitrate (NO3−-N) and phosphate (PO43−-P). Samples of 70 mL were taken from the influent and effluent water flow as well as from the intermediary sampling port (between the CW-MFC and BSF) for analysis every three days. The COD concentrations of the water samples were measured in the Water Quality Laboratory at the University of Cape Town using standard titrimetric methods. The ammonia, nitrite, nitrate and phosphate concentrations were analysed using an automated photometric analyser (Gallery, ThermoFisher Scientific, United States) according to its standard operating procedures. The pH of the water was monitored using a pH-meter (Accsen PH 8 Basic, Lasec, South Africa). The flowrate through the BSF and the plant development were additionally monitored on a daily basis. Finally, the electrical performance of the CWMFC was monitored by continually measuring the potential across the external resistance by means of a data acquisition device (USB-6000, National Instruments, Hungary).
CE =
IM FbqΔCOD
(3) −1
is the molecular weight of oxygen, where, M =32 g mol F = 96,485 C mol−1 is Faraday’s constant, b =4 mol e−/mol O2 is the number of electrons exchanged per mole of oxygen, q is the influent flowrate in L s−1 and ΔCOD is the difference between the influent and effluent COD in g L−1. 3. Results and discussion 3.1. Water treatment efficiency The investigations displayed promising results in terms of the ability of the system to remove organic material, nutrients and pathogens from the greywater. The study provided evidence that the biological and physical processes taking place in the CW-MFC were the primary mechanisms for removing chemical oxygen demand (COD) from the greywater, accounting for the removal of 81% of the organic material before the greywater entered the BSF. The BSF, however, was found to contribute further to the removal of COD from the greywater with final average treatment efficiencies of 91% and 86% recorded for the two testing periods. In an effort to improve the treatment efficiency of the integrated system, the design was modified to include a granular activated carbon filter at the final stage of the treatment chain (for 4
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C.R. Bolton and D.G. Randall
Fig. 2. COD concentrations and removal efficiencies for two different greywaters tested in this study with concentrations of 807 mg L−1 and 432 mg L−1.
the E. coli concentrations in the effluent were found to satisfy the requirement in the standard (undetectable count in 100 mL), failure to comply with any alternative microbiological requirement as stipulated in the code could imply an unacceptable risk to human health. In further investigations, all microbiological determinands should be monitored in order to verify compliance. Investigations into appropriate disinfection treatment processes should also be conducted to ensure that the bacteriological limits as listed in the standard are achieved on a continuous basis.
greywater 2 only) which was found to effectively remove almost all residual organics from the greywater. This resulted in a 99% COD removal efficiency for the modified system design. The results recorded over the duration of the experiment are illustrated in Fig. 2. The removal efficiency for greywater 1 was observed to decrease in the MFC while the removal efficiency of the BSF was observed to decrease for greywater 2. This could likely be attributed to the GAC in the anode becoming saturated with pollutants, the slow acclimatisation of bacteria in the MFC to the wastewater, a reduction in dissolved oxygen levels in the BSF or damage to the biofilm layer. There are, however, no concrete explanations for these observations and it is recommended that they be investigated further. The CW-MFC and BSF was additionally observed to contribute to the removal of nutrients from the synthetic greywater. The system showed a promising potential for the removal of nitrate and phosphate from the greywater with removal efficiencies up to 63% (effluent concentration of 0.31 mg N L−1) and 75% (effluent concentration of 0.07 mg P L−1) being recorded for the compounds respectively. It was further noted that despite removal efficiencies being low in comparison to COD removal, the effluent nutrient concentrations were consistently found to comply with South African national standards (SANS 2411:2015) for drinking water quality. Finally, the CW-MFC and BSF was observed to achieve complete bacterial removal for a 4-log concentration of E. coli in the influent greywater. The colony forming units were enumerated using MacConkey’s Agar, a specialized media that is selective for gram-negative bacteria and can differentiate those bacteria which are able to ferment lactose such as E. coli [20]. The samples were tested at various concentrations, as seen in Fig. 3, to ensure that the bacteria concentrations were in the appropriate range for obtaining a viable plate count. Results showed that an initial increase in the E. coli concentrations could be observed after the CW-MFC, however, no microbial colonies could be detected in the effluent after the greywater had passed through the BSF. Inclusion of the BSF in the design of the greywater treatment system was, thus, determined to be an integral component of the system design for the purpose of removing microbiological contaminants from the greywater. This is due to microbes in the biofilm which are responsible for degrading the pathogens in the greywater. However, any pathogens that pass through the system to the non-biological zone in the BSF are expected to die naturally due to the lack of oxygen and nutrients in this region. The result of the study illustrated the ability of the system to reduce the concentrations of pollutants in the greywater. The effluent was found to consistently satisfy the selected requirements for drinking water which suggests that the system has the potential to treat handwashing greywater back to a level that is safe for human contact. While,
3.2. Bioelectricity generation performance Polarization curves were determined for the purpose of electrochemically characterising the CW-MFC operating under two different organic loading conditions. The maximum power and corresponding current densities were recorded as 2.42 mW m−3 and 35.80 mA m−3 in the case of the system fed greywater with an influent COD of 807 ± 26 mg L-1 and 4.33 mW m−3 and 22.12 mA m−3 in the case that influent COD was 432 ± 16 mg L-1 as shown in Fig. 4.6a and Fig. 4.6b. The maximum power densities obtained in this study were found to be significantly lower than those reported in similar studies which were recorded as 302 mW m−3 [21], 321 mW m−3 [14] and 276 mW m−3 [22]. The reason for the variation in power output may be attributed to the system configuration and corresponding potential losses observed in the CW-MFC. The internal resistance for this system was estimated by the slope of the linear region of the polarisation curve and determined to be 2244 Ω and 2992 Ω under the two different organic loading conditions respectively which is equivalent to an internal resistance approximately 10 times greater than those reported by various researchers [21,22]. The internal resistance of an MFC is defined as the sum of the activation losses, bacterial metabolic losses, ohmic losses and concentration losses observed in the system [23]. Ohmic losses describe the resistance of flow of electrons through the electrodes and interconnections [23] and while other losses such as the activation losses may be reduced by optimising the electrode size and material, the most effective way to lower the internal resistance of a CW-MFC has been achieved by reducing the spacing between the anode and cathode and as such minimizing measured ohmic losses [24]. Xu and co-workers [24] reported that there is a strong dependence of the internal resistance on electrode spacing and were able to achieve a maximum power density of 1300 mW m−3, higher than most values previously reported for wetland microbial fuel cells, by reducing the spacing between the electrodes to 70 mm (250 mm smaller than the spacing used in this study). This finding is in agreement with Oon and co-workers [25] who also reported that maximum voltage, power density and coulombic efficiency could be observed at the smallest electrode 5
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Fig. 3. Colonies of E. coli on MacConkey’s Agar media from the (a) influent (b) middle and (c) effluent water samples of the CW-MFC and BSF system.
