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Membrane bioreactor technology: A novel approach to the treatment of compost leachate
Waste Management xxx (2013) xxx–xxx
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Membrane bioreactor technology: A novel approach to the treatment of compost leachate Kayleigh Brown a, Avik J. Ghoshdastidar a, Jillian Hanmore a, James Frazee b, Anthony Z. Tong a,⇑ a b
Department of Chemistry, Acadia University, Wolfville, NS, Canada B4P 2R6 E&Q Consulting and Associates Limited, Wolfville, NS, Canada B4P 2R1
a r t i c l e
i n f o
Article history: Received 14 May 2012 Accepted 16 April 2013 Available online xxxx Keywords: Compost leachate Membrane bioreactor Biodegradation Organic waste Caffeine Heavy metals
a b s t r a c t Compost leachate forms during the composting process of organic material. It is rich in oxidizable organics, ammonia and metals, which pose a risk to the environment if released without proper treatment. An innovative method based on the membrane bioreactor (MBR) technology was developed to treat compost leachate over 39 days. Water quality parameters, such as pH, dissolved oxygen, ammonia, nitrate, nitrite and chemical oxygen demand (COD) were measured daily. Concentrations of caffeine and metals were measured over the course of the experiment using gas chromatography – mass spectrometry (GC/MS) and inductively coupled plasma – mass spectrometry (ICP–MS) respectively. A decrease of more than 99% was achieved for a COD of 116 g/L in the initial leachate. Ammonia was decreased from 2720 mg/ L to 0.046 mg/L, while the nitrate concentration in the effluent rose to 710 mg/L. The bacteria in the MBR system adjusted to the presence of the leachate, and increased 4 orders of magnitude. Heavy metals were removed by at least 82.7% except copper. These successful results demonstrated the membrane bioreactor technology is feasible, efficient method for the treatment of compost leachate. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction Due to increasing public concerns on the quantities of solid waste being disposed in landfills and its environmental and health impact, many provinces and states in North America and beyond have implemented strategies for solid waste management (Nova Scotia Environment, 1995). The main goal of solid waste management is to divert the greatest amount of solid waste from landfills through source reduction, material reuse, recycling, business development, and composting (Nova Scotia Environment, 1995). Composting is the controlled decomposition of organic material by microorganisms, such as bacteria and fungi, forming a stable soil called humus (Epstein, 1997). Material that is composted, known as feedstock, is separated into two categories: residential and IC&I (industrial, commercial and institutional). Residential waste consists mainly of left-over food such as meat, fruits and vegetables. IC&I waste includes farming, fishing and wood processing wastes, yard wastes and paper. Many municipalities operate centralized composting facilities. For example, there are 17 major composting facilities in Nova Scotia that process about 186,200 tons/year of various organic wastes (Nova Scotia Environment, 2010). The composting process not only diverts organic ⇑ Corresponding author. Address: Department of Chemistry, Acadia University, 6 University Avenue, Wolfville, NS, Canada B4P 2R6. Tel.: +1 902 585 1355; fax: +1 902 585 1114. E-mail address: [email protected] (A.Z. Tong).
waste from landfills, but also produces two value-added fertilizer products: (1) a solid form, referred to as processing soil or screened compost, and (2) a liquid form, referred to as compost tea. A major issue associated with composting is the formation of leachate. Compost may consist of material with high water content and water is also produced during biodegradation processes. As degradation takes place, this wastewater seeps out and forms what is known as leachate. Some composting facilities, especially those with feedstock from residential and agricultural sources, generate considerable amounts of leachate. For example, Northridge Farms Limited (Aylesford, Nova Scotia), the industrial partner of this study, processes compost material for the Annapolis Valley with a population of about 80,000, and it produces leachate at an average of about 4000 L/day. Compost leachate contains any material that was extracted, suspended or dissolved in the compost pile. Depending on the feedstock, the leachate may contain high levels of metals, biological material, pesticides, phthalates, polychlorinated biphenyls (PCBs) or dioxins (Epstein, 1997; Jarecki et al., 2005; Trujillo et al., 2006). Compost leachate has large quantities of total oxidizable organics that can be measured as chemical oxygen demand (COD). High values of COD are problematic because release of this leachate will deplete dissolved oxygen in receiving waters. The resulting anoxic environments cause fatalities of plants, fish and other aquatic organisms. Many toxic organic compounds exist in the leachate, meaning if it is not treated properly, surface and groundwater may become contaminated placing the
0956-053X/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.wasman.2013.04.006
Please cite this article in press as: Brown, K., et al. Membrane bioreactor technology: A novel approach to the treatment of compost leachate. Waste Management (2013), http://dx.doi.org/10.1016/j.wasman.2013.04.006
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K. Brown et al. / Waste Management xxx (2013) xxx–xxx
public and local ecosystems at risk (Cumar and Nagaraja, 2011). Since leachate is harmful, composting facilities in most municipalities are required to have a leachate management system in place. A proper system consists of infrastructure and monitoring systems designed to collect, monitor, control and treat leachate before being released to the environment (Nova Scotia Environment, 2010). There are very few research publications on treatment of compost leachate. Liu et al. used continuous flow packed zeolite columns to remove ammonia and achieved more than 98% of efficiency (Liu and Lo, 2001a, b). By using Fenton’s reagent, Trujillo et al. showed COD removal rates of 77% and 75% for leachates from wastewater sludge composting and municipal solid wastes, respectively (Trujillo et al., 2006). However, these methods were either focused on one particular water quality parameter or showed low efficiency. Further, these physical or chemical treatment methods lack flexibility, need high maintenance and may introduce new contaminants. One current popular methodology for treating compost leachate is engineered or constructed wetlands. Engineered wetlands are a man-made wetland system containing water, saturated substrates, emergent and submergent plants and animals that are designed to imitate natural wetlands (Hammer, 1989; Stottmeister et al., 2006). Most engineered wetlands resemble marshes; those with herbaceous emergent and submergent plants show the most promise for wastewater treatment, where cattails, rush and giant reed are acclimatized to changing water and nutrient quantities and are more capable of processing large pollutant loads (Hammer, 1989). Engineered wetlands utilize a negligible amount of additional energy, and need minimal operation and maintenance, which makes this technology a low-cost treatment method. Some disadvantages of engineering wetlands include low efficiency, long development time, large space requirement, and reduced efficiency at sub-zero temperatures. A phenomenon called soil saturation may also occur, where the soil in the wetland becomes unable to absorb further amounts of leachate, which renders the treatment by vegetation and bacteria ineffective. Membrane bioreactor (MBR) technology provides biological treatment with membrane separation (Judd, 2006; Meng et al., 2009; Stephenson et al., 2000). The system consists of an aerated water-filled tank containing activated sludge and multiple capillary-form membrane tubes. The pores of ultrafiltration membranes are approximately 20–50 nm in diameter, which effectively retains microorganisms, macromolecules and suspended solids. Microorganisms use the contaminants as nutrients for growth and metabolism. The contaminants are thus broken down into less harmful substances (Judd, 2006). Membrane bioreactor technology has many advantages over conventional activated sludge treatment processes. The secondary clarifiers normally used are able to be replaced by the membrane system, which reduces the space required for the treatment process. The overall retention time of the activated sludge is longer in the MBR, which increases contact opportunities of bacteria with contaminants, and subsequently leads to high efficiency (Ghoshdastidar et al., 2012; Le-Clech, 2010). Another advantage of the membrane bioreactor technology is removal of microorganisms to below detection limit from the MBR effluent due to ultra-filtration. A MBR system is very compact, especially compared to the acres required by engineered wetlands. A further advantage of MBR technology over engineered wetlands is that the system is unaffected by freezing caused by sub-zero temperatures and thus can be used during all times of year. The MBR technology has been applied to the treatment of contaminants in both municipal and industrial wastewater (Ghoshdastidar and Tong, 2013; Radjenovic´ et al., 2008), but our research was the first attempt at using MBR as a treatment method for compost leachate. An MBR system was built and optimized
according to the characteristics of compost leachate. Various wastewater quality parameters were monitored to demonstrate the feasibility and efficiency of the MBR treatment method. 2. Materials and methods 2.1. Compost leachate sampling and preparation Compost leachate samples were collected from Northridge Farms Limited, a composting facility located in Aylesford, Nova Scotia. The leachate was pumped out of a holding tank into a barrel, and allowed to settle for two days to remove major suspended solids. The supernatant in this barrel was then pumped into PVC containers. These containers were transported to the laboratory and stored in a cold room at 4 °C for the duration of the experiment. About 2 L of the leachate was transferred daily to a glass jar and pumped into an MBR for treatment. The same leachate stored in the cold room was used throughout the experiment to avoid variation of characteristics of the initial leachate. 2.2. Membrane bioreactor setup An MBR system was built in-house using SuperUF membrane modules provided by Superstring MBR Technology Corp. (Brooklyn, NY). Five rows of membrane modules were grouped in this MBR and provided a total membrane surface area of 1.10 m2. Membrane pore size diameter ranged from 20 to 30 nm. The capacity of the MBR tank used in this project was 114 L. A continuous supply of air was introduced to the tank at 1100 L/h using copper spargers below the membrane. Using peristaltic pumps, the influent and effluent were pumped into and out of the MBR at a volumetric flow rate of 0.83 mL/min that was calibrated daily. As composting facilities only generate small amounts of leachate, this experimental flow rate can be proportionally expanded to industrial applications. The temperature of the MBR was maintained at a constant 25 ± 1 °C throughout the experiment period. This MBR system was initiated with 5 L of activated sludge taken from Mill Cove municipal wastewater treatment plant in Halifax, Nova Scotia. No additional chemical additives or nutrients were introduced during the course of project. It was noticed that the amount of sludge in MBR increased during the experiment period. In this lab-scale experiment, solids removal was not conducted due to the relatively large capacity of the bioreactor in comparison to the influent volume. However, it is expected that periodic solids removal should be incorporated in industrial-scale applications. Compost leachate was continuously pumped into the MBR system for 39 days, and wastewater quality parameters were monitored daily during this time period. A biological treatment system may be considered in stationary condition after 2 sludge retention times (SRTs) and most MBR facilities have SRT between 10 and 20 days (Delgado et al., 2011). At the end of the 39-day experiment period, major water quality parameters become stable and the MBR system reached a steady-state. 2.3. Test methods of common wastewater parameters A Hach HQd 40 probe was used to measure the pH, electrical conductivity and dissolved oxygen of effluent. These tests were completed by obtaining 200 mL of effluent sample and immediately analyzing the sample, to avoid inaccuracies due to temperature changes and the loss of oxygen. The appropriate probe was attached to the meter, placed in the sample, which was stirred at a rate of 30 rpm/min. A Hach DR2800 spectrophotometer was used to measure chemical oxygen demand (COD), ammonia, nitrate and nitrite. These water quality parameters were measured using
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K. Brown et al. / Waste Management xxx (2013) xxx–xxx
standard HACH methods: the reactor digestion method with TNT 821 LR vials, salicylate method with TNT 830 ULR vials, dimethylphenol method with TNT 835 LR vials, and diazotization method with TNT 839 LR vials, respectively. Microbiological analysis was completed daily to monitor the population of heterotrophic bacteria in the MBR. The measurements were completed in triplicate; three independent samples of activated sludge were obtained and quantitated. Agar plates were prepared according to the Standard Methods for the Examination of Water and Wastewater (Clesceri et al., 1989). The agar mixture was autoclaved at 121 °C for 60 min, poured into plastic Petri dishes and allowed to set for 48 h before use. Approximately 200 mL of activated sludge was obtained from the MBR, and 100 lL were quickly removed using a micropipette and placed in one of the test tubes containing saline solution. Any further dilutions were made in 10 increments. The plate was then inoculated with 100 lL of the final dilution mixture, which was spread evenly on the plate using the metal spreader. Plates were placed in the incubator at 36 °C for 48 h. The total number of colonies on the plate was counted and calculated in colony forming units (CFU/mL) according to the level of dilution.
