Environmental and cost life cycle assessment of different alternatives for improvement of wastewater treatment plants in developing countries

Science of the Total Environment 660 (2019) 57–68

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Science of the Total Environment journal homepage: www.elsevier.com/locate/scitotenv

Environmental and cost life cycle assessment of different alternatives for improvement of wastewater treatment plants in developing countries Hamdy Awad, Mohamed Gar Alalm ⁎, Hisham Kh. El-Etriby Department of Public Works Engineering, Faculty of Engineering, Mansoura University, Mansoura 35516, Egypt

H I G H L I G H T S

G R A P H I C A L

A B S T R A C T

• Environmental impacts of four different scenarios to improve WWTPs were studied. • Energy consumption in WWTPs has a great influence on the environmental impacts. • Tertiary treatment was beneficial to all impact categories due to water reuse. • Merging anaerobic digestion and tertiary treatment attained maximum benefits.

a r t i c l e

i n f o

Article history: Received 18 October 2018 Received in revised form 23 December 2018 Accepted 25 December 2018 Available online 03 January 2019 Editor: Deyi Hou Keywords: Life cycle assessment Wastewater treatment Anaerobic digestion Tertiary treatment

a b s t r a c t Most of wastewater treatment plants (WWTPs) in developing countries comprised primary and secondary treatment without including any tertiary treatment or sludge processing. Decision makers think that additional treatment is costly and the gained environmental benefits are limited. This study aims to investigate the environmental and economic benefits of improving current conventional WWTPs in developing countries by adding tertiary treatment and/or anaerobic digestion of sludge. For this purpose, life cycle assessment (LCA) for four different scenarios was studied for a wastewater plant located in Gamasa, Egypt. The 1st scenario is the plant in its current state. The 2nd scenario is the addition of anaerobic digestion of sludge. The 3rd scenario is the addition of a tertiary treatment stage. The 4th scenario is the addition of anaerobic digestion of sludge and tertiary treatment stage. CML 2000 method was used for assessing the environmental impacts of the four scenarios. The 4th scenario attained maximum environmental benefits for all categories due to the energy saving and the prospect of water reuse. The application of the 4th scenario achieved environmental benefits in some important categories such as ozone layer depletion. According to the economic evaluation, the addition of tertiary treatment leads to gain financial profits due to the value of the reusable produced water. This study underlines the importance of considering LCA in development of WWTPs in developing countries. © 2019 Elsevier B.V. All rights reserved.

1. Introduction Disposal of domestic wastewater without sufficient treatment is increasingly concerned in developing countries. The improper design and operating of wastewater treatment plants (WWTPs) may cause severe ⁎ Corresponding author. E-mail address: [email protected] (M. Gar Alalm).

https://doi.org/10.1016/j.scitotenv.2018.12.386 0048-9697/© 2019 Elsevier B.V. All rights reserved.

environmental problems on local and global scales (Sabeen et al., 2018; Xiong et al., 2018a). Moreover, many developing countries are not totally served by wastewater treatment plants. Primitive methods are still used to mitigate the direct impacts of untreated wastewater on human health, but many environmental and health impacts are unbeatable. In addition, most of wastewater treatment plants in developing countries only include primary (physical treatment) and secondary (biological treatment) stages without tertiary treatment or