electrode as the electron acceptor but instead used alternative high potential compounds (these are any substance present in the anode with more positive redox potentials than the electrode) as electron acceptors [23]. Furthermore, the reduction in COD concentration was observed to be met with an increase in CE which suggested that the system is more efficient under lower organic loads. Zhao and coworkers [27] similarly reported that an increase in CE is not proportional to the increase in substrate availability. Fig. 5 depicts a representative cell voltage pattern recorded for a typical 96 -h period during which the treatment system was fed synthetic handwashing greywater with an average COD concentration of 432 ± 16 mg L−1. Fig. 5 shows that the cell voltage was dependent on the conditions under which the system was operated at different times over the course of the day. Fluctuations in the light/dark cycle (which were simulated by turning on and off the plant growing lamp) and pumping scheme were observed to have a direct influence on the recorded voltage variations across the CW-MFC. The observed cell voltage pattern is consistent with observations made in previous studies that have recorded daily oscillations of MFC voltage as a result of photosynthetic activity of the wetland plants [28]. Photosynthetic activities are expected to improve the electrical output of the CW-MFC during sunlight hours due to the increased level of substrate available for energy production and oxygen released by the plant roots [26]. However, water losses as a result of evapotranspiration have also been reported to
spacing. The primary focus of this work was on treatment and reuse and hence power optimisation was not investigated further. However, the system has potential and methods for improving power production should be considered in future work. Both polarisation curves presented in Fig. 4 show an initial steep decrease in potential for low current densities which represents the activation losses which occur during the electron transfer at the electrode [23]. Low activation energies are reportedly achieved by increasing the surface area of the electrode and allowing for the establishment of a biofilm on the surface [23]. It is, therefore, recommended that, in addition to investigating the effect of reducing the electrode spacing on the power output of the CW-MFC, that alternative electrode configurations be investigated. This might include, but is not limited to, investigating the use of a carbon rod current collector in the anode as done by Srivatava and co-workers [14] and increasing the size of the cathode as investigated by Corbella and co-workers [26]. Coulombic efficiencies (CEs) for the system operated at average organic loadings of 807 mg L−1 and 432 mg L−1 were recorded as 0.003% and 0.007% respectively. The CE values obtained in this study are lower than the CE values that have been previously reported for wetland microbial fuel cells – ranging between 0.05 and 3.9% [17]. The low CEs obtained indicate that most of the organic matter degraded in the CWMFC was not contributing to power generation in the cell. This observation suggests that the bacteria were unable to utilise the
Fig. 4. Polarisation and power curves for (a) Greywater 1–807 ± 26 mg L−1 and (b) Greywater 2 - 432 ± 16 mg L-1. 6
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Fig. 5. Fluctuations in potential across the CW-MFC over a typical 96 -h period.
influence the performance of CW-MFCs [26]. In this study the cathode was placed at the surface of the water and as such any water loss in the CW-MFC would result in the drying out of the cathode which may hamper the flow of electrons and correspond to a subsequent decline in the recorded voltage. Cell voltages were observed to remain high during the night when evapotranspiration was low with a slight decline being observed after 07h00 when the lights were switched on. The adverse effects of water loss due to evapotranspiration during the sunlight hours were, however, mitigated by operating the system under continuous upflow conditions. This was done to maintain the water level at an optimum height throughout the day (09h00 to 17h00) by allowing the greywater to flow directly from the CW-MFC into the BSF via the connecting outlet pipe. The maximum daily cell voltage recorded over the second testing period was 99.3 mV with an average cell voltage of 86.7 ± 3.3 mV.
Fig. 6. Proposed CW-MFC bathroom set-up for powering 3 W LED light bulb.
The greywater treatment system could be designed to comprise of separate modular units to allow easy access for maintenance purposes and for the potential replacement of materials such as the GAC. This process could be simplified by adopting a cartridge removal system. Considering future developments in lighting technology that require lower power inputs and CW-MFC technology that produce higher power outputs it will soon be possible to simultaneously recycle bathroom greywater while producing sufficient energy to meet bathroom lighting power requirements. At this time, the primary focus for the development of this technology remains the potential for on-site recycling of handwashing greywater but the added potential for simultaneously producing electrical energy must not be down played. Meanwhile, further pilot scale studies are required to determine the exact power output and treatment efficiency of such a system. It is recommended that these investigations also focus on determining the environmental and economic impacts associated with implementation, developing a life-cycle assessment as well as compiling a detailed operation and maintenance plan for the system to be implemented in different urban contexts. Future developments of the technology may even see similar planted MFC systems incorporated into vertical gardens to supplement power supply in the building or even in outdoor mini wetlands to improve stormwater quality while also powering street lights.
3.3. Design implications The outcome of the research showed that the use of CW-MFCs for the treatment of handwashing greywater presents the opportunity to harvest electrical energy from the microbial breakdown of organic matter in the greywater. The results of the research were, therefore, used to analyse the practical feasibility of implementing CW-MFC technology in a commercial bathroom setting for the purpose of generating sufficient energy to power a bathroom light. While energy surplus provided by CW-MFCs has been explored thoroughly in previous literature, there is very little in the way of developing practical and innovative solutions for the application and harvesting of such power. Since the present study did not include any design optimisation, a theoretical maximum power density of 1300 mW m−3 as reported by Xu et al. (2018) was used to determine the potential system set-up required to power one bathroom light. Furthermore, with rapid developments in light-emitting diodes (LED) as an energy efficient alternative to traditional lighting solutions, ceiling light fittings with wattages as low as 3 W are now readily available in the market place and as such a power requirement of 3 W was considered for similar purposes. Assuming that the maximum power density of the system, obtained under laboratory conditions, could be scaled up with 100% efficiency, then a constructed wetland with a volume of 2.3 m3 would be required to generate sufficient electrical energy to power the 3 W LED light. This volume could, theoretically, be housed within a standard restroom vanity unit (900 × 900 x 3000 mm) that comprises of 3 handwashing basins as shown in Fig. 6.