3
cess, only excess of compost leachate is discharged and its flow rate is usually less than several cubic meters for a medium size composting facility. The treatment method used in this study was designed according to leachate’s characteristics of high waste load and low flow rate. In this lab-scale experiment, a relatively large bioreactor (114 L) and a small flow rate (0.83 mL/min) provided a long biodegradation period. Air at a high flow rate (1100 L/h) was pumped into the bioreactor to ensure adequate oxygen for aerobic digestion. Organic loading rate (OLR), hydraulic retention time (HRT) and membrane flux were 50.9 mg/L/h, 95 days and 0.756 mL/cm2/min, respectively. Trans-membrane pressure was kept below 30 kPa and no membrane cleaning was necessary during the experiment period. 3.2. Chemical oxygen demand
3. Results and discussion
COD was monitored very closely as an important wastewater quality parameter (Fig. 1), because it was indicative of the overall organic waste load of the MBR system and the adaptation of the microorganisms to the presence of the leachate. COD was studied rather than biochemical oxygen demand (BOD) because COD measurements provide better precision and require less time to conduct than BOD measurements. The precision of spectrophotometric method for COD was ±3 mg/L as determined by the supplier, Hach. As noticed in Fig. 1, COD values varied more for later experiment days; this increased uncertainty was introduced by the necessary effluent dilution in order to bring COD values into the detection range of the Hach method. The COD of the effluent shows a smooth and gradual increase before reaching a plateau. This increase can be modeled using a reversed exponential decay curve and a nonlinear curve fit (dash line) was performed. The time constant of the exponential curve was determined to be 15.9 days. Consequently, the half-life time can be calculated as 11.0 days, which indicates that the MBR system reached its half treatment capacity in 11 days. The curve fit also yields an asymptotic limit at 437 mg/L (the dotted line in Fig. 1). The initial COD of the compost leachate was 116 g/L. Such a significant COD value indicated that the compost leachate was saturated with organic waste. At the end of experiment (Day 39), COD reached 400 mg/L thereby achieving a 99.7% reduction of the initial COD. That being said, the final COD value is still relatively high and may exceed the wastewater effluent guidelines (ABL Environmental Consultants Ltd., 2006). In future experiments, an additional aerated tank/clarifier can be added to further reduce the COD. Further, sludge/solids removal and membrane cleaning were not performed in this lab-scale experiment. The increase of COD during the experiment period might be related to excessive sludge or membrane fouling. Regularly scheduled sludge removal and membrane cleaning can help to reduce COD and improve the system performance in large-scale applications. COD can be regarded as the total nutrient concentration in the MBR system. The rate of biodegradation of organic waste, R, depends on COD values of the influent (CODi) and the effluent (CODe). This relationship can be written as (Smith, 2006; Stephenson et al., 2000),
3.1. Characteristics of compost leachate
R¼
Compost leachate was comprehensively characterized in this study (general water quality parameters in Table 1 and results of heavy metals in Table 2). All organic and inorganic parameters were very high, which posed a significant challenge to the treatment method. Due to its high nutrient content (Table 1), compost leachate is usually sprayed back to compost piles to accelerate the composting process if the compost is too dry. Because of this recycling pro-
where V, / and t are the volume of the MBR tank, volumetric flow rate and time, respectively. When a steady state is reached, the derivative, dCODe/dt, equals to zero. Hence, the equilibrium rate of biodegradation, Re, becomes
2.4. Individual chemical analysis Caffeine was chosen as an indicator of organic substances because it is commonly present in compost materials and leachate. Its calibration standard (HPLC grade) was obtained from Sigma– Aldrich (Oakville, Ontario). Caffeine in the MBR effluent was extracted for analysis using solid phase extraction (SPE). Each SPE column (ChromabondÓ HR-P, manufactured by Machery–Nagel) was first conditioned using 3 mL of methanol, followed by 3 mL of distilled water. The effluent sample was acidified to a pH of approximately 2 using 20% (v/v) sulfuric acid and then 100 mL of the sample was aspirated under vacuum through a cartridge. The cartridge was eluted with 4 mL of ethyl acetate. For quality control, each batch consisted of one blank sample, one control sample, and two effluent samples (duplicates). PhenanthreneD10 (Accustandard) was used as the internal standard in calibration. The extract was then analyzed by gas chromatography – mass spectrometry (GC–MS, Agilent 7890/5975). The GC column used was a DB-5MS column (Agilent) with 20 m length, 0.18 lm coating thickness, and 0.18 mm internal diameter. The GC oven temperature program consisted of an initial temperature of 100 °C, held for 1 min, increased to 220 °C at 10 °C/min, continuously increased to a final temperature of 280 °C at 20 °C/min which was held for another 3 min. Selective ion monitoring was used to examine the target analytes; ions of 188 and 194 (mass-to-charge ratio) were used for phenanthrene-D10 and caffeine, respectively. Weekly effluent samples were collected, prepared and sent to AGAT Laboratories Limited in Dartmouth, Nova Scotia, for metal analysis using inductively coupled plasma – mass spectrometry (ICP–MS).
V dCODe ðCODi CODe Þ ; / dt
Re ¼
V ðCODi CODe Þ /
Please cite this article in press as: Brown, K., et al. Membrane bioreactor technology: A novel approach to the treatment of compost leachate. Waste Management (2013), http://dx.doi.org/10.1016/j.wasman.2013.04.006
ð1Þ
ð2Þ
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Table 1 Characteristics of initial compost leachate. Parameter
Value
Parameter
Value
pH Saturation pH (20 °C) Saturation pH (4 °C) Langelier index (20 °C) Langelier index (4 °C) True color Turbidity Electrical conductivity Calculated TDS Bicarb. alkalinity (as CaCO3) Carb. alkalinity (as CaCO3) Alkalinity Hardness Total organic carbon COD
5.10 4.17 4.49 0.93 0.61 9940 TCU 1370 NTU 82.6 mS/cm 29.1 g/L 7289 mg/L <10 mg/L 7290 mg/L 20.0 g/L 44.0 g/L 116 g/L
Ammonia as N Nitrite as N Nitrate as N Chloride Fluoride Sulfate Ortho-phosphate as P Total phosphorous Hydroxide Total sodium Total potassium Total calcium Total magnesium Anion sum Cation sum
2720 mg/L <5 mg/L <5 mg/L 3720 mg/L 3010 mg/L 1340 mg/L 0.37 mg/L 485 mg/L <5 mg/L 3810 mg/L 4580 mg/L 6310 mg/L 1020 mg/L 279 me/L 894 me/L
Table 2 Concentrations of caffeine and total metals in the initial leachate and MBR effluent. Unit: lg/L
Caffeine Aluminum Antimony Arsenic Barium Beryllium Bismuth Boron Cadmium Chromium Cobalt Copper Iron Lead Manganese Molybdenum Nickel Selenium Silver Strontium Thallium Tin Titanium Uranium Vanadium Zinc
RDL
0.4 10 2 2 5 2 2 5 0.3 2 1 2 50 0.5 2 2 2 2 0.5 5 0.1 2 2 0.1 2 5
Leachate
1330 39,800 <100 634 1190 <100 <100 4890 20.0 401 220 113 297,000 811.0 51,000 147 589 <100 <25 27,100 <5 <100 1370 <5 136 13,500
Experiment day
% Red.