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advanced sludge processing. Decisions about wastewater projects in developing countries are primarily influenced by direct capital and operating costs as long as the design is meeting the local standards, while life-cycle cost and life-cycle environmental impacts are rarely considered. Achievement of integrated sustainability in wastewater treatment projects requires the consideration of life cycle perspective, which is not only limited to water remediation benefits (Song et al., 2017; Zang et al., 2015). Accordingly, many recent studies reported the significance of life cycle assessment (LCA) in the evaluation of wastewater treatment facilities (Benetto et al., 2009; Corbella et al., 2017; Liu et al., 2013; Muñoz et al., 2009). LCA is a standardized, integrated, and complicated tool to compile and evaluate the environmental impacts for a product including all inputs and outputs from cradle to grave (Finkbeiner et al., 2006; Tang et al., 2018). This assessment is mainly carried out by assembling an inventory of the most related inputs and outputs to the system and determining the possible impacts related to the inventory (inputs and outputs). The outcomes of the inventory analysis and impact assessment stages are then interpreted relative to the purposes of the study (Foteinis et al., 2018). There are mainly two types of life cycle assessment procedures: the first one gives emphasis to energy and materials flow in a manufacturing process and is called the conventional process-based life cycle assessment and the second type highlights environmental data in a manufacturing process and is called the input-output life cycle assessment (Zhang et al., 2010). Input-output life cycle assessment is mostly used for evaluation of environmental impacts in the studies emphasizing on sustainable development like energy and water production (Chen et al., 2012; Lorenzo-Toja et al., 2016). Development of wastewater treatment technologies is recently considered an important role to achieve upcoming water security in a world that has an increase in water stress (Gar Alalm et al., 2016; Xiong et al., 2018b). Wastewater treatment plants are considered as units aiming the effective removal of nutrients and organic matter from contaminated wastewaters to avoid the accumulation of these pollutants in the environment. Although these benefits, WWTPs have been found to consume a great portion of energy, resulting in increasing of greenhouse gases emissions and other impacts on the environment (Yoshida et al., 2014; Zou et al., 2018). More than 50% of the energy consumption in WWTPs is caused by mechanical aeration (Au et al., 2013; Gar Alalm et al., 2018). Therefore, it is necessary to study the improvement of WWTPs from the life cycle perspective to maximize the environmental benefits and minimize the undesired impacts, especially for the WWTPs that have been constructed without taking the sustainability requirements into account. This study aims to study the environmental performance of different scenarios for the improvement of existing WWTPs in developing countries. Complete input-output LCA study is carried out for suggested scenarios to develop an existing WWTP located in Gamasa, Egypt. CML 2000 method was used for the determination of seven different impact categories for all scenarios. 2. Methodology 2.1. Goal and scope The goal of this study is to determine the environmental impacts for improvement of a conventional municipal wastewater treatment plant in developing countries. A conventional wastewater treatment plant in Gamasa city, Egypt was taken as a case study. The information and data of the plant were collected from the official documents of the Egyptian Company of Water and Wastewater. The plant was put into operation in 2010. The plant consists of two grit removal chambers, four primary settling tanks, four aeration tanks, four secondary settling tanks, a chlorination tank, two sludge thickeners, and 88 sludge drying beds. The maximum capacity of the plant is 40,000 m3/d. Four scenarios were assessed to determine which scenario has the least impacts on the

environment. The first scenario is the plant in its current status (Fig. 1a). The second scenario is the plant in its current status but with the addition of anaerobic digester for sludge fermentation and biogas production (Fig. 1b). The anaerobic digestion takes place in a cylindrical tank with a fixed roof and the tank floor is conical with a bottom sloping to the center. The produced gas is utilized in electricity production to partially or completely operate the plant. The third scenario is the plant in its current status with the addition of a tertiary treatment process. The tertiary treatment includes conventional chemical coagulation and sand filtration process (Fig. 1c). The fourth scenario is a combination of scenarios 2 and 3 that includes the addition of both tertiary treatment and anaerobic digestion of the sludge (Fig. 1d). 2.2. Framework for LCA analysis The functional unit for comparison of the impacts of different scenarios is 1 m3 of treated wastewater. The four scenarios were assessed using inventory data from Ecoinvent 2.0 database (Frischknecht et al., 2007). For LCA analysis of different scenarios, the boundary of this study includes primary, secondary and tertiary treatment stages. Both construction and operation phases were taken into account. All materials and consumed energy were included in the two phases. The output from the main system (Gamasa wastewater treatment plant) to the environment principally includes liquid discharge such as the secondary effluent, unusable sludge generated in the operation phase, and gaseous emissions such as CO2 from the construction and operation phase. The other three scenarios include products such as the finally treated water (tertiary effluent) for reuse, usable sludge (digestate) from the anaerobic digestion process, which can be recycled for agriculture application and biogas production from the anaerobic digester which is utilized in electricity generation. Following the basic principles of ISO14040 (Finkbeiner et al., 2006), a framework was obtained as shown in Fig. 2 which gives details of the systems for LCA analysis, together with the input and output items. 2.3. Life cycle inventory Life cycle inventories (LCI) were mainly the materials and energy consumption in both construction and operation phases. The material and energy consumption of Gamasa wastewater treatment plant were collected during technical visits and meetings with the managers and the technicians of the treatment plant. The LCI analysis for the anaerobic digester and tertiary treatment process were calculated according to Metcalf et al. (2003). In addition, some factors and parameters such as retention time, biogas production rate, and composition of collected gases were taken from other reports in the literature (Muñoz et al., 2009; Y.J. Suh and Rousseaux, 2002; Zhang et al., 2010). The retention time is important to calculate the volume of the anaerobic tank, while the gas quantity and composition are essential to calculate the power that could be generated from the biogas. 2.3.1. Construction stage In the construction phase, the main materials used for plant construction were concrete and steel. The inventory of construction materials for the plant is shown in Table 1. The plant occupied an area about (225 ∗ 350 m). In the case of the second scenario, 88 drying beds are removed with and one tank is added for the anaerobic digestion process. For the third and fourth scenario, it is assumed that 90% of the secondary effluent from Gamasa plant will be used in tertiary treatment. The tertiary treatment facilities occupied about 4765 m2. All the data for the construction stage are for the whole design period 30 years. 2.3.2. The operation phase For the operation of Gamasa wastewater treatment plant, the most important element is energy consumption and the effluent