3.4. Water savings and financial implications An obvious benefit that could be realised as a result of installing the CW-MFC and BSF system in commercial restrooms is that significant 7
Journal of Environmental Chemical Engineering 7 (2019) 103249
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Fig. 7. Water cost savings and potential payback period. Costing was based on local South African prices and converted to USD using an exchange rate of 1 USD = R13.82 as of 18 January 2019.
removal for a 4-log E. coli influent load and a 99% COD removal was achieved by including a final GAC filter for a 432 mg L−1 organic load. The results obtained in this study agree with existing research on CWMFCs which indicate that COD removals for CW-MFCs typically range from 60 to 100% [29] - where the upper limit was achieved by Oon and co-workers in 2015 using a vertical upflow device with artificial aeration of the cathode [25]. The final effluent quality, in this study, was further found to comply with the selected requirements stipulated in South African National Standards (SANS 241-1:2015) for drinking water quality which illustrated the potential for the system to return greywater to potable quality required for handwashing purposes. This is a significant observation for a system that has been designed for the closed-loop recycling of a water source as noted by Gassie and Englehardt [30]. While the bioelectricity generation in the system was observed to be low with a maximum power density of 4.33 mW m-3, the study successfully demonstrated that the energy released during the microbial breakdown of organics in handwashing greywater may be recovered as electrical energy using CW-MFC technology. Furthermore, assuming a cost of USD145/unit, the cost of installing the proposed systems in a building with 1000 occupants could be recovered after just over 1 year of operation. The findings captured in the study were used to discuss a practical means of implementing the technology in a commercial environment by developing a potential system set-up that would produce sufficient electrical energy to power a 3 W LED light bulb. Moreover, this study showed that development of an on-site greywater treatment system that integrates CW-MFC and BSF technology could allow for the sustainable and potentially profitable management of freshwater resources. This could be one way in which the commercial sector could effectively contribute to water saving initiatives. Srivastava and co-workers suggest that up to now most studies have used synthetic greywater and primarily focus on the validation of CW-MFC technology for bio-electricity generation [29]. It is, therefore, recommended that future investigations focus on determining the performance and maintenance requirements of a pilot-scale system operated using real handwashing greywater and that appropriate methods for disinfecting the effluent water before reuse be determined. Finally, further developments to the research should aim to determine the economic and environmental impacts of the technology, expected lifecycle behaviour of the system and a detailed operation and maintenance plan.
amounts of potable water may be saved in comparison to traditional wash basins. This benefit was quantified and evaluated by determining the annual water cost savings that could be expected according to the City of Cape Town water and sanitation tariffs for level 4, 5 and 6 water restriction as shown in Fig. 7. A representative office building catering for 1000 employees was used to determine the potential financial feasibility of developing the greywater treatment system. An office building of this size would require a minimum of 20 wash basins to be provided in the restroom facilities in order to comply with the South African Government Notice 943 of 2013. Offsetting the initial capital investment for the 20 units with the expected water savings would mean a financial payback period for the entire system at level 6 restrictions after only 3 years. This was determined assuming a costly piece of equipment (USD376/unit) such as the one constructed for the laboratory investigations. Further calculations showed that by reducing the cost of each unit from USD376/ unit to USD145/unit, because of economies of scale and use of cheaper material, the cost of installing the system could be recovered after just over 1 year of operation. Although this study was limited to a high-level analysis of the water saving potential of the CW-MFC and BSF system, it successfully illustrated the potential profitability of developing technology of this kind for implementation in a commercial office building environment and an ever-increasing water sensitive future. 4. Conclusions New approaches to addressing the sustainable management of existing water reserves are emerging with a growing global interest in exploiting the reuse potential of greywater resources. This study aimed to address the challenge of realigning imbalances in freshwater supply and demand through the development of an on-site greywater treatment system that could be implemented in a commercial office building environment. The installation investigated in this study was designed to combine constructed wetland microbial fuel cell (CW-MFC) and biological sand filter (BSF) technology to allow for the continuous recycling of handwashing greywater while simultaneously producing electrical power. The CW-MFC was planted with Cyperus papyrus ‘nana’, a wetland plant indigenous to southern and eastern Africa, which showed high resilience to applied greywater conditions. The performance of the system was evaluated by regularly monitoring the organic material, nutrient and E. coli removal efficiencies as well as the cell voltage generation. The system was observed to achieve a complete bacterial 8
Journal of Environmental Chemical Engineering 7 (2019) 103249
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Conflict of interest [13]
Nothing declared. Acknowledgements
[14]
The authors would like to gratefully acknowledge the University of Cape Town and the Water Research Commission for their financial support. We also wish to thank Njabulo Thela and Hector Mafungwa from the Water Quality Lab for all their technical support with the experiments as well as Charles Nicholas for building the experimental rig. The help from Marilyn Krige (UCT Molecular and Cell Biology Department) is also appreciated. We also acknowledge the receipt of the 2018 Greenovate Engineering Award for the project.
[15] [16] [17]
[18] [19]
[20]
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Contents lists available at ScienceDirect
Journal of Environmental Chemical Engineering journal homepage: www.elsevier.com/locate/jece
Development of an integrated wetland microbial fuel cell and sand filtration system for greywater treatment Chloe Rose Bolton, Dyllon Garth Randall
T
⁎
Civil Engineering Department, University of Cape Town, 7700 Cape Town, South Africa
A R T I C LE I N FO
A B S T R A C T
Keywords: Bacteria Cape Town Drought Handwashing Water reuse
This research focused on the design and operation of a system combining a constructed wetland microbial fuel cell (CW-MFC) and a biological sand filter (BSF) for the continuous treatment and recycling of handwashing greywater. The technology was assessed both in terms of its ability to treat the greywater and to generate bioelectricity. The performance was evaluated by regularly monitoring the organic material, nutrient and E. coli removal efficiencies as well as the power generation. The system was observed to achieve a complete bacterial removal for a 4-log E. coli influent load and a 99% COD removal was achieved with a 432 mg L−1 organic load. The final effluent quality was found to comply with South African standards for drinking water quality, illustrating the potential for reuse. Moreover, a maximum power density of 4.33 mW m-3 was achieved by the system thus demonstrating the ability of the process to recover power from handwashing greywater. It was recommended that future investigations focus on determining the performance and maintenance requirements of a pilot-scale system operated using real handwashing greywater, appropriate methods for disinfection of the effluent, improved power generation and the expected life-cycle behaviour of the greywater treatment system.
1. Introduction South Africa as a nation is no stranger to threats on water security – Cape Town having experienced the lowest recorded rainfall in the past century in both 2015 and 2017 [1]. As a result, Level 6B water restrictions were implemented in Cape Town as of 1 February 2018 although later relaxed to level 5 restrictions as of 1 October 2018. At its peak, the target was set by the City to reduce water usage to 450 ML day−1 with residents being limited to just 50 litres per person per day whether at home, work or elsewhere [1]. Furthermore, all non-residential properties were instructed to cut down water use by 45% compared to the permissible pre-drought consumption or risked facing strict penalties [2]. The level 6 tariffs for the provision of commercial water and sanitation (USD4.12/kL and USD3.20/kL respectively) saw an increase of more than double the level 4 tariffs of 2017 [2]. This tariff increase had significant financial implications for businesses if they did not take immediate action to manage their water usage. While many people have adopted measures to cut down on freshwater consumption on an individual level, the question remains as to how the commercial sector can contribute effectively to water saving initiatives. The water demand of commercial buildings may be reduced through the incorporation of innovative water efficiency measures. Examples of such measures that have successfully been implemented in commercial ⁎
buildings across the globe include the collection and use of rainwater, on-site collection and treatment of effluent for reuse and integrated building water management plans [3]. The largest portion of non-domestic urban water use is attributed to office and public buildings [4]. Between 20 and 40% of the indoor water used in these commercial buildings is generated as greywater from washbasins and kitchenettes while the rest may be attributed to the flushing of toilets and urinals [4]. By incorporating greywater treatment systems in commercial office buildings, the light greywater generated may be collected, treated and reused. This can reduce the freshwater consumption of office buildings by up to 50% [4] which has positive impacts on the environment and economic benefits for businesses where water and sanitation tariffs are fixed – as is the case for South Africa. In addition, technologies such as waterless fertilizer-producing urinals can also be implemented to reduce water consumption in buildings [5–7]. Greywater is urban indoor wastewater that is generated from several different activities but specifically excludes any wastewater generated from the flushing of toilets. Common contaminants in greywater include substances such as food particles, soap, microorganisms, salt, oil and dirt [8]. Since greywater can be made up of any combination of these wastewater sources and as such concentration of contaminants, the chemical composition of greywater is considered to be highly variable [8]. It is, however, apparent that the greywater produced in an
Corresponding author. E-mail address: [email protected] (D.G. Randall).