0
6
14
22
28
38
<0.4 <10 <2 <2 31 <2 <2 13 <0.3 <2 <1 82 <50 <0.5 5 <2 <2 <2 <0.5 153 <0.1 <2 <2 <0.1 <2 51
2.0 49 <2 8 52 <2 <2 167 <0.3 2 2 2370 191 1 308 <2 17 4 <0.5 929 <0.1 7 2 0.1 <2 57
1.0 68 4 14 21 <2 <2 364 <0.3 4 3 3600 338 1.1 211 3 34 8 <0.5 1590 <0.1 11 5 <0.1 <2 29
1.1 26 5 19 16 <2 <2 575 <0.3 4 3 3610 241 1.1 20 10 45 9 <0.5 2230 <0.1 12 5 0.5 2 35
0.4 22 5 18 14 <2 <2 660 <0.3 5 4 3430 323 1 20 10 52 11 <0.5 2430 <0.1 11 6 0.4 2 49
0.6 26 6 19 12 <2 <2 844 <0.3 5 6 3170 392 0.8 26 10 66 13 <0.5 2930 <0.1 13 7 0.4 2 67
99.95% 99.93% NC 97.00% 98.99% NC NC 82.74% NC 98.75% 97.27% NA 99.87% 99.90% 99.95% 93.20% 88.79% NC NC 89.19% NC NC 99.49% NC 98.53% 99.50%
RDL: reported detection limit. NC: not calculated. NA: not applicable.
Using this equation, Re of our laboratory MBR system was determined to be 50.4 mg L1 h1. This rate is comparable to that in industrial-size systems (Stephenson et al., 2000). Hence, this laboratory method can be readily expanded to eventual industrial applications for compost leachate. 3.3. Ammonia, nitrite and nitrate Ammonia, nitrite and nitrate are naturally occurring compounds, and are part of the nitrogen cycle. In an aerobic environment, nitrifying bacteria convert ammonia to nitrite and then to nitrate. These three main inorganic nitrogen compounds in the effluent were monitored during the experiment (Fig. 2). Nitrate (NO3–N) showed significantly higher concentrations than ammonia (NH3–N) and nitrite (NO2–N). Ammonia is formed when organic nitrogen compounds are decomposed and it is used by microorganisms for nutrient (Grigor-
opoulou et al., 1999). Ammonia is a concern in wastewater because it is toxic to fish (World Health Organization, 2003). The ammonia concentration was relatively stable with an average of 0.093 mg/L during the experiment. Day 7 showed a small increase, which might be attributed to variation of bacterial concentration in the MBR system. The ammonia concentration of the initial compost leachate was 2720 mg/L, which indicates that nearly 100% of ammonia was removed during the MBR treatment process. The nitrite levels in the MBR effluent remained quite low and consistent with an average of 0.063 mg/L. A 1-day spike was observed on Day 22. This might be introduced by a system fluctuation, because the effluent flow was stopped for a few hours for maintenance on that day. Nitrate concentration increased continuously throughout the experiment and reached 727 mg/L on Day 39 before gradually reaching a plateau. This upward trend in the concentration of nitrate was observed, because nitrate is the final product in the oxidation of nitrogen compounds by microorgan-
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K. Brown et al. / Waste Management xxx (2013) xxx–xxx 450 400 350
250 200 150 Experimental data Exponential fit Asymptote
100 50 0 0
5
10
15
20
25
30
35
40
Experiment day Fig. 1. Chemical oxygen demand (COD) results of daily effluent samples. The dashed line is the fitted curve and the dotted line is the asymptote.
700
Ammonia nitrogen Nitrite nitrogen Nitrate nitrogen
600 500 400 300 200 100
0.5 0.4 0.3 0.2 0.1 0.0
0 10
20
30
Experiment day Fig. 2. Concentrations of ammonia, nitrite and nitrate in daily effluent samples.
isms, which caused an accumulation of nitrate in the aerobic bioreactor. As for COD, nitrate fluctuated towards the end of the experiment which can be attributed to the uncertainty introduced by serial dilutions. Experimental results indicated that microorganisms in MBR became acclimatized to compost leachate at the end of experiment and the system was capable to handle the heavy load of organic waste including the nitrogen compounds. The initial leachate had a high level of ammonia (2720 mg/L) but low levels of nitrite and nitrate (less than 5 mg/L). After treatment, ammonia was almost completely removed, but nitrate concentration increased gradually. It is noticed that the nitrogen mass was not balanced between the influent and effluent. This nitrogen stripping phenomena was related to environmental working conditions. Considering a high flux of air (1100 L/h) was pumped into the bioreactor, a portion of ammonia was purged out due to its high volatility. Further, some nitrogen mass was mineralized and incorporated into biomass. In future experiment, complete analysis of nitrogen compounds and alkalinity of influent, sludge, effluent, and ambient air will be conducted to study the kinetics of nitrogen mass balance and transformation.
5
10
15
20
25
30
35
40
10
40
Dissolved O2 (mg/L)
0
8 6 4
(c)
2 0 10
Conductivity (mS/cm)
Nitrogen concentration (mg/L)
800
8 6 4
(b)
2 0 10 9
pH
COD (mg/L)
300
which was slightly acidic, while the average pH of the effluent was around 8.12. The compost leachate had a very high alkalinity at 7290 mg/L. During the treatment, acidic organic compounds present in the leachate might be more effectively digested than basic compounds, which made the effluent basic. It was noticed that the pH returned back to neutral in several days once leachate introduction was stopped, indicating the alkaline effluent was caused by biodegradation. In future research or industrial application, this alkalinity instability may be balanced by using pH buffer solution or sulfuric acid for neutralization. Electrical conductivity is indicative of the presence of ions in the effluent. Although polarized organic compounds may have some contribution, the conductivity mainly depends on inorganic ions including nitrate, phosphate, metals, etc. The electrical conductivity in the MBR effluent increased steadily during the experiment (Fig. 3b); the trend was similar to that seen with the COD and nitrate data in Figs. 1 and 2. As COD increased, more organic compounds were available to be decomposed and oxidized to inorganic ions. Nitrate is a major component of total dissolved solids; it increased together with the conductivity. When the leachate was initially introduced to the system, the dissolved oxygen (DO) decreased from 7.66 mg/L on Day 0 to 3.95 mg/L on Day 3 (Fig. 3c). After Day 3, the air flow into the MBR was increased from 1200 L/h to 1700 L/h, which helped raise the oxygen content and create better environment for aerobic biodegradation. Due to this adjustment, the dissolved oxygen level became stabilized and gradually increased to 6.89 mg/L at the end of experiment. The continuing upward trend indicated microorganisms adapted to the leachate and breakdown of the leachate was more efficient in its use of oxygen. A drop of DO was observed on Day 16 which was correlated with COD data in the same day. The ambient temperature on Day 16 was higher than 25 °C and might be responsible for an increase of micro-
8 7
(a)
6 5
3.4. pH, conductivity and dissolved oxygen The pH of the MBR effluent remained stable over the course of the experiment (Fig. 3a). The pH of the compost leachate was 5.10,
0
5
10
15
20
25
30
35
40
Experiment day Fig. 3. Experimental results of pH, electrical conductivity and dissolved oxygen in MBR effluent.
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bial degradation of organics in the MBR. This resulted in a high consumption of dissolved oxygen. The air flow rate in this study was very high for a 114 L MBR. This flow rate with respect to the size of the bioreactor may not be economically sustainable for larger MBRs in industrial applications. However, the height of industrial MBRs is usually more than 2 m, which provides a long pathway for oxygen diffusion into water. Hence, industrial MBRs are more efficient in using air and can achieve the same DO level with less air flow than the laboratory MBR.
identified, which will give further information on bacterial activities during the treatment process. Because of membrane filtration, total suspended solids in MBR effluent were less than 10 mg/L during the experiment. While this study mainly focused on changes of chemical parameters, mixed liquor suspended solids (MLSSs) of the bioreactor is an indicator of biomass mineralization and bacterial population growth. In next phase of research, suspended solids in influent, processing water and effluent will be studied to further explore the kinetics and efficiency of biodegradation. 3.6. Caffeine and metals
3.5. Heterotrophic bacteria population The total population of heterotrophic bacteria in MBR was measured throughout the experiment (Fig. 4). As the data spans cross 4 orders of magnitude, a logarithmic scale is used to give a better illustration of changes. If colony counts were slightly more than 300 in all plates in the same day; 300 was used in calculation with respective dilution factor; so no error bar is shown in this case. If colony counts were noticeably more than 300, higher ratio dilution was used to bring counts down to a measurable range. Due to the logarithmic scale, error bars in the minus direction look larger than those in the plus direction. The initial bacterial population was 6.17 104 CFU/mL. Due to significant amount of nutrients introduced by compost leachate, rapid growth of the population was observed until Day 10 at 2.95 106 CFU/mL. At Day 10, the bacteria, originally present in the MBR, reached its maximum population. However, these bacteria had not adapted to the changing chemical composition in the MBR. Further introduction of compost leachate started to reduce the bacterial population. After Day 10, the bacterial population continuously dropped to 3.93 104 CFU/mL on Day 19. It was noticed that COD, nitrate and conductivity levels exhibited spikes on Day 19. A change in the species of bacteria present in the MBR was observed visually with colonies of a new shape and color present on the agar plates for that day. This change may be attributed to the presence of hibernating bacterial spores in the leachate that started to grow and eventually outnumbered the initial types of bacteria in the MBR. A dramatic growth was recorded after Day 19 and the bacterial population reached a peak at 5.03 108 CFU/ mL on Day 30. It is interesting to notice that the population started to decrease again after Day 30. This indicates another stationary stage was reached on Day 30. Various types of bacteria thrived and fell to reach their equilibrium in the MBR treatment system. In future research, bacterial species present in the MBR should be
Bacterial population (CFU/mL)
1010
10 9
10 8
10 7
10 6
10 5
10 4 0
10
20
30
Experiment day Fig. 4. Total heterotrophic bacteria counts in the MBR.