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a) Primary settling tank

Disinfection tank

Drying beds

Secondary settling tank Secondary sludge

Secondary effluent

Aeration tank Secondary sludge

Grit removal

Primary sludge

Influent

Thickener

b) Sludge from the activated sludge process

Thickening

Anaerobic digestion

Electricity generation

Biogas

Fertilizer for agriculture use

Digestate

c) Secondary effluent from Gamasa WWTP Receiving

Flocculationsedimentation tank

Coagulation

Pump

Tank

Tertiary

Treated water tank

Pump effluent

Sand filter

d) Gamasa wastewater treatment plant

Secondary effluent

Electricity

Sludge

Tertiary treatment process

Anaerobic digestion process

Tertiary effluent

Fig. 1. a) Scenario 1: Gamasa wastewater treatment plant; b) scenario 2: current plant plus the anaerobic digestion process; c) scenario 3: current plant plus the tertiary treatment process; d) scenario 4: current plant plus the anaerobic digestion process and the tertiary treatment process.

characteristics. In the first and third scenario, the plant is operated using electricity from non-renewable energy (electrical grid), while in the second and fourth scenario, it is operated using electricity from both electrical grid and generated electricity from the anaerobic digestion of sludge. The average daily sludge is about 100 m3/d according to the collected data from Gamasa treatment plant. The dry weight percentage of solid residues from anaerobic digestion process is assumed 25% according to the literature (Y. Suh and Rousseaux, 2002), so the average daily sludge in the second scenario has been assumed 25 m3/d. The methane gas production was calculated and found to be 1330 m3/d according to the rates in the literature (Metcalf et al., 2003). Only about 35% of the CH4 energy might be converted into electricity, and the remaining 65% is given off as heat (McCarty et al., 2011). The most important output of the tertiary treatment process is water, which can be used

for industrial and domestic purposes. The characteristics of the tertiary treated effluent are assumed according to the reported removal efficiencies for the same tertiary treatment procedure in the literature (Hamoda et al., 2004; Licciardello et al., 2018; Rajput, 2006; Zhang et al., 2010). Table 2 shows the life cycle inventory of the operation stage. The higher pH in tertiary treatment is attributed to the decrease of dissolved CO2 concentration due to the reduction of organic matter by oxidation (Colmenarejo et al., 2006). 2.3.3. The output items The outputs of the four scenarios are illustrated within the framework of study (Fig. 2). The effluents and gaseous emissions of the plant were considered as outputs for all scenarios. Solid waste and gaseous emissions are calculated according to relevant reports in the