https://doi.org/10.1016/j.jece.2019.103249 Received 24 May 2019; Received in revised form 26 June 2019; Accepted 29 June 2019 Available online 02 July 2019 2213-3437/ © 2019 Elsevier Ltd. All rights reserved.
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technology since they involved the slow treatment of water and required significant land area but recent studies have shown that by combining wetland and innovative microbial fuel cell technology, these challenges can be overcome [14]. Microbial fuel cells (MFCs) are a category of fuel cells which produce electricity from biomass and bacteria and have the potential application as an alternative renewable energy source [15]. Organic matter is degraded by microorganisms inside of the MFC anodic chamber and in the process electrons are released [16]. These electrons are collected and conveyed through an external electric circuit from the anode (negative terminal) to the cathode (positive terminal) and as such useful electrical energy can be produced [15]. Since wastewater contains stored energy in the form of organic matter it stands to reason that MFCs are capable of generating electricity from wastewater sources [15]. CWs and MFCs are considered to be compatible technologies as the redox conditions that are required for the development of MFCs are known to develop naturally in wetlands. Both systems additionally rely on the actions of bacteria to remove contaminants from water [17]. Studies have shown that the combination of the systems can improve the performance of the wetland in treating wastewater by 27–49% while simultaneously allowing for the generation of electrical energy [14]. Srivastava and co-workers [14] found that closed circuit CWMFCs outperform normal constructed wetlands, removing 86% of chemical oxygen demand (COD) in comparison to just 63.8% under an organic load of 0.75 g L−1. This enhanced performance is attributed to the improved microbial electron transfer from the anaerobic zone to the aerobic zone through the external electric circuit [14]. Anaerobic treatment pathways are known to be dominant in wetland systems, however, they are described as being both slower and less efficient processes for the oxidation of organic material due to the scarcity of electron acceptors. The same study showed that the maximum power density and current density that was produced by the CW-MFC system were 321 mW m-3 and 422 mA m-3 respectively. Variations in reported power outputs for CW-MFCs may be attributed to the redox conditions within the cell which are established naturally. These conditions are influenced by the set-up and operational conditions as well as the characteristics of the wastewater used. The primary aim of this work was to design and test a system combining CW-MFC and BSF technology such that handwashing water could be continuously recycled on-site. A secondary focus of the research was to investigate if the system could simultaneously produce bio-electricity. The research aimed to investigate the potential for the advancement of biotechnologies and on-site greywater treatment systems in an effort to determine affordable means of reducing the water demand of commercial buildings.
office environment is likely to be primarily attributed to handwashing activities. It is estimated that soap is the most significant pollutant in handwashing greywater, accounting for 90% of all contamination [9]. The physical and chemical composition of the greywater produced in these buildings is, therefore, believed to be unique in the sense that it can somewhat be controlled through the soap products that are provided for use [9]. Commercial buildings are expected to be suitable for the development and application of on-site greywater treatment systems. There are a number of different treatment technologies that have been developed and tested for the purpose of treating greywater. These systems are classified in terms of the primary mechanism employed for the treatment of the water and include physical, biological, chemical, natural and hybrid designs [10]. The latter refers to systems that make use of a combination of different treatment methods to improve the treatment efficiency as well as the quality of the effluent [10]. Research suggests that the optimal design for greywater treatment systems consists of multiple stages including biological treatment, filtration and disinfection [4,10]. Slow Sand Filtration (SSF), which could be used to treat greywater, is one of the earliest known methods of water treatment and has been used by humans since the development of the first sand filtration systems in the early 19th century [11]. These systems were used extensively over Europe, however, lost popularity in the 20th century with the development of other treatment processes that could operate on smaller footprints and handle greater variations in influent quality [11]. In the past 30 years there has been a renewed interest in the development of SSFs and biological sand filters (BSF) for domestic use in rural contexts [11]. This is attributed to the simplicity of design as well as its low energy and chemical requirements. The main difference between a SSF and BSF is the development of a biofilm layer in the latter which allows for biological treatment processes to remove harmful microorganisms from the wastewater. A typical BSF is comprised of a layer of graded filter sand that is supported by layers of gravel material. The system is designed to allow water to flow vertically downwards on a gravity basis. BSFs mimic naturally occurring physical and biological water purification processes for the removal of contaminants such as pathogens, organic matter and suspended particles [11]. The influent water is introduced at the surface of the filter and allowed to percolate through the sand layers [10]. As the water filters through the layers, pollutants are removed from the water by way of mechanical action (such as absorption, diffusion and sedimentation) as well as biological action (such as metabolic breakdown) [11]. BSF systems have been adapted from traditional SSF designs for the purpose of intermittent domestic use. In addition to the standard filter sand and support layers, designs include a diffusion plate for the homogenous supply of influent water without disrupting the biofilm and an outlet pipe which is fixed at a specific height so as to control the standing water level within the system [12]. Constructed wetlands (CW) are systems that have, similarly, been developed for the purpose of treating wastewater. They are engineered wetlands that are designed to mimic natural processes for water treatment [13]. They consist of vegetation, soils, microorganisms and water and use physical, chemical and biological processes to remove pollutants from a water source. The application of constructed wetlands for water treatment has gained popularity due to the potential it holds to offer a cost effective and low-energy solution to treating wastewater [13]. The major disadvantages presented by the technology as it stands, however, is that wetlands are outdoor systems that are land intensive and may be sensitive to climate change [10]. CW systems have emerged as an option for low cost treatment of wastewater and have been readily tested for their performance in removing contaminants such as heavy metals, dye, nutrients and microbes from a range of domestic, industrial, urban and agricultural wastewater [14]. CWs have in the past shown some limitations as a
2. Material and methods 2.1. System design and set-up In this study a laboratory scale CW-MFC and BSF system was designed and constructed for the purpose of treating synthetic handwashing greywater. The system was designed to comprise of two cylindrical columns, the first a wetland microbial fuel cell and the second a biological sand filter as illustrated in Fig. 1. The design of the CW-MFC was chosen based on the system reported by Srivastava and co-workers [14] to perform optimally both with respect to wastewater treatment and electrical energy production. The BSF was constructed as an adaptation to the traditional slow sand filter as a smaller unit designed for the purpose of intermittent use [12]. The system was later modified to include a granular activated carbon (GAC) filter (diameter 65 mm, height 400 mm) at the final stage of the treatment chain to investigate a means of improving the removal efficiency of pollutants from the wastewater. The modification was made after the first testing period after observing lower than required removal 2
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done to maintain a standing water level at a height that would prevent the filter sand from drying out as a means of protecting the biofilm from damage. Finally, a diffuser plate constructed from stainless steel was installed 140 mm above the filter sand layer to ensure even distribution of the influent greywater into the BSF unit. 2.3. Constructed wetland microbial fuel cell assembly The vertical upflow CW-MFC was fabricated in a transparent acrylic plastic column with an internal diameter of 100 mm, a height of 650 mm and a final liquid volume of 2.2 L. The emergent plant species employed in this study was Cyperus papyrus ‘nana’. The plant, which is indigenous to eastern and southern Africa, was chosen based on a study which reported that the plant displays high resistance to applied greywater conditions [18]. The first layer in the cell consisted of washed LECA (lightweight expanded clay aggregate) with a pellet size of 8–16 mm that was placed to a height of 100 mm from the base of the column, acting as both protection for the inlet pipe and supporting layer for the anode. The cell was equipped with an anode constructed from granular activated carbon (GAC) (1.7–3 mm) that was compacted around a cylindrical steel mesh electron collector (diameter 30 mm) in a nylon fabric sleeve for ease of placement. The anodic chamber extended to a height of 230 mm above the supporting layer and was overlain by a 20 mm glass wool separating layer and 300 mm layer of LECA making up the cathodic chamber. The cathode was constructed from a 70 x 70 mm platinum (Pt) coated carbon paper (0.4 mg cm−2) and placed such that the lower end of the cathode was always in contact with the wastewater. Finally, the electrodes were connected across a 1000 Ω resistor via nickel-coated copper wires that were properly sealed by epoxy material.