40
Because of its high consumption by humans, caffeine has been used as an indicator of domestic sewage contamination in ground and surface water (Seiler et al., 1999). Municipal wastewater is usually rich in caffeine. For example, it was found at 17.0 lg/L in the effluent of the Mill Cove sewage treatment plant (Halifax, Nova Scotia) even after vigorous 3-stage treatment (Crouse et al., 2012). It was used in this study as an indicator of organic substances. The calculated concentration of caffeine in the leachate was determined to be 1.33 mg/L. The concentration of caffeine in the effluent increased briefly with the introduction of compost leachate (Table 2). As the experiment progressed, the levels of caffeine decreased gradually to 0.6 lg/L at the end of experiment. An impressive reduction of 99.9% was observed in the concentration of caffeine. The results of total metal analysis include over 20 heavy metals (Table 2). Six days were chosen over the course of the experiment to highlight the trend in metal concentrations over time and the progress of the MBR in removing these elements. Due to its heavy load of contamination, the initial compost leachate was diluted considerably during ICP-MS analysis, which increased the reported detection limits (RDLs) of the leachate sample accordingly. Percent reductions were not calculated for heavy metals with concentrations that were less than RDLs. Initial concentrations of five metals: aluminum, iron, manganese, strontium and zinc were particularly high in the compost leachate. Of the high major metal contaminants, all were reduced to 89% of their initial concentrations or greater at the end of experiment. The reductions may be attributed to incorporation of metals into biomass within the MBR. To avoid excessive accumulation of metals in activated sludge, removal of the activated sludge periodically is required for continuous operation beyond this proof-of-concept study. It is important to understand how heavy metals in MBR are related to the MLSS concentration. In future research, heavy metals in suspended solids will be investigated to study the kinetics of absorption and their partition in MLSS. Many of the metals studied have Health Canada mandated maximum acceptable concentrations (MACs) for drinking water (Federal-Provincial-Territorial Committee on Drinking Water, 2010). Initial concentrations of barium, cadmium, chromium, lead, manganese, exceeded these maximum acceptable concentrations; however the effectiveness of the treatment yielded dramatically reduced concentrations, some of which have exceeded wastewater standards and even met drinking water quality standards. The arsenic level (634 lg/L) was also well above the drinking water quality standard (10 lg/L) before treatment and was drastically reduced to 19 lg/L, nearly meeting the drinking water standard. Health Canada also regulates metals for aesthetic considerations. Metals such as aluminum, iron, manganese and zinc all exceeded the aesthetic guidelines as set by Health Canada. All four metals were reduced to below acceptable drinking water guidelines. The only metal that showed noticeable increases in the effluent was copper. Copper tubing was used for the collection of effluent from the MBR membrane modules. Copper tubing was also used for the air spargers that were present below the MBR modules. Copper pipes were corroded by the leachate and released copper into the MBR system. The increase of the copper concentration
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was visually observable through the apparent color of the effluent over time. Health Canada has established an aesthetic quality guideline for copper in drinking water (1.0 mg/L) (Federal-Provincial-Territorial Committee on Drinking Water, 2010). The copper concentration in the MBR effluent (3.17 mg/L) only slightly exceeds this guideline. 4. Conclusions The treatment of compost leachate using membrane bioreactor technology was studied for 39 days at which the MBR treatment reached a steady state. Numerous water quality parameters were examined during the experiment period. The COD of the effluent was successfully reduced by more than 99% from an initial leachate COD of 116 g/L. High reductions were also observed for ammonia, caffeine and many of the metals studied. This experiment demonstrated MBR is a very efficient method for treating compost leachate. Acknowledgements The authors would like to acknowledge the financial support from Acadia University and the Natural Sciences and Engineering Research Council of Canada (NSERC). In addition, the authors would like to thank Dwight Horsnell (Northridge Farms Limited), Dr. Yiming Zeng (Superstring MBR Technology, Corp.) and Dr. Martin Tango (School of Engineering, Acadia University) for their technical assistance and helpful discussion in this project. References ABL Environmental Consultants Ltd., 2006. Atlantic Canada Wastewater Guidelines Manual. Environment Canada. Clesceri, L.S., Greenberg, A.E., Trussell, R.R., 1989. Standard Methods for the Examination of Water and Wastewater. American Public Health Association, Washington, DC. Crouse, B., Ghoshdastidar, A.J., Tong, A.Z., 2012. The presence of acidic and neutral drugs in treated sewage effluents and receiving waters of the Cornwallis and Annapolis River watersheds and Mill Cove WPCC in Nova Scotia. Environmental Research 112, 92–99. Cumar, S., Nagaraja, B., 2011. Environmental impact of leachate characteristics on water quality. Environmental Monitoring and Assessment 178, 499–505. Delgado, S., Villarroel, R., Gonzalez, E., Morales, M., 2011. Aerobic Membrane Bioreactor for Wastewater Treatment – Performance Under Substrate-Limited Conditions. InTech.
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Epstein, E., 1997. The Science of Composting. CRC Press LLC, Boca Raton. Federal-Provincial-Territorial Committee on Drinking Water, 2010. Guidelines for Canadian Drinking Water Quality. Health Canada. Ghoshdastidar, A.J., Tong, A.Z., 2013. Treatment of 2,4-D, mecoprop, and dicamba using membrane bioreactor technology. Environmental Science and Pollution Research, doi: http://dx.doi.org/10.1007/s11356-013-1498-z. Ghoshdastidar, A.J., Saunders, J.E., Brown, K.H., Tong, A.Z., 2012. Membrane bioreactor treatment of commonly used organophosphate pesticides. Journal of Environmental Science and Health, Part B 47, 742–750. Grigoropoulou, H., Sotirakou, E., Kladitis, G., Diamantis, N., 1999. Ammonia and phosphorous removal in municipal wastewater treatment plant with extended aeration. Global Nest: The International Journal 1, 47–53. Hammer, D.A., 1989. Constructed Wetlands for Wastewater Treatment: Municipal, Industrial, and Agricultural. Lewis Publishers, Chelsea. Jarecki, M.K., Chong, C., Voroney, R.P., 2005. Evaluation of compost leachates for plant growth in hydroponic culture. Journal of Plant Nutrition 28, 651–667. Judd, S., 2006. Principles and Applications of Membrane Bioreactors in Water and Wastewater Treatment. Elsevier Ltd., Oxford. Le-Clech, P., 2010. Membrane bioreactors and their uses in wastewater treatments. Applied Microbiology and Biotechnology 88, 1253–1260. Liu, C.-H., Lo, K.V., 2001a. Ammonia removal from compost lachate using zeolite. II. A study using continuous flow packed columns. Journal of Environmental Science and Health, Part B 36, 667–675. Liu, C.-H., Lo, K.V., 2001b. Ammonia removal from composting leachate using zeolite. I. Characterization of the zeolite. Journal of Environmental Science and Health, Part A 36, 1671–1688. Meng, F., Chae, S.-R., Drews, A., Kraume, M., Shin, H.-S., Yang, F., 2009. Recent advances in membrane bioreactors (MBRs): membrane fouling and membrane material. Water Research 43, 1489–1512. Nova Scotia Environment, 1995. Solid Waste Resource Management Strategy. Province of Nova Scotia, pp. 3–11. Nova Scotia Environment, 2010. Organic Composting Facilities in Nova Scotia. Province of Nova Scotia, pp. 7–9. Radjenovic´, J., Matošic´, M., Mijatovic´, I., Petrovic´, M., Barceló, D., 2008. In: Barceló, D., Petrovic, M. (Eds.), Membrane Bioreactor (MBR) as an Advanced Wastewater Treatment Technology; Emerging Contaminants from Industrial and Municipal Waste. Springer, Berlin/Heidelberg, pp. 37–101. Seiler, R.L., Zaugg, S.D., Thomas, J.M., Howcroft, D.L., 1999. Caffeine and pharmaceuticals as indicators of waste water contamination in wells. Ground Water 37, 405–410. Smith, H.L., 2006. Bacterial Growth. Arizona State University, Tempe. Stephenson, T., Brindle, K., Judd, S., Jefferson, B., 2000. Membrane Bioreactors for Wastewater Treatment. IWA Publishing, London. Stottmeister, U., Buddhawong, S., Kuschk, P., Wiessner, A., Mattusch, J., 2006. Constructed wetlands and their performance for treatment of water contaminated with arsenic and heavy metals. In: Twardowska, I., Allen, H.E., Häggblom, M.M., Stefaniak, S. (Eds.), Soil and Water Pollution Monitoring, Protection and Remediation. Springer, Dordrecht, pp. 417–432. Trujillo, D., Font, X., Sánchez, A., 2006. Use of Fenton reaction for the treatment of leachate from composting of different wastes. Journal of Hazardous Materials 138, 201–204. World Health Organization, 2003. Ammonia in Drinking Water. World Health, Organization, pp. 2–3.