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System under study

Input 1- Construction stage

Scenario 1: Gamasa WWTP

Materials and energy for

Scenario 2: Gamasa WWTP +

-Construction work -Manufacturing

anaerobic digester Scenario 3: Gamasa WWTP + tertiary

-etc.

treatment

2-Operation stage Scenario 4: Gamasa WWTP + -Energy consumption

Output 1-Gaseous emissions -Co2 -N2O -CH4 2- Solid waste -sludge 3-products -Secondary effluent -Tertiary effluent -Biogas -Digestate

anaerobic digester + tertiary treatment

Fig. 2. Framework of the system for the LCA study.

literature (Flores-Alsina et al., 2011; Snip et al., 2009). The greenhouse gases (GHG) that have been considered in this study are CO2, CH4, and N2O. The details of the calculations of the emissions are illustrated in part S8 in the supplementary file. The effluents from all scenarios are finally discharged to a natural stream (drainage 2, Gamasa), and therefore they contribute in many impacts on the environment such as eutrophication. Additionally, water reuse after the tertiary treatment in industry and non-drinking purposes may reduce the impacts of all categories because it saves tap and fresh waters. The energy consumptions of the additional units are assumed according to relevant reports in the literature (Tchobanoglous et al., 2014; Zhang et al., 2010). The effluent characteristics from tertiary treatment are complying with the guidelines of the Environmental Protection Agency (EPA) for the water reuse (EPA, 2012). Accordingly, the effluents in the 3rd and 4th scenarios are utilized for the non-drinking purpose to save the tap and freshwaters. The water reuse scenarios are expected to be more economical and beneficial to the environment.

• Eutrophication potential - EP (kg PO4−3 eq.): it describes the possibility of nutritious substances to cause eutrophication in surface water. • Photochemical oxidation - PHO (kg of formed ozone eq.): it describes the potential of gaseous emissions to form photo-oxidant substances by photochemical reactions. • Depletion of abiotic resources - DAR (kg of antimony eq.): it describes the contribution of various emissions to deplete natural energy resources such as iron ore and crude oil. • Ozone layer depletion potential - ODP (kg CFC-11 eq.): it describes the potential of substances to cause depletion of the ozone layer. • Terrestrial ecotoxicity potential - TAETP (kg 1,4 DCB eq.): it describes the potential of substances to affect human, flora, and fauna. Most of these substances include heavy metals or bio-recalcitrant organics. • Freshwater aquatic ecotoxicity – FETP (kg 1,4 DCB eq.): it refers to the impact on freshwater ecosystems (streams, rivers, and lakes that have a salinity of less than 0.05%), due to emissions of toxic substances to air, water, and soil.

2.4. Life cycle impact assessment method The environmental impacts of the four scenarios were determined by CML 2000 baseline method. This method was developed by the Institute of Environmental Sciences at Leiden University (Vrecko and Vilanova, 2015). This method includes the most important impact categories for domestic wastewater such as eutrophication potential and global warming potential. The following impact categories are considered in this study. • Acidification potential – AP (kg SO2 eq.): it describes the potential of substances to acidify the water. These substances may cause acid rains which are harmful to terrestrial and aquatic species. • Global warming potential - GWP (kg CO2 eq.): it describes the potential of gaseous emissions to cause global warming.

Table 1 Life cycle inventory of construction materials for the four scenarios. Item

Unit

Scenario 1

Scenario 2

Scenario 3

Scenario 4

Cement Steel Sand Gravel Land occupation

Ton Ton Ton Ton m2

2100 425 3781 7142 78,750

1974 395 3655 6891 62,395

3845 839 6921 13,072 89,556

3719 706 6795 12,821 71,901

Table 2 Life cycle inventory of operation stage for the four scenarios. Item

Unit

Scenario 1

Processing Sludge

m3/d

100

km KWh KWH m3/d

Transportation of sludge Electricity, conventional procedures Electricity, biogas Water for reuse purposes (industrial and domestic)