Fig. 1. Schematic of CW-MFC and BSF system [(1) Cyperus papyrus ‘nana’ plant species, (2) Cathode, (3) Anode, (4) Glass wool, (5) 1 000 Ω Resistor, (6) Multimeter, (7) Sampling port, (8) Diffuser plate, (9) Standing water layer, (10) Outlet, (11) peristaltic pump] used in this study.
efficiencies, since the intention of the system was to return the greywater back to its original quality fit for handwashing purposes. The system design was chosen based on literature [12,14] to maximise treatment efficiency of the unit by allowing for both physical and biological treatment processes to take place. An integrated greywater treatment system such as the one designed in this study is believed never to have been tested for the intended purpose of recycling handwashing greywater on-site.
2.4. Synthetic greywater 2.2. Biological sand filter assembly For the purpose of this study, a synthetic greywater recipe was designed as a modification of the recipes developed and used for testing by various researchers [9,19]. The synthetic recipe was developed to recreate the physical and chemical characteristics of a generic handwashing greywater. The greywater was designed for the purpose of testing the treatment unit under laboratory conditions and was, therefore, required to mimic real handwashing composition, provide a matrix that would allow for the survival of micro-organisms and be reproducible to obtain consistent quality between batches [19]. Generic handwashing greywater was simulated using Earth Sap East India Islands Liquid Soap (which is biodegradable and contains no petrochemicals or synthetic fragrance) and municipal tap water. Three alternative synthetic greywater recipes were tested to investigate the effect of providing different organic loading rates to the CW-MFC and BSF as well as to investigate the performance of the treatment unit in removing microbiological contaminants from the greywater. The composition and parameters of the different greywaters that were used have been provided in Table 1. The greywater was prepared in 25 L batches every three days to correlate with each testing period over the duration of the experiments. To prepare the synthetic handwashing greywater, the soap required for a 5 L solution was weighed and mixed into 500 mL of municipal tap water using a magnetic stirrer (MSH20, Labcon, South Africa) at low speed for one minute or until visibly dissolved. The concentrated soap
The biological sand filter unit was constructed within a cylindrical transparent acrylic plastic column with an internal diameter of 290 mm and a total height of 800 mm and a working volume of approximately 14.8 L. A transparent system was used for the purpose of monitoring the formation of the biofilm and filtration process. The BSF consisted of layers of selected filter media that were sieved and washed in accordance with the CAWST Biosand Filter Manual [12]. The bottom most layer was constructed from 12 mm crushed rock aggregate that was placed to a height of 50 mm above the base of the BSF. This layer was provided as a drainage layer to ensure the free flow of water from within the BSF into the outlet pipe (nominal diameter 10 mm) fixed at the base of the column. The second layer was constructed from 6 mm crushed rock aggregate with a total depth of 50 mm and was placed as a separating layer between the filter sand and drainage layer. Builder’s sand was selectively sieved using standard sieves no. 16, 30, 50, and 100 to allow for the removal of unwanted debris and large particles. The portion of sand passing through the 1.18 mm sieve (no. 16) was used as the filter medium in the present study. The filter sand was placed to a height of 400 mm above the separating gravel layer. A separator of fine woven polyester fabric was placed above the separating layer to prevent the sand from drawing down into the gravel to protect against potential clogging of the outlet pipe. The outlet pipe was fixed at a level 50 mm above the top of the fine filter sand layer. This was Table 1 Composition of the greywaters used for this study. Component
Greywater 1
Greywater 2
Greywater 3
Soap (Earthsap) Tap Water Escherichia coli (E. coli)
1.2 g/handwash 300 mL/handwash –
1.2 g/handwash 600 mL/handwash –
1.2 g/handwash 600 mL/handwash 4 μL/L at 0.674 OD600nm
3
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2.6. Electrochemical characterisation
solution was added to a further 4.5 L of room temperature tap water in a clean container to obtain the required greywater concentration. The greywater was mixed thoroughly by hand before it was added to the 25 L feed tank. This process was repeated five times to obtain the desired quantity for the three-day test period. The different concentrations of soap meant the COD concentration could be fixed for each type of greywater. The handwashing greywater 3 was inoculated with a saturated overnight culture of wild-type E. coli to account for microbial and pathogenic components of real greywater. This was done for the purpose of monitoring the potential for the system to remove microbiological contaminants from the wastewater. For bacterial dosing, 0.1 mL of E. coli inoculum was mixed with every 25 L batch of greywater. Samples were taken, using autoclaved sample cups, from the inlet and outlet flows as well as the intermediary sampling port after 5 days for analysis. The tests were conducted over a period greater than the hydraulic retention time (HRT) of 2.2 days to ensure that the measured concentrations of E. coli in the effluent were not affected by any standing water in the system which would not have contained the bacteria. The samples, once collected, were kept at 4 °C, until such time they could be analysed, as a means of stopping any further bacterial growth. MacConkey’s Agar plates were used to enumerate the colony forming units (CFU) present in the water samples.