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Membrane bioreactor technology: A novel approach to the treatment of compost leachate Kayleigh Brown a, Avik J. Ghoshdastidar a, Jillian Hanmore a, James Frazee b, Anthony Z. Tong a,⇑ a b
Department of Chemistry, Acadia University, Wolfville, NS, Canada B4P 2R6 E&Q Consulting and Associates Limited, Wolfville, NS, Canada B4P 2R1
a r t i c l e
i n f o
Article history: Received 14 May 2012 Accepted 16 April 2013 Available online xxxx Keywords: Compost leachate Membrane bioreactor Biodegradation Organic waste Caffeine Heavy metals
a b s t r a c t Compost leachate forms during the composting process of organic material. It is rich in oxidizable organics, ammonia and metals, which pose a risk to the environment if released without proper treatment. An innovative method based on the membrane bioreactor (MBR) technology was developed to treat compost leachate over 39 days. Water quality parameters, such as pH, dissolved oxygen, ammonia, nitrate, nitrite and chemical oxygen demand (COD) were measured daily. Concentrations of caffeine and metals were measured over the course of the experiment using gas chromatography – mass spectrometry (GC/MS) and inductively coupled plasma – mass spectrometry (ICP–MS) respectively. A decrease of more than 99% was achieved for a COD of 116 g/L in the initial leachate. Ammonia was decreased from 2720 mg/ L to 0.046 mg/L, while the nitrate concentration in the effluent rose to 710 mg/L. The bacteria in the MBR system adjusted to the presence of the leachate, and increased 4 orders of magnitude. Heavy metals were removed by at least 82.7% except copper. These successful results demonstrated the membrane bioreactor technology is feasible, efficient method for the treatment of compost leachate. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction Due to increasing public concerns on the quantities of solid waste being disposed in landfills and its environmental and health impact, many provinces and states in North America and beyond have implemented strategies for solid waste management (Nova Scotia Environment, 1995). The main goal of solid waste management is to divert the greatest amount of solid waste from landfills through source reduction, material reuse, recycling, business development, and composting (Nova Scotia Environment, 1995). Composting is the controlled decomposition of organic material by microorganisms, such as bacteria and fungi, forming a stable soil called humus (Epstein, 1997). Material that is composted, known as feedstock, is separated into two categories: residential and IC&I (industrial, commercial and institutional). Residential waste consists mainly of left-over food such as meat, fruits and vegetables. IC&I waste includes farming, fishing and wood processing wastes, yard wastes and paper. Many municipalities operate centralized composting facilities. For example, there are 17 major composting facilities in Nova Scotia that process about 186,200 tons/year of various organic wastes (Nova Scotia Environment, 2010). The composting process not only diverts organic ⇑ Corresponding author. Address: Department of Chemistry, Acadia University, 6 University Avenue, Wolfville, NS, Canada B4P 2R6. Tel.: +1 902 585 1355; fax: +1 902 585 1114. E-mail address: [email protected] (A.Z. Tong).
waste from landfills, but also produces two value-added fertilizer products: (1) a solid form, referred to as processing soil or screened compost, and (2) a liquid form, referred to as compost tea. A major issue associated with composting is the formation of leachate. Compost may consist of material with high water content and water is also produced during biodegradation processes. As degradation takes place, this wastewater seeps out and forms what is known as leachate. Some composting facilities, especially those with feedstock from residential and agricultural sources, generate considerable amounts of leachate. For example, Northridge Farms Limited (Aylesford, Nova Scotia), the industrial partner of this study, processes compost material for the Annapolis Valley with a population of about 80,000, and it produces leachate at an average of about 4000 L/day. Compost leachate contains any material that was extracted, suspended or dissolved in the compost pile. Depending on the feedstock, the leachate may contain high levels of metals, biological material, pesticides, phthalates, polychlorinated biphenyls (PCBs) or dioxins (Epstein, 1997; Jarecki et al., 2005; Trujillo et al., 2006). Compost leachate has large quantities of total oxidizable organics that can be measured as chemical oxygen demand (COD). High values of COD are problematic because release of this leachate will deplete dissolved oxygen in receiving waters. The resulting anoxic environments cause fatalities of plants, fish and other aquatic organisms. Many toxic organic compounds exist in the leachate, meaning if it is not treated properly, surface and groundwater may become contaminated placing the
0956-053X/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.wasman.2013.04.006
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public and local ecosystems at risk (Cumar and Nagaraja, 2011). Since leachate is harmful, composting facilities in most municipalities are required to have a leachate management system in place. A proper system consists of infrastructure and monitoring systems designed to collect, monitor, control and treat leachate before being released to the environment (Nova Scotia Environment, 2010). There are very few research publications on treatment of compost leachate. Liu et al. used continuous flow packed zeolite columns to remove ammonia and achieved more than 98% of efficiency (Liu and Lo, 2001a, b). By using Fenton’s reagent, Trujillo et al. showed COD removal rates of 77% and 75% for leachates from wastewater sludge composting and municipal solid wastes, respectively (Trujillo et al., 2006). However, these methods were either focused on one particular water quality parameter or showed low efficiency. Further, these physical or chemical treatment methods lack flexibility, need high maintenance and may introduce new contaminants. One current popular methodology for treating compost leachate is engineered or constructed wetlands. Engineered wetlands are a man-made wetland system containing water, saturated substrates, emergent and submergent plants and animals that are designed to imitate natural wetlands (Hammer, 1989; Stottmeister et al., 2006). Most engineered wetlands resemble marshes; those with herbaceous emergent and submergent plants show the most promise for wastewater treatment, where cattails, rush and giant reed are acclimatized to changing water and nutrient quantities and are more capable of processing large pollutant loads (Hammer, 1989). Engineered wetlands utilize a negligible amount of additional energy, and need minimal operation and maintenance, which makes this technology a low-cost treatment method. Some disadvantages of engineering wetlands include low efficiency, long development time, large space requirement, and reduced efficiency at sub-zero temperatures. A phenomenon called soil saturation may also occur, where the soil in the wetland becomes unable to absorb further amounts of leachate, which renders the treatment by vegetation and bacteria ineffective. Membrane bioreactor (MBR) technology provides biological treatment with membrane separation (Judd, 2006; Meng et al., 2009; Stephenson et al., 2000). The system consists of an aerated water-filled tank containing activated sludge and multiple capillary-form membrane tubes. The pores of ultrafiltration membranes are approximately 20–50 nm in diameter, which effectively retains microorganisms, macromolecules and suspended solids. Microorganisms use the contaminants as nutrients for growth and metabolism. The contaminants are thus broken down into less harmful substances (Judd, 2006). Membrane bioreactor technology has many advantages over conventional activated sludge treatment processes. The secondary clarifiers normally used are able to be replaced by the membrane system, which reduces the space required for the treatment process. The overall retention time of the activated sludge is longer in the MBR, which increases contact opportunities of bacteria with contaminants, and subsequently leads to high efficiency (Ghoshdastidar et al., 2012; Le-Clech, 2010). Another advantage of the membrane bioreactor technology is removal of microorganisms to below detection limit from the MBR effluent due to ultra-filtration. A MBR system is very compact, especially compared to the acres required by engineered wetlands. A further advantage of MBR technology over engineered wetlands is that the system is unaffected by freezing caused by sub-zero temperatures and thus can be used during all times of year. The MBR technology has been applied to the treatment of contaminants in both municipal and industrial wastewater (Ghoshdastidar and Tong, 2013; Radjenovic´ et al., 2008), but our research was the first attempt at using MBR as a treatment method for compost leachate. An MBR system was built and optimized