Scenario 2

Scenario 3

Scenario 4

100

25

20 2290

25 (digestate) 20 893.5

20 2862

20 893.5

– –

1396.5 –

– 36,000

1396.5 36,000

Emissions to water (drainage 2, Gamasa) BOD mg/L 50 COD mg/L 72 TSS mg/L 43 TDS mg/L 9168 pH 7.6 NH+ mg/L 14 4 Nitrite mg/l 0.8 Nitrate mg/l 1.2 PO4 mg/l 5 Free chlorine mg/l 0.52

50 72 43 9168 7.6 14 0.8 1.2 0 0.52

Water reuse 1.5 1.5 2.16 2.16 1.29 1.29 1274.3 1274.3 7.8 7.8 0.28 0.28 0.4 0.4 0.6 0.6 0 0 0.5 0.5

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2.5. Economic evaluation The total costs (TC) for improvement of Gamasa wastewater plant by anaerobic digestion and/or tertiary treatment were calculated per each treated m3 of wastewater. The total costs are the summation of actual amortization costs (AC) and operating costs (OC) based on Eq. (1) (Gar Alalm et al., 2014a) TC ¼ AC þ OC

ð1Þ

The initial amortization costs (iAC) are calculated by summation of the costs of all the construction materials and activities divided by the amount of treated wastewater during a lifespan (L) of 30 yrs. The actual amortization costs (AC) are then calculated based on a continuous interest rate (i) of 8% (Average interest rate in developing countries) as given in Eq. (2) (Gar Alalm and Nasr, 2018) AC ¼ iAC

ð1 þ iÞL i

!

ð1 þ iÞL −1

ð2Þ

The operating costs are calculated taking into account the materials, energy consumption, and maintenance. The maintenance costs are assumed 2% of the amortization costs (Gar Alalm et al., 2014b). The labor costs are not considered in this study for the simplicity of the calculations and because the labor costs are varied according to the plant location. The net total costs are calculated after considering the benefits of utilizing produced biogas and water saving. 3. Results and discussion 3.1. Environmental impacts of conventional activated sludge system (scenario 1) Gamasa wastewater treatment plant in its current state (scenario 1) causes considerable effects in all analyzed impact categories as shown in Fig. 3(a). In the conventional system, the high electricity consumption and the effluent of secondary treatment in operation stage were the main contributors to environmental impacts. On the other hand, the impacts of the construction stage were relatively small as they were assigned to the functional unit of 1 m3 of treated water. This finding was also reported in many LCA studies for conventional wastewater treatment plants (Sabeen et al., 2018), and highrate anaerobic-aerobic digestion (Postacchini et al., 2016). The activities of the operation stage contribute with higher than 80% to all impact categories, except for the freshwater aquatic ecotoxicity, they contribute with about 42%. This result is attributed to the harmful products and emissions of cement and steel industries as shown in Fig. 3 (c) and as previously interpreted in the literature (Liu et al., 2013). According to the normalized impacts (Fig. 3(b)), the most significant environmental impact is the GWP. The high GWP (0.954 kg CO2 eq.) is mainly due to the gaseous emissions like CO2, and NO2, besides the consumption of energy from nonrenewable sources such as thermal combustion of fossil fuels. In addition, the production of construction materials (mainly steel and cement) contributes to GWP. The eutrophication potential is 0.0013 kg PO4 eq. which considered the second significant impact (27.4%). The eutrophication is mainly due to the remaining nutrients in the effluent, while the eutrophication occurred during the construction stage could be neglected. The abiotic depletion is the third significant impact category (4.63%). This is due to the consumption of resources such as fossil fuels, minerals, and metals in the construction and operation phases. The impacts on acidification potential, photochemical oxidation, ozone layer depletion, and terrestrial ecotoxicity are relatively small and the major contributions are operation and treatment processes. The electricity consumption is the main contributor to the environmental impacts of the conventional activated sludge plant as shown in Fig. 3(b). The high electricity consumption is