The overall electrical performance of the CW-MFC was evaluated based on the power output and coulombic efficiency (CE) of the system. The power output was calculated as shown in Eq. 1. (1)
P = I Ecell
where, P is the power in Watts (W), I is the current in Amperes (A) and Ecell is the cell voltage in Volts (V). The voltage was measured across a fixed external resistance (Rext) of 1000 Ω and recorded every 10 min using a data acquisition device (USB-6000, National Instruments, Hungary). The current across the resistance was thus determined according to Ohm’s law as shown in Eq. 2.
I=
Ecell R ext
(2)
The direct measure of power across a fixed resistance was calculated by expressing the power as a function of the cell voltage and external resistance. The maximum obtainable power, however, was determined by obtaining the polarisation and power curves for the system under each organic loading condition. The polarisation curve represents voltage as a function of current and was obtained by varying the external resistance over the CWMFC from 100 to 1,000,000 Ω and recording the steady state voltage. The internal resistance of the CW-MFC was obtained by determining the slope of the linear potion of the polarisation curve (Rint=-ΔEcell/ΔI). The power curve was obtained by representing the power as a function of the current across the external resistance with the maximum obtainable power occurring at the point where Rext=Rint (Logan et al., 2006). The power density (W m−3) and current density (A m−3) were normalised to the anodic volume for the purpose of comparing the power output of the system to different systems investigated in the literature. This was achieved by dividing the observed power by the volume of the anodic chamber. Furthermore, the coulombic efficiency, which is a measure of the proportion of electrons transferred to the anode from the substrate, was calculated on the basis of current generated at steady state as shown in Eq. 3 (Logan et al., 2006):
2.5. Operation, sampling and monitoring The CW-MFC and BSF unit was operated under continuous mode with the organic loading being the variable that was changed over the duration of the experiment. The study was divided into three distinct testing periods. The first period comprised of the system being fed greywater with an average influent chemical oxygen demand (COD) of 807 ± 26 mg L−1 (Greywater 1), which was seen to represent handwashing greywater during times of restricted water use, for a period of 12 days. During the second period the system was fed greywater with an average influent COD of 432 ± 16 mg L−1 for 12 days (Greywater 2), which represented general handwashing greywater. During the third period the greywater (Greywater 3) was inoculated with wild-type E. coli as a surrogate bacteria for the purpose of monitoring the potential of the system to remove pathogens and viruses from the wastewater. This testing period lasted 5 days. The synthetic greywater was fed into the system at 16 mL/min using a peristaltic pump (MasterFlex L/S, Cole-Parmer, United States) operated between 09h00 and 17h00 daily. This established a hydraulic retention time (HRT) of 2.3 h for the CWMFC and 2.2 days for the integrated treatment system. Furthermore, the experiments were conducted in a 12:12 h light/dark cycle with ambient light provided using a 50 W COB LED grow light to simulate natural growth conditions for the wetland plant. The ability of the CW-MFC and BSF unit to treat handwashing greywater was evaluated in terms of its removal efficiencies of total COD, ammonia (NH3-N), nitrite (NO2−-N), nitrate (NO3−-N) and phosphate (PO43−-P). Samples of 70 mL were taken from the influent and effluent water flow as well as from the intermediary sampling port (between the CW-MFC and BSF) for analysis every three days. The COD concentrations of the water samples were measured in the Water Quality Laboratory at the University of Cape Town using standard titrimetric methods. The ammonia, nitrite, nitrate and phosphate concentrations were analysed using an automated photometric analyser (Gallery, ThermoFisher Scientific, United States) according to its standard operating procedures. The pH of the water was monitored using a pH-meter (Accsen PH 8 Basic, Lasec, South Africa). The flowrate through the BSF and the plant development were additionally monitored on a daily basis. Finally, the electrical performance of the CWMFC was monitored by continually measuring the potential across the external resistance by means of a data acquisition device (USB-6000, National Instruments, Hungary).
CE =
IM FbqΔCOD
(3) −1
is the molecular weight of oxygen, where, M =32 g mol F = 96,485 C mol−1 is Faraday’s constant, b =4 mol e−/mol O2 is the number of electrons exchanged per mole of oxygen, q is the influent flowrate in L s−1 and ΔCOD is the difference between the influent and effluent COD in g L−1. 3. Results and discussion 3.1. Water treatment efficiency The investigations displayed promising results in terms of the ability of the system to remove organic material, nutrients and pathogens from the greywater. The study provided evidence that the biological and physical processes taking place in the CW-MFC were the primary mechanisms for removing chemical oxygen demand (COD) from the greywater, accounting for the removal of 81% of the organic material before the greywater entered the BSF. The BSF, however, was found to contribute further to the removal of COD from the greywater with final average treatment efficiencies of 91% and 86% recorded for the two testing periods. In an effort to improve the treatment efficiency of the integrated system, the design was modified to include a granular activated carbon filter at the final stage of the treatment chain (for 4
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Fig. 2. COD concentrations and removal efficiencies for two different greywaters tested in this study with concentrations of 807 mg L−1 and 432 mg L−1.
the E. coli concentrations in the effluent were found to satisfy the requirement in the standard (undetectable count in 100 mL), failure to comply with any alternative microbiological requirement as stipulated in the code could imply an unacceptable risk to human health. In further investigations, all microbiological determinands should be monitored in order to verify compliance. Investigations into appropriate disinfection treatment processes should also be conducted to ensure that the bacteriological limits as listed in the standard are achieved on a continuous basis.