according to the characteristics of compost leachate. Various wastewater quality parameters were monitored to demonstrate the feasibility and efficiency of the MBR treatment method. 2. Materials and methods 2.1. Compost leachate sampling and preparation Compost leachate samples were collected from Northridge Farms Limited, a composting facility located in Aylesford, Nova Scotia. The leachate was pumped out of a holding tank into a barrel, and allowed to settle for two days to remove major suspended solids. The supernatant in this barrel was then pumped into PVC containers. These containers were transported to the laboratory and stored in a cold room at 4 °C for the duration of the experiment. About 2 L of the leachate was transferred daily to a glass jar and pumped into an MBR for treatment. The same leachate stored in the cold room was used throughout the experiment to avoid variation of characteristics of the initial leachate. 2.2. Membrane bioreactor setup An MBR system was built in-house using SuperUF membrane modules provided by Superstring MBR Technology Corp. (Brooklyn, NY). Five rows of membrane modules were grouped in this MBR and provided a total membrane surface area of 1.10 m2. Membrane pore size diameter ranged from 20 to 30 nm. The capacity of the MBR tank used in this project was 114 L. A continuous supply of air was introduced to the tank at 1100 L/h using copper spargers below the membrane. Using peristaltic pumps, the influent and effluent were pumped into and out of the MBR at a volumetric flow rate of 0.83 mL/min that was calibrated daily. As composting facilities only generate small amounts of leachate, this experimental flow rate can be proportionally expanded to industrial applications. The temperature of the MBR was maintained at a constant 25 ± 1 °C throughout the experiment period. This MBR system was initiated with 5 L of activated sludge taken from Mill Cove municipal wastewater treatment plant in Halifax, Nova Scotia. No additional chemical additives or nutrients were introduced during the course of project. It was noticed that the amount of sludge in MBR increased during the experiment period. In this lab-scale experiment, solids removal was not conducted due to the relatively large capacity of the bioreactor in comparison to the influent volume. However, it is expected that periodic solids removal should be incorporated in industrial-scale applications. Compost leachate was continuously pumped into the MBR system for 39 days, and wastewater quality parameters were monitored daily during this time period. A biological treatment system may be considered in stationary condition after 2 sludge retention times (SRTs) and most MBR facilities have SRT between 10 and 20 days (Delgado et al., 2011). At the end of the 39-day experiment period, major water quality parameters become stable and the MBR system reached a steady-state. 2.3. Test methods of common wastewater parameters A Hach HQd 40 probe was used to measure the pH, electrical conductivity and dissolved oxygen of effluent. These tests were completed by obtaining 200 mL of effluent sample and immediately analyzing the sample, to avoid inaccuracies due to temperature changes and the loss of oxygen. The appropriate probe was attached to the meter, placed in the sample, which was stirred at a rate of 30 rpm/min. A Hach DR2800 spectrophotometer was used to measure chemical oxygen demand (COD), ammonia, nitrate and nitrite. These water quality parameters were measured using
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standard HACH methods: the reactor digestion method with TNT 821 LR vials, salicylate method with TNT 830 ULR vials, dimethylphenol method with TNT 835 LR vials, and diazotization method with TNT 839 LR vials, respectively. Microbiological analysis was completed daily to monitor the population of heterotrophic bacteria in the MBR. The measurements were completed in triplicate; three independent samples of activated sludge were obtained and quantitated. Agar plates were prepared according to the Standard Methods for the Examination of Water and Wastewater (Clesceri et al., 1989). The agar mixture was autoclaved at 121 °C for 60 min, poured into plastic Petri dishes and allowed to set for 48 h before use. Approximately 200 mL of activated sludge was obtained from the MBR, and 100 lL were quickly removed using a micropipette and placed in one of the test tubes containing saline solution. Any further dilutions were made in 10 increments. The plate was then inoculated with 100 lL of the final dilution mixture, which was spread evenly on the plate using the metal spreader. Plates were placed in the incubator at 36 °C for 48 h. The total number of colonies on the plate was counted and calculated in colony forming units (CFU/mL) according to the level of dilution.
3
cess, only excess of compost leachate is discharged and its flow rate is usually less than several cubic meters for a medium size composting facility. The treatment method used in this study was designed according to leachate’s characteristics of high waste load and low flow rate. In this lab-scale experiment, a relatively large bioreactor (114 L) and a small flow rate (0.83 mL/min) provided a long biodegradation period. Air at a high flow rate (1100 L/h) was pumped into the bioreactor to ensure adequate oxygen for aerobic digestion. Organic loading rate (OLR), hydraulic retention time (HRT) and membrane flux were 50.9 mg/L/h, 95 days and 0.756 mL/cm2/min, respectively. Trans-membrane pressure was kept below 30 kPa and no membrane cleaning was necessary during the experiment period. 3.2. Chemical oxygen demand
3. Results and discussion
COD was monitored very closely as an important wastewater quality parameter (Fig. 1), because it was indicative of the overall organic waste load of the MBR system and the adaptation of the microorganisms to the presence of the leachate. COD was studied rather than biochemical oxygen demand (BOD) because COD measurements provide better precision and require less time to conduct than BOD measurements. The precision of spectrophotometric method for COD was ±3 mg/L as determined by the supplier, Hach. As noticed in Fig. 1, COD values varied more for later experiment days; this increased uncertainty was introduced by the necessary effluent dilution in order to bring COD values into the detection range of the Hach method. The COD of the effluent shows a smooth and gradual increase before reaching a plateau. This increase can be modeled using a reversed exponential decay curve and a nonlinear curve fit (dash line) was performed. The time constant of the exponential curve was determined to be 15.9 days. Consequently, the half-life time can be calculated as 11.0 days, which indicates that the MBR system reached its half treatment capacity in 11 days. The curve fit also yields an asymptotic limit at 437 mg/L (the dotted line in Fig. 1). The initial COD of the compost leachate was 116 g/L. Such a significant COD value indicated that the compost leachate was saturated with organic waste. At the end of experiment (Day 39), COD reached 400 mg/L thereby achieving a 99.7% reduction of the initial COD. That being said, the final COD value is still relatively high and may exceed the wastewater effluent guidelines (ABL Environmental Consultants Ltd., 2006). In future experiments, an additional aerated tank/clarifier can be added to further reduce the COD. Further, sludge/solids removal and membrane cleaning were not performed in this lab-scale experiment. The increase of COD during the experiment period might be related to excessive sludge or membrane fouling. Regularly scheduled sludge removal and membrane cleaning can help to reduce COD and improve the system performance in large-scale applications. COD can be regarded as the total nutrient concentration in the MBR system. The rate of biodegradation of organic waste, R, depends on COD values of the influent (CODi) and the effluent (CODe). This relationship can be written as (Smith, 2006; Stephenson et al., 2000),
3.1. Characteristics of compost leachate
R¼
Compost leachate was comprehensively characterized in this study (general water quality parameters in Table 1 and results of heavy metals in Table 2). All organic and inorganic parameters were very high, which posed a significant challenge to the treatment method. Due to its high nutrient content (Table 1), compost leachate is usually sprayed back to compost piles to accelerate the composting process if the compost is too dry. Because of this recycling pro-
where V, / and t are the volume of the MBR tank, volumetric flow rate and time, respectively. When a steady state is reached, the derivative, dCODe/dt, equals to zero. Hence, the equilibrium rate of biodegradation, Re, becomes
2.4. Individual chemical analysis Caffeine was chosen as an indicator of organic substances because it is commonly present in compost materials and leachate. Its calibration standard (HPLC grade) was obtained from Sigma– Aldrich (Oakville, Ontario). Caffeine in the MBR effluent was extracted for analysis using solid phase extraction (SPE). Each SPE column (ChromabondÓ HR-P, manufactured by Machery–Nagel) was first conditioned using 3 mL of methanol, followed by 3 mL of distilled water. The effluent sample was acidified to a pH of approximately 2 using 20% (v/v) sulfuric acid and then 100 mL of the sample was aspirated under vacuum through a cartridge. The cartridge was eluted with 4 mL of ethyl acetate. For quality control, each batch consisted of one blank sample, one control sample, and two effluent samples (duplicates). PhenanthreneD10 (Accustandard) was used as the internal standard in calibration. The extract was then analyzed by gas chromatography – mass spectrometry (GC–MS, Agilent 7890/5975). The GC column used was a DB-5MS column (Agilent) with 20 m length, 0.18 lm coating thickness, and 0.18 mm internal diameter. The GC oven temperature program consisted of an initial temperature of 100 °C, held for 1 min, increased to 220 °C at 10 °C/min, continuously increased to a final temperature of 280 °C at 20 °C/min which was held for another 3 min. Selective ion monitoring was used to examine the target analytes; ions of 188 and 194 (mass-to-charge ratio) were used for phenanthrene-D10 and caffeine, respectively. Weekly effluent samples were collected, prepared and sent to AGAT Laboratories Limited in Dartmouth, Nova Scotia, for metal analysis using inductively coupled plasma – mass spectrometry (ICP–MS).
V dCODe ðCODi CODe Þ ; / dt
Re ¼
V ðCODi CODe Þ /
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ð1Þ
ð2Þ
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Table 1 Characteristics of initial compost leachate. Parameter
Value
Parameter
Value
pH Saturation pH (20 °C) Saturation pH (4 °C) Langelier index (20 °C) Langelier index (4 °C) True color Turbidity Electrical conductivity Calculated TDS Bicarb. alkalinity (as CaCO3) Carb. alkalinity (as CaCO3) Alkalinity Hardness Total organic carbon COD
5.10 4.17 4.49 0.93 0.61 9940 TCU 1370 NTU 82.6 mS/cm 29.1 g/L 7289 mg/L <10 mg/L 7290 mg/L 20.0 g/L 44.0 g/L 116 g/L
Ammonia as N Nitrite as N Nitrate as N Chloride Fluoride Sulfate Ortho-phosphate as P Total phosphorous Hydroxide Total sodium Total potassium Total calcium Total magnesium Anion sum Cation sum
2720 mg/L <5 mg/L <5 mg/L 3720 mg/L 3010 mg/L 1340 mg/L 0.37 mg/L 485 mg/L <5 mg/L 3810 mg/L 4580 mg/L 6310 mg/L 1020 mg/L 279 me/L 894 me/L
Table 2 Concentrations of caffeine and total metals in the initial leachate and MBR effluent. Unit: lg/L
Caffeine Aluminum Antimony Arsenic Barium Beryllium Bismuth Boron Cadmium Chromium Cobalt Copper Iron Lead Manganese Molybdenum Nickel Selenium Silver Strontium Thallium Tin Titanium Uranium Vanadium Zinc
RDL
0.4 10 2 2 5 2 2 5 0.3 2 1 2 50 0.5 2 2 2 2 0.5 5 0.1 2 2 0.1 2 5
Leachate
1330 39,800 <100 634 1190 <100 <100 4890 20.0 401 220 113 297,000 811.0 51,000 147 589 <100 <25 27,100 <5 <100 1370 <5 136 13,500
Experiment day
% Red.