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mainly attributed to the mechanical aeration (Tan et al., 2011). The transportation impacts are less than 5% for all categories, except for ozone layer depletion, it contributes 24% of the total impact. Similar findings were reported for other wastewater treatment case studies in the literature (Garfí et al., 2017). 3.2. Environmental impacts after the addition of anaerobic digestion (scenario 2) The addition of the anaerobic digestion process to Gamasa treatment plant has resulted in the reduction of environmental impacts in all considered categories except photochemical oxidation as shown in Fig. 4 (a). The rise of photochemical oxidation is attributed to CH4 emissions from the digestion process. The GWP in the operation stage was slightly reduced from 0.954 to 0.951 kg CO2 eq. This finding is mostly due to the biogas production which has been utilized to provide a part of electricity for the plant instead of complete dependence on the electrical grid. On the other hand, the gaseous emissions from the anaerobic digestion process led to the non-significance of this reduction. The addition of anaerobic digestion has a dual effect on the terrestrial ecotoxicity. It has a good effect due to the reducing of electricity consumption from conventional procedures and a bad effect due to the production of biosolids and digestate liquid. The digestate could be separated into solid and liquid fractions. The solid fraction is usually 20% of the mass and it could be composted and used as fertilizers. The utilization of liquid fraction is limited due to several challenges such as handling, transportation, and nutrient recovery especially in developing countries (Kizito et al., 2017). Accordingly, the utilization of the liquid fraction of digestate is not considered as a beneficial activity in this study. The eutrophication potential is still the second significant impact category according to the normalization results shown in Fig. 4(b). The eutrophication potential is mainly attributed to influent quality, and since the influent quality is not changed in this scenario, there is no significant change in eutrophication potential. The depletion of abiotic resources was also reduced from 0.00027 to 0.00015 kg antimony eq. This reduction is due to the saving of crude oil by depending on the produced biogas to provide the electricity for the plant. The ozone depletion potential has been also decreased because the amount of transported sludge has been reduced. The acidification potential has been reduced by about 47%. The terrestrial ecotoxicity potential has been also reduced from 0.0002 to 0.00014 kg 1,4 DCB eq. The cause of these reductions is the dependence on green technology for electricity production as could be understood from Figs. 3(c) and 4(c). 3.3. Environmental impacts after addition of tertiary treatment stage (scenario 3) The tertiary treatment process (scenario 3) has significant effects in all impact categories as depicted in Fig. 5. The benefits from the reuse of the tertiary treated effluent are much higher than the energy and materials consumption in the tertiary treatment stage. The energy consumption has the greatest effect on the environmental impacts and due to the energy saving from water reuse in nondrinking and industrial purposes. Fig. 5 depicts that the gained reusable water from the tertiary treatment has great negative impacts in all categories. The negative impact means a benefit for the environment. For instance, the net abiotic depletion potential for constructing and operating the plant is - 0.0011882 kg Sb eq., which means that the plant in this scenario is beneficial for reducing this impact category. The same finding was observed for the photochemical oxidation and ozone layer depletion potentials. Moreover, the GWP is reduced from 0.953 to 0.907 kg CO2 eq. due to saving water and power in water treatment plants. The eutrophication potential is significantly reduced from 0.0013 to 0.00017 kg PO4 eq. because a limited amount of effluents are discharged to the drains and other water bodies.

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Fig. 3. LCA of the 1st scenario (a) the environmental impacts and the contributions of operation and construction stages, (b) normalized impacts of different categories, (c) influence of different activities.

It is clear from the above results that the percentage of construction stage effect on the environmental impacts has been increased due to the construction of the second stage (tertiary treatment plant), and consequently the construction environmental impacts have been increased in many categories. On the contrary, the tertiary

treatment process caused a significant reduction in the operation environmental impacts, and since the operation stage has the greatest effect on the environmental impacts of Gamasa wastewater treatment plant, this reduction in the operation stage impacts led to a significant reduction in the total impacts of the treatment plant.

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Fig. 4. LCA of the 2nd scenario (a) the environmental impacts and the contributions of operation and construction stages, (b) normalized impacts of different categories, (c) influence of different activities.