greywater 2 only) which was found to effectively remove almost all residual organics from the greywater. This resulted in a 99% COD removal efficiency for the modified system design. The results recorded over the duration of the experiment are illustrated in Fig. 2. The removal efficiency for greywater 1 was observed to decrease in the MFC while the removal efficiency of the BSF was observed to decrease for greywater 2. This could likely be attributed to the GAC in the anode becoming saturated with pollutants, the slow acclimatisation of bacteria in the MFC to the wastewater, a reduction in dissolved oxygen levels in the BSF or damage to the biofilm layer. There are, however, no concrete explanations for these observations and it is recommended that they be investigated further. The CW-MFC and BSF was additionally observed to contribute to the removal of nutrients from the synthetic greywater. The system showed a promising potential for the removal of nitrate and phosphate from the greywater with removal efficiencies up to 63% (effluent concentration of 0.31 mg N L−1) and 75% (effluent concentration of 0.07 mg P L−1) being recorded for the compounds respectively. It was further noted that despite removal efficiencies being low in comparison to COD removal, the effluent nutrient concentrations were consistently found to comply with South African national standards (SANS 2411:2015) for drinking water quality. Finally, the CW-MFC and BSF was observed to achieve complete bacterial removal for a 4-log concentration of E. coli in the influent greywater. The colony forming units were enumerated using MacConkey’s Agar, a specialized media that is selective for gram-negative bacteria and can differentiate those bacteria which are able to ferment lactose such as E. coli [20]. The samples were tested at various concentrations, as seen in Fig. 3, to ensure that the bacteria concentrations were in the appropriate range for obtaining a viable plate count. Results showed that an initial increase in the E. coli concentrations could be observed after the CW-MFC, however, no microbial colonies could be detected in the effluent after the greywater had passed through the BSF. Inclusion of the BSF in the design of the greywater treatment system was, thus, determined to be an integral component of the system design for the purpose of removing microbiological contaminants from the greywater. This is due to microbes in the biofilm which are responsible for degrading the pathogens in the greywater. However, any pathogens that pass through the system to the non-biological zone in the BSF are expected to die naturally due to the lack of oxygen and nutrients in this region. The result of the study illustrated the ability of the system to reduce the concentrations of pollutants in the greywater. The effluent was found to consistently satisfy the selected requirements for drinking water which suggests that the system has the potential to treat handwashing greywater back to a level that is safe for human contact. While,
3.2. Bioelectricity generation performance Polarization curves were determined for the purpose of electrochemically characterising the CW-MFC operating under two different organic loading conditions. The maximum power and corresponding current densities were recorded as 2.42 mW m−3 and 35.80 mA m−3 in the case of the system fed greywater with an influent COD of 807 ± 26 mg L-1 and 4.33 mW m−3 and 22.12 mA m−3 in the case that influent COD was 432 ± 16 mg L-1 as shown in Fig. 4.6a and Fig. 4.6b. The maximum power densities obtained in this study were found to be significantly lower than those reported in similar studies which were recorded as 302 mW m−3 [21], 321 mW m−3 [14] and 276 mW m−3 [22]. The reason for the variation in power output may be attributed to the system configuration and corresponding potential losses observed in the CW-MFC. The internal resistance for this system was estimated by the slope of the linear region of the polarisation curve and determined to be 2244 Ω and 2992 Ω under the two different organic loading conditions respectively which is equivalent to an internal resistance approximately 10 times greater than those reported by various researchers [21,22]. The internal resistance of an MFC is defined as the sum of the activation losses, bacterial metabolic losses, ohmic losses and concentration losses observed in the system [23]. Ohmic losses describe the resistance of flow of electrons through the electrodes and interconnections [23] and while other losses such as the activation losses may be reduced by optimising the electrode size and material, the most effective way to lower the internal resistance of a CW-MFC has been achieved by reducing the spacing between the anode and cathode and as such minimizing measured ohmic losses [24]. Xu and co-workers [24] reported that there is a strong dependence of the internal resistance on electrode spacing and were able to achieve a maximum power density of 1300 mW m−3, higher than most values previously reported for wetland microbial fuel cells, by reducing the spacing between the electrodes to 70 mm (250 mm smaller than the spacing used in this study). This finding is in agreement with Oon and co-workers [25] who also reported that maximum voltage, power density and coulombic efficiency could be observed at the smallest electrode 5
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Fig. 3. Colonies of E. coli on MacConkey’s Agar media from the (a) influent (b) middle and (c) effluent water samples of the CW-MFC and BSF system.
electrode as the electron acceptor but instead used alternative high potential compounds (these are any substance present in the anode with more positive redox potentials than the electrode) as electron acceptors [23]. Furthermore, the reduction in COD concentration was observed to be met with an increase in CE which suggested that the system is more efficient under lower organic loads. Zhao and coworkers [27] similarly reported that an increase in CE is not proportional to the increase in substrate availability. Fig. 5 depicts a representative cell voltage pattern recorded for a typical 96 -h period during which the treatment system was fed synthetic handwashing greywater with an average COD concentration of 432 ± 16 mg L−1. Fig. 5 shows that the cell voltage was dependent on the conditions under which the system was operated at different times over the course of the day. Fluctuations in the light/dark cycle (which were simulated by turning on and off the plant growing lamp) and pumping scheme were observed to have a direct influence on the recorded voltage variations across the CW-MFC. The observed cell voltage pattern is consistent with observations made in previous studies that have recorded daily oscillations of MFC voltage as a result of photosynthetic activity of the wetland plants [28]. Photosynthetic activities are expected to improve the electrical output of the CW-MFC during sunlight hours due to the increased level of substrate available for energy production and oxygen released by the plant roots [26]. However, water losses as a result of evapotranspiration have also been reported to
spacing. The primary focus of this work was on treatment and reuse and hence power optimisation was not investigated further. However, the system has potential and methods for improving power production should be considered in future work. Both polarisation curves presented in Fig. 4 show an initial steep decrease in potential for low current densities which represents the activation losses which occur during the electron transfer at the electrode [23]. Low activation energies are reportedly achieved by increasing the surface area of the electrode and allowing for the establishment of a biofilm on the surface [23]. It is, therefore, recommended that, in addition to investigating the effect of reducing the electrode spacing on the power output of the CW-MFC, that alternative electrode configurations be investigated. This might include, but is not limited to, investigating the use of a carbon rod current collector in the anode as done by Srivatava and co-workers [14] and increasing the size of the cathode as investigated by Corbella and co-workers [26]. Coulombic efficiencies (CEs) for the system operated at average organic loadings of 807 mg L−1 and 432 mg L−1 were recorded as 0.003% and 0.007% respectively. The CE values obtained in this study are lower than the CE values that have been previously reported for wetland microbial fuel cells – ranging between 0.05 and 3.9% [17]. The low CEs obtained indicate that most of the organic matter degraded in the CWMFC was not contributing to power generation in the cell. This observation suggests that the bacteria were unable to utilise the
Fig. 4. Polarisation and power curves for (a) Greywater 1–807 ± 26 mg L−1 and (b) Greywater 2 - 432 ± 16 mg L-1. 6
Journal of Environmental Chemical Engineering 7 (2019) 103249
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Fig. 5. Fluctuations in potential across the CW-MFC over a typical 96 -h period.
influence the performance of CW-MFCs [26]. In this study the cathode was placed at the surface of the water and as such any water loss in the CW-MFC would result in the drying out of the cathode which may hamper the flow of electrons and correspond to a subsequent decline in the recorded voltage. Cell voltages were observed to remain high during the night when evapotranspiration was low with a slight decline being observed after 07h00 when the lights were switched on. The adverse effects of water loss due to evapotranspiration during the sunlight hours were, however, mitigated by operating the system under continuous upflow conditions. This was done to maintain the water level at an optimum height throughout the day (09h00 to 17h00) by allowing the greywater to flow directly from the CW-MFC into the BSF via the connecting outlet pipe. The maximum daily cell voltage recorded over the second testing period was 99.3 mV with an average cell voltage of 86.7 ± 3.3 mV.
Fig. 6. Proposed CW-MFC bathroom set-up for powering 3 W LED light bulb.