0
6
14
22
28
38
<0.4 <10 <2 <2 31 <2 <2 13 <0.3 <2 <1 82 <50 <0.5 5 <2 <2 <2 <0.5 153 <0.1 <2 <2 <0.1 <2 51
2.0 49 <2 8 52 <2 <2 167 <0.3 2 2 2370 191 1 308 <2 17 4 <0.5 929 <0.1 7 2 0.1 <2 57
1.0 68 4 14 21 <2 <2 364 <0.3 4 3 3600 338 1.1 211 3 34 8 <0.5 1590 <0.1 11 5 <0.1 <2 29
1.1 26 5 19 16 <2 <2 575 <0.3 4 3 3610 241 1.1 20 10 45 9 <0.5 2230 <0.1 12 5 0.5 2 35
0.4 22 5 18 14 <2 <2 660 <0.3 5 4 3430 323 1 20 10 52 11 <0.5 2430 <0.1 11 6 0.4 2 49
0.6 26 6 19 12 <2 <2 844 <0.3 5 6 3170 392 0.8 26 10 66 13 <0.5 2930 <0.1 13 7 0.4 2 67
99.95% 99.93% NC 97.00% 98.99% NC NC 82.74% NC 98.75% 97.27% NA 99.87% 99.90% 99.95% 93.20% 88.79% NC NC 89.19% NC NC 99.49% NC 98.53% 99.50%
RDL: reported detection limit. NC: not calculated. NA: not applicable.
Using this equation, Re of our laboratory MBR system was determined to be 50.4 mg L1 h1. This rate is comparable to that in industrial-size systems (Stephenson et al., 2000). Hence, this laboratory method can be readily expanded to eventual industrial applications for compost leachate. 3.3. Ammonia, nitrite and nitrate Ammonia, nitrite and nitrate are naturally occurring compounds, and are part of the nitrogen cycle. In an aerobic environment, nitrifying bacteria convert ammonia to nitrite and then to nitrate. These three main inorganic nitrogen compounds in the effluent were monitored during the experiment (Fig. 2). Nitrate (NO3–N) showed significantly higher concentrations than ammonia (NH3–N) and nitrite (NO2–N). Ammonia is formed when organic nitrogen compounds are decomposed and it is used by microorganisms for nutrient (Grigor-
opoulou et al., 1999). Ammonia is a concern in wastewater because it is toxic to fish (World Health Organization, 2003). The ammonia concentration was relatively stable with an average of 0.093 mg/L during the experiment. Day 7 showed a small increase, which might be attributed to variation of bacterial concentration in the MBR system. The ammonia concentration of the initial compost leachate was 2720 mg/L, which indicates that nearly 100% of ammonia was removed during the MBR treatment process. The nitrite levels in the MBR effluent remained quite low and consistent with an average of 0.063 mg/L. A 1-day spike was observed on Day 22. This might be introduced by a system fluctuation, because the effluent flow was stopped for a few hours for maintenance on that day. Nitrate concentration increased continuously throughout the experiment and reached 727 mg/L on Day 39 before gradually reaching a plateau. This upward trend in the concentration of nitrate was observed, because nitrate is the final product in the oxidation of nitrogen compounds by microorgan-
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250 200 150 Experimental data Exponential fit Asymptote
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Experiment day Fig. 1. Chemical oxygen demand (COD) results of daily effluent samples. The dashed line is the fitted curve and the dotted line is the asymptote.
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Experiment day Fig. 2. Concentrations of ammonia, nitrite and nitrate in daily effluent samples.
isms, which caused an accumulation of nitrate in the aerobic bioreactor. As for COD, nitrate fluctuated towards the end of the experiment which can be attributed to the uncertainty introduced by serial dilutions. Experimental results indicated that microorganisms in MBR became acclimatized to compost leachate at the end of experiment and the system was capable to handle the heavy load of organic waste including the nitrogen compounds. The initial leachate had a high level of ammonia (2720 mg/L) but low levels of nitrite and nitrate (less than 5 mg/L). After treatment, ammonia was almost completely removed, but nitrate concentration increased gradually. It is noticed that the nitrogen mass was not balanced between the influent and effluent. This nitrogen stripping phenomena was related to environmental working conditions. Considering a high flux of air (1100 L/h) was pumped into the bioreactor, a portion of ammonia was purged out due to its high volatility. Further, some nitrogen mass was mineralized and incorporated into biomass. In future experiment, complete analysis of nitrogen compounds and alkalinity of influent, sludge, effluent, and ambient air will be conducted to study the kinetics of nitrogen mass balance and transformation.
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which was slightly acidic, while the average pH of the effluent was around 8.12. The compost leachate had a very high alkalinity at 7290 mg/L. During the treatment, acidic organic compounds present in the leachate might be more effectively digested than basic compounds, which made the effluent basic. It was noticed that the pH returned back to neutral in several days once leachate introduction was stopped, indicating the alkaline effluent was caused by biodegradation. In future research or industrial application, this alkalinity instability may be balanced by using pH buffer solution or sulfuric acid for neutralization. Electrical conductivity is indicative of the presence of ions in the effluent. Although polarized organic compounds may have some contribution, the conductivity mainly depends on inorganic ions including nitrate, phosphate, metals, etc. The electrical conductivity in the MBR effluent increased steadily during the experiment (Fig. 3b); the trend was similar to that seen with the COD and nitrate data in Figs. 1 and 2. As COD increased, more organic compounds were available to be decomposed and oxidized to inorganic ions. Nitrate is a major component of total dissolved solids; it increased together with the conductivity. When the leachate was initially introduced to the system, the dissolved oxygen (DO) decreased from 7.66 mg/L on Day 0 to 3.95 mg/L on Day 3 (Fig. 3c). After Day 3, the air flow into the MBR was increased from 1200 L/h to 1700 L/h, which helped raise the oxygen content and create better environment for aerobic biodegradation. Due to this adjustment, the dissolved oxygen level became stabilized and gradually increased to 6.89 mg/L at the end of experiment. The continuing upward trend indicated microorganisms adapted to the leachate and breakdown of the leachate was more efficient in its use of oxygen. A drop of DO was observed on Day 16 which was correlated with COD data in the same day. The ambient temperature on Day 16 was higher than 25 °C and might be responsible for an increase of micro-
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3.4. pH, conductivity and dissolved oxygen The pH of the MBR effluent remained stable over the course of the experiment (Fig. 3a). The pH of the compost leachate was 5.10,
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Experiment day Fig. 3. Experimental results of pH, electrical conductivity and dissolved oxygen in MBR effluent.
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bial degradation of organics in the MBR. This resulted in a high consumption of dissolved oxygen. The air flow rate in this study was very high for a 114 L MBR. This flow rate with respect to the size of the bioreactor may not be economically sustainable for larger MBRs in industrial applications. However, the height of industrial MBRs is usually more than 2 m, which provides a long pathway for oxygen diffusion into water. Hence, industrial MBRs are more efficient in using air and can achieve the same DO level with less air flow than the laboratory MBR.