The negative sign for some of the impact categories means that the beneficial effect of the tertiary treatment process is bigger than the bad effect of the conventional activated sludge system. The secondary effluent from Gamasa wastewater treatment plant is discharged into a drain and it still contains residual contaminants

that may cause harmful effects on the surface water and the aquatic environment. The advantage from reuse of the effluent of the tertiary treatment process was assessed in two sides. Firstly, the secondary treatment effluent will not be discharged to a surface stream. Therefore, benefits can be gained by reducing the residual pollutants

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Fig. 5. LCA of the 3rd scenario (a) the environmental impacts and the contributions of operation and construction stages, (b) normalized impacts of different categories, (c) influence of different activities.

discharging to the surface stream. Secondly, the effluent of the tertiary treatment process will be used to replace tap water for industrial and domestic purposes. Thus, the benefit of water saving is considered to be equivalent to energy saving that otherwise would be consumed for the same quantity of tap water production (Zhang et al., 2010).

3.4. Environmental impacts after addition of anaerobic digester and tertiary treatment stage (scenario 4) LCA analysis of Gamasa wastewater treatment plant after adding an anaerobic digester (scenario 2) and the tertiary treatment (scenario 3) showed great positive effects in all impact categories.

H. Awad et al. / Science of the Total Environment 660 (2019) 57–68

Accordingly, merging both scenarios is an integrated solution to obtain the best environmental scenario on a life-cycle basis. Negative impact potentials have been observed for acidification, abiotic depletion, fresh water aquatic ecotoxicity, photochemical oxidation and ozone layer depletion as shown in Fig. 6. It is clear that the reusable water

65

(for non-drinking and industrial purposes) from the tertiary treatment process and the reduction of power consumption from the electricity grid were the main reasons for the improvement of the environmental performance of the plant on a life-cycle basis as shown in Fig. 6(c). The global warming potential was reduced from 0.954 to 0.746 kg CO2 eq

Fig. 6. LCA of the 4th scenario (a) the environmental impacts and the contributions of operation and construction stages, (b) normalized impacts of different categories, (c) influence of different activities.

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Fig. 7. Comparison of the environmental impacts of the four scenarios.

which means that more beneficial effects were earned by combining the anaerobic digestion of sludge with tertiary treatment. Acidification potential had positive value in the 3rd scenario, but it became negative in the 4th scenario due to the reduction of power consumption from the electricity grid. 3.5. Comparison of the environmental impacts of the four scenarios The traditional activated sludge system without any improvements has the greatest environmental impact potentials compared to the other three scenarios as shown in Fig. 7. The main reasons for this finding are the complete dependence on the electricity grid and the discharge of contaminated effluents to surface water bodies. The addition of sludge digester and/or tertiary treatment increased the environmental impact potentials in the construction phase. However, great environmental benefits were attained due to the dependence on the electricity generated from biogas, and water reuse in non-drinking purposes. For the eutrophication impact category, the best-treated water characteristics are scenarios 3 and 4 because the residual nutrients from the secondary stage are not transferred to surface water. In terms of terrestrial ecotoxicity, the second scenario has a lower impact than the first scenario because of the reduction of electricity consumption and the utilization of the solid fraction of the digestate to displace synthetic

fertilizers. Moreover, combining tertiary treatment and sludge digestion (4th scenario) reduced the terrestrial ecotoxicity potential due to the water reuse and water saving. The acidification potential was reduced in the 2nd scenario to 54% of the 1st scenario potential due to the saving of non-renewable energy, and it was almost zero after addition of tertiary treatment (3rd scenario). Moreover, the acidification potential was negative in the 4th scenario due to combining the advantages of energy saving and water reuse. The 3rd and 4th scenarios were beneficial for abiotic depletion, photochemical oxidation, and ozone layer depletion, but the 4th scenario has greater beneficial values. Accordingly, the best scenario in this study is the 4th scenario which includes tertiary treatment and anaerobic digestion of sludge. 3.6. Economic evaluation The total and net costs of the three development scenarios are illustrated in Table 3. More calculation details are illustrated in Table S6. 1st scenario is considered as the current state while the scenarios 2, 3, and 4 are considered the developed state scenarios. The aim of the costs estimation study is to give the benefits of developing the current WWTPs in developing countries in the long run. The cost of 1 kWh is assumed 1.45 Egyptian pound (EGP) (1 EGP ≈ 0.056 $) according to the Egyptian ministry of power and electricity. The benefit of 1 m3 of reused water