The greywater treatment system could be designed to comprise of separate modular units to allow easy access for maintenance purposes and for the potential replacement of materials such as the GAC. This process could be simplified by adopting a cartridge removal system. Considering future developments in lighting technology that require lower power inputs and CW-MFC technology that produce higher power outputs it will soon be possible to simultaneously recycle bathroom greywater while producing sufficient energy to meet bathroom lighting power requirements. At this time, the primary focus for the development of this technology remains the potential for on-site recycling of handwashing greywater but the added potential for simultaneously producing electrical energy must not be down played. Meanwhile, further pilot scale studies are required to determine the exact power output and treatment efficiency of such a system. It is recommended that these investigations also focus on determining the environmental and economic impacts associated with implementation, developing a life-cycle assessment as well as compiling a detailed operation and maintenance plan for the system to be implemented in different urban contexts. Future developments of the technology may even see similar planted MFC systems incorporated into vertical gardens to supplement power supply in the building or even in outdoor mini wetlands to improve stormwater quality while also powering street lights.
3.3. Design implications The outcome of the research showed that the use of CW-MFCs for the treatment of handwashing greywater presents the opportunity to harvest electrical energy from the microbial breakdown of organic matter in the greywater. The results of the research were, therefore, used to analyse the practical feasibility of implementing CW-MFC technology in a commercial bathroom setting for the purpose of generating sufficient energy to power a bathroom light. While energy surplus provided by CW-MFCs has been explored thoroughly in previous literature, there is very little in the way of developing practical and innovative solutions for the application and harvesting of such power. Since the present study did not include any design optimisation, a theoretical maximum power density of 1300 mW m−3 as reported by Xu et al. (2018) was used to determine the potential system set-up required to power one bathroom light. Furthermore, with rapid developments in light-emitting diodes (LED) as an energy efficient alternative to traditional lighting solutions, ceiling light fittings with wattages as low as 3 W are now readily available in the market place and as such a power requirement of 3 W was considered for similar purposes. Assuming that the maximum power density of the system, obtained under laboratory conditions, could be scaled up with 100% efficiency, then a constructed wetland with a volume of 2.3 m3 would be required to generate sufficient electrical energy to power the 3 W LED light. This volume could, theoretically, be housed within a standard restroom vanity unit (900 × 900 x 3000 mm) that comprises of 3 handwashing basins as shown in Fig. 6.
3.4. Water savings and financial implications An obvious benefit that could be realised as a result of installing the CW-MFC and BSF system in commercial restrooms is that significant 7
Journal of Environmental Chemical Engineering 7 (2019) 103249
C.R. Bolton and D.G. Randall
Fig. 7. Water cost savings and potential payback period. Costing was based on local South African prices and converted to USD using an exchange rate of 1 USD = R13.82 as of 18 January 2019.
removal for a 4-log E. coli influent load and a 99% COD removal was achieved by including a final GAC filter for a 432 mg L−1 organic load. The results obtained in this study agree with existing research on CWMFCs which indicate that COD removals for CW-MFCs typically range from 60 to 100% [29] - where the upper limit was achieved by Oon and co-workers in 2015 using a vertical upflow device with artificial aeration of the cathode [25]. The final effluent quality, in this study, was further found to comply with the selected requirements stipulated in South African National Standards (SANS 241-1:2015) for drinking water quality which illustrated the potential for the system to return greywater to potable quality required for handwashing purposes. This is a significant observation for a system that has been designed for the closed-loop recycling of a water source as noted by Gassie and Englehardt [30]. While the bioelectricity generation in the system was observed to be low with a maximum power density of 4.33 mW m-3, the study successfully demonstrated that the energy released during the microbial breakdown of organics in handwashing greywater may be recovered as electrical energy using CW-MFC technology. Furthermore, assuming a cost of USD145/unit, the cost of installing the proposed systems in a building with 1000 occupants could be recovered after just over 1 year of operation. The findings captured in the study were used to discuss a practical means of implementing the technology in a commercial environment by developing a potential system set-up that would produce sufficient electrical energy to power a 3 W LED light bulb. Moreover, this study showed that development of an on-site greywater treatment system that integrates CW-MFC and BSF technology could allow for the sustainable and potentially profitable management of freshwater resources. This could be one way in which the commercial sector could effectively contribute to water saving initiatives. Srivastava and co-workers suggest that up to now most studies have used synthetic greywater and primarily focus on the validation of CW-MFC technology for bio-electricity generation [29]. It is, therefore, recommended that future investigations focus on determining the performance and maintenance requirements of a pilot-scale system operated using real handwashing greywater and that appropriate methods for disinfecting the effluent water before reuse be determined. Finally, further developments to the research should aim to determine the economic and environmental impacts of the technology, expected lifecycle behaviour of the system and a detailed operation and maintenance plan.
amounts of potable water may be saved in comparison to traditional wash basins. This benefit was quantified and evaluated by determining the annual water cost savings that could be expected according to the City of Cape Town water and sanitation tariffs for level 4, 5 and 6 water restriction as shown in Fig. 7. A representative office building catering for 1000 employees was used to determine the potential financial feasibility of developing the greywater treatment system. An office building of this size would require a minimum of 20 wash basins to be provided in the restroom facilities in order to comply with the South African Government Notice 943 of 2013. Offsetting the initial capital investment for the 20 units with the expected water savings would mean a financial payback period for the entire system at level 6 restrictions after only 3 years. This was determined assuming a costly piece of equipment (USD376/unit) such as the one constructed for the laboratory investigations. Further calculations showed that by reducing the cost of each unit from USD376/ unit to USD145/unit, because of economies of scale and use of cheaper material, the cost of installing the system could be recovered after just over 1 year of operation. Although this study was limited to a high-level analysis of the water saving potential of the CW-MFC and BSF system, it successfully illustrated the potential profitability of developing technology of this kind for implementation in a commercial office building environment and an ever-increasing water sensitive future. 4. Conclusions New approaches to addressing the sustainable management of existing water reserves are emerging with a growing global interest in exploiting the reuse potential of greywater resources. This study aimed to address the challenge of realigning imbalances in freshwater supply and demand through the development of an on-site greywater treatment system that could be implemented in a commercial office building environment. The installation investigated in this study was designed to combine constructed wetland microbial fuel cell (CW-MFC) and biological sand filter (BSF) technology to allow for the continuous recycling of handwashing greywater while simultaneously producing electrical power. The CW-MFC was planted with Cyperus papyrus ‘nana’, a wetland plant indigenous to southern and eastern Africa, which showed high resilience to applied greywater conditions. The performance of the system was evaluated by regularly monitoring the organic material, nutrient and E. coli removal efficiencies as well as the cell voltage generation. The system was observed to achieve a complete bacterial 8
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Conflict of interest [13]
Nothing declared. Acknowledgements
[14]
The authors would like to gratefully acknowledge the University of Cape Town and the Water Research Commission for their financial support. We also wish to thank Njabulo Thela and Hector Mafungwa from the Water Quality Lab for all their technical support with the experiments as well as Charles Nicholas for building the experimental rig. The help from Marilyn Krige (UCT Molecular and Cell Biology Department) is also appreciated. We also acknowledge the receipt of the 2018 Greenovate Engineering Award for the project.
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