identified, which will give further information on bacterial activities during the treatment process. Because of membrane filtration, total suspended solids in MBR effluent were less than 10 mg/L during the experiment. While this study mainly focused on changes of chemical parameters, mixed liquor suspended solids (MLSSs) of the bioreactor is an indicator of biomass mineralization and bacterial population growth. In next phase of research, suspended solids in influent, processing water and effluent will be studied to further explore the kinetics and efficiency of biodegradation. 3.6. Caffeine and metals
3.5. Heterotrophic bacteria population The total population of heterotrophic bacteria in MBR was measured throughout the experiment (Fig. 4). As the data spans cross 4 orders of magnitude, a logarithmic scale is used to give a better illustration of changes. If colony counts were slightly more than 300 in all plates in the same day; 300 was used in calculation with respective dilution factor; so no error bar is shown in this case. If colony counts were noticeably more than 300, higher ratio dilution was used to bring counts down to a measurable range. Due to the logarithmic scale, error bars in the minus direction look larger than those in the plus direction. The initial bacterial population was 6.17 104 CFU/mL. Due to significant amount of nutrients introduced by compost leachate, rapid growth of the population was observed until Day 10 at 2.95 106 CFU/mL. At Day 10, the bacteria, originally present in the MBR, reached its maximum population. However, these bacteria had not adapted to the changing chemical composition in the MBR. Further introduction of compost leachate started to reduce the bacterial population. After Day 10, the bacterial population continuously dropped to 3.93 104 CFU/mL on Day 19. It was noticed that COD, nitrate and conductivity levels exhibited spikes on Day 19. A change in the species of bacteria present in the MBR was observed visually with colonies of a new shape and color present on the agar plates for that day. This change may be attributed to the presence of hibernating bacterial spores in the leachate that started to grow and eventually outnumbered the initial types of bacteria in the MBR. A dramatic growth was recorded after Day 19 and the bacterial population reached a peak at 5.03 108 CFU/ mL on Day 30. It is interesting to notice that the population started to decrease again after Day 30. This indicates another stationary stage was reached on Day 30. Various types of bacteria thrived and fell to reach their equilibrium in the MBR treatment system. In future research, bacterial species present in the MBR should be
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Because of its high consumption by humans, caffeine has been used as an indicator of domestic sewage contamination in ground and surface water (Seiler et al., 1999). Municipal wastewater is usually rich in caffeine. For example, it was found at 17.0 lg/L in the effluent of the Mill Cove sewage treatment plant (Halifax, Nova Scotia) even after vigorous 3-stage treatment (Crouse et al., 2012). It was used in this study as an indicator of organic substances. The calculated concentration of caffeine in the leachate was determined to be 1.33 mg/L. The concentration of caffeine in the effluent increased briefly with the introduction of compost leachate (Table 2). As the experiment progressed, the levels of caffeine decreased gradually to 0.6 lg/L at the end of experiment. An impressive reduction of 99.9% was observed in the concentration of caffeine. The results of total metal analysis include over 20 heavy metals (Table 2). Six days were chosen over the course of the experiment to highlight the trend in metal concentrations over time and the progress of the MBR in removing these elements. Due to its heavy load of contamination, the initial compost leachate was diluted considerably during ICP-MS analysis, which increased the reported detection limits (RDLs) of the leachate sample accordingly. Percent reductions were not calculated for heavy metals with concentrations that were less than RDLs. Initial concentrations of five metals: aluminum, iron, manganese, strontium and zinc were particularly high in the compost leachate. Of the high major metal contaminants, all were reduced to 89% of their initial concentrations or greater at the end of experiment. The reductions may be attributed to incorporation of metals into biomass within the MBR. To avoid excessive accumulation of metals in activated sludge, removal of the activated sludge periodically is required for continuous operation beyond this proof-of-concept study. It is important to understand how heavy metals in MBR are related to the MLSS concentration. In future research, heavy metals in suspended solids will be investigated to study the kinetics of absorption and their partition in MLSS. Many of the metals studied have Health Canada mandated maximum acceptable concentrations (MACs) for drinking water (Federal-Provincial-Territorial Committee on Drinking Water, 2010). Initial concentrations of barium, cadmium, chromium, lead, manganese, exceeded these maximum acceptable concentrations; however the effectiveness of the treatment yielded dramatically reduced concentrations, some of which have exceeded wastewater standards and even met drinking water quality standards. The arsenic level (634 lg/L) was also well above the drinking water quality standard (10 lg/L) before treatment and was drastically reduced to 19 lg/L, nearly meeting the drinking water standard. Health Canada also regulates metals for aesthetic considerations. Metals such as aluminum, iron, manganese and zinc all exceeded the aesthetic guidelines as set by Health Canada. All four metals were reduced to below acceptable drinking water guidelines. The only metal that showed noticeable increases in the effluent was copper. Copper tubing was used for the collection of effluent from the MBR membrane modules. Copper tubing was also used for the air spargers that were present below the MBR modules. Copper pipes were corroded by the leachate and released copper into the MBR system. The increase of the copper concentration
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K. Brown et al. / Waste Management xxx (2013) xxx–xxx
was visually observable through the apparent color of the effluent over time. Health Canada has established an aesthetic quality guideline for copper in drinking water (1.0 mg/L) (Federal-Provincial-Territorial Committee on Drinking Water, 2010). The copper concentration in the MBR effluent (3.17 mg/L) only slightly exceeds this guideline. 4. Conclusions The treatment of compost leachate using membrane bioreactor technology was studied for 39 days at which the MBR treatment reached a steady state. Numerous water quality parameters were examined during the experiment period. The COD of the effluent was successfully reduced by more than 99% from an initial leachate COD of 116 g/L. High reductions were also observed for ammonia, caffeine and many of the metals studied. This experiment demonstrated MBR is a very efficient method for treating compost leachate. Acknowledgements The authors would like to acknowledge the financial support from Acadia University and the Natural Sciences and Engineering Research Council of Canada (NSERC). In addition, the authors would like to thank Dwight Horsnell (Northridge Farms Limited), Dr. Yiming Zeng (Superstring MBR Technology, Corp.) and Dr. Martin Tango (School of Engineering, Acadia University) for their technical assistance and helpful discussion in this project. References ABL Environmental Consultants Ltd., 2006. Atlantic Canada Wastewater Guidelines Manual. Environment Canada. Clesceri, L.S., Greenberg, A.E., Trussell, R.R., 1989. Standard Methods for the Examination of Water and Wastewater. American Public Health Association, Washington, DC. Crouse, B., Ghoshdastidar, A.J., Tong, A.Z., 2012. The presence of acidic and neutral drugs in treated sewage effluents and receiving waters of the Cornwallis and Annapolis River watersheds and Mill Cove WPCC in Nova Scotia. Environmental Research 112, 92–99. Cumar, S., Nagaraja, B., 2011. Environmental impact of leachate characteristics on water quality. Environmental Monitoring and Assessment 178, 499–505. Delgado, S., Villarroel, R., Gonzalez, E., Morales, M., 2011. Aerobic Membrane Bioreactor for Wastewater Treatment – Performance Under Substrate-Limited Conditions. InTech.
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Epstein, E., 1997. The Science of Composting. CRC Press LLC, Boca Raton. Federal-Provincial-Territorial Committee on Drinking Water, 2010. Guidelines for Canadian Drinking Water Quality. Health Canada. Ghoshdastidar, A.J., Tong, A.Z., 2013. Treatment of 2,4-D, mecoprop, and dicamba using membrane bioreactor technology. Environmental Science and Pollution Research, doi: http://dx.doi.org/10.1007/s11356-013-1498-z. Ghoshdastidar, A.J., Saunders, J.E., Brown, K.H., Tong, A.Z., 2012. Membrane bioreactor treatment of commonly used organophosphate pesticides. Journal of Environmental Science and Health, Part B 47, 742–750. Grigoropoulou, H., Sotirakou, E., Kladitis, G., Diamantis, N., 1999. Ammonia and phosphorous removal in municipal wastewater treatment plant with extended aeration. Global Nest: The International Journal 1, 47–53. Hammer, D.A., 1989. Constructed Wetlands for Wastewater Treatment: Municipal, Industrial, and Agricultural. Lewis Publishers, Chelsea. Jarecki, M.K., Chong, C., Voroney, R.P., 2005. Evaluation of compost leachates for plant growth in hydroponic culture. Journal of Plant Nutrition 28, 651–667. Judd, S., 2006. Principles and Applications of Membrane Bioreactors in Water and Wastewater Treatment. Elsevier Ltd., Oxford. Le-Clech, P., 2010. Membrane bioreactors and their uses in wastewater treatments. Applied Microbiology and Biotechnology 88, 1253–1260. Liu, C.-H., Lo, K.V., 2001a. Ammonia removal from compost lachate using zeolite. II. A study using continuous flow packed columns. Journal of Environmental Science and Health, Part B 36, 667–675. Liu, C.-H., Lo, K.V., 2001b. Ammonia removal from composting leachate using zeolite. I. Characterization of the zeolite. Journal of Environmental Science and Health, Part A 36, 1671–1688. Meng, F., Chae, S.-R., Drews, A., Kraume, M., Shin, H.-S., Yang, F., 2009. Recent advances in membrane bioreactors (MBRs): membrane fouling and membrane material. Water Research 43, 1489–1512. Nova Scotia Environment, 1995. Solid Waste Resource Management Strategy. Province of Nova Scotia, pp. 3–11. Nova Scotia Environment, 2010. Organic Composting Facilities in Nova Scotia. Province of Nova Scotia, pp. 7–9. Radjenovic´, J., Matošic´, M., Mijatovic´, I., Petrovic´, M., Barceló, D., 2008. In: Barceló, D., Petrovic, M. (Eds.), Membrane Bioreactor (MBR) as an Advanced Wastewater Treatment Technology; Emerging Contaminants from Industrial and Municipal Waste. Springer, Berlin/Heidelberg, pp. 37–101. Seiler, R.L., Zaugg, S.D., Thomas, J.M., Howcroft, D.L., 1999. Caffeine and pharmaceuticals as indicators of waste water contamination in wells. Ground Water 37, 405–410. Smith, H.L., 2006. Bacterial Growth. Arizona State University, Tempe. Stephenson, T., Brindle, K., Judd, S., Jefferson, B., 2000. Membrane Bioreactors for Wastewater Treatment. IWA Publishing, London. Stottmeister, U., Buddhawong, S., Kuschk, P., Wiessner, A., Mattusch, J., 2006. Constructed wetlands and their performance for treatment of water contaminated with arsenic and heavy metals. In: Twardowska, I., Allen, H.E., Häggblom, M.M., Stefaniak, S. (Eds.), Soil and Water Pollution Monitoring, Protection and Remediation. Springer, Dordrecht, pp. 417–432. Trujillo, D., Font, X., Sánchez, A., 2006. Use of Fenton reaction for the treatment of leachate from composting of different wastes. Journal of Hazardous Materials 138, 201–204. World Health Organization, 2003. Ammonia in Drinking Water. World Health, Organization, pp. 2–3.
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