Table 3 Economic evaluation of different categories. Scenario Scenario 2 Scenario 3 Scenario 4

AC (EGP/m3)

OC (including maintenance) (EGP/m3)

TC (EGP/m3)

Energy saving (EGP/m3)

Water saving (EGP/m3)

Net costs (EGP/m3)

0.057 0.048 0.105

0.033 0.022 0.055

0.09 0.07 0.16

−0.051 0 −0.051

0 −0.702 −0.702

0.039 −0.632 −0.593

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is assumed 0.78 EGP (considered as 30% of the cost of 1 m3 of produced fresh water). The calculated net costs of the addition of anaerobic digestion (2nd scenario) are about 0.039 EGP/m3 which are less than 3% of the current costs of 1 m3 of treated wastewater (1.37 EGP/m3 according to the Egyptian company of water and wastewater). Moreover, the costs may be reduced if the value of the space resulted from the cancellation of drying beds is considered. The net costs for the addition of tertiary treatment to a current plant (3rd scenario) are - 0.632 EGP/m3. The negative value means that the plant will earn money in the long run, which can compensate for the initial amortization costs. The profits of the 4th scenario are a little bit less than the 3rd scenario due to the net costs of anaerobic digestion. However, considering the environmental benefits along with financial benefits may lead to consider the 4th scenario as the optimum scenario for decision makers. 4. Conclusions Four different scenarios of Gamasa wastewater plant were environmentally evaluated by LCA technique based on CML 2000 baseline method. The LCA showed that the influence associated with the operational activities is more important than the influence of construction materials and activities. In addition, the results show that the gaseous emissions and energy consumption have the greatest effects on environmental impacts. The conventional activated sludge system with anaerobic digestion of sludge showed lower impacts on the environment compared to the conventional system without sludge digestion due to the replacement of electricity produced from conventional procedures by the power generated from anaerobic digestion process. Adding tertiary treatment was very beneficial for all impact categories due to water and energy saving gained from water reuse. As a consequence, merging anaerobic digestion and tertiary treatment attained maximum environmental benefits for all categories due to the improvement of energy efficiency and water saving. The costs estimation for the development scenarios revealed that financial benefits of 0.632 and 0.593 EGP/m3 could be attained by application of the 3rd and 4th scenarios respectively. The only limitation that could be considered in the decision making for developing of current WWTPs is the abundance of amortization costs. Acknowledgment The authors gratefully acknowledge the support from the Science and Technology Development Fund (STDF), Egypt through the project No 262279. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.scitotenv.2018.12.386. References Au, M.T., Pasupuleti, J., Chua, K.H., 2013. Strategies to improve energy efficiency in sewage treatment plants. 4th International Conference on Energy and Environment, pp. 1–4 https://doi.org/10.1088/1755-1315/16/1/012033. Benetto, E., Nguyen, D., Lohmann, T., Schmitt, B., Schosseler, P., 2009. Life cycle assessment of ecological sanitation system for small-scale wastewater treatment. Sci. Total Environ. 407, 1506–1516. https://doi.org/10.1016/j.scitotenv.2008.11.016. Chen, Z., Ngo, H.H., Guo, W., 2012. A critical review on sustainability assessment of recycled water schemes. Sci. Total Environ. 426, 13–31. https://doi.org/10.1016/j. scitotenv.2012.03.055. Colmenarejo, M.F., Rubio, A., Sánchez, E., Vicente, J., García, M.G., Borja, R., 2006. Evaluation of municipal wastewater treatment plants with different technologies at Las Rozas, Madrid (Spain). J. Environ. Manag. 81, 399–404. https://doi.org/10.1016/j. jenvman.2005.11.007. Corbella, C., Puigagut, J., Garfí, M., 2017. Life cycle assessment of constructed wetland systems for wastewater treatment coupled with microbial fuel cells. Sci. Total Environ. 584–585, 355–362. https://doi.org/10.1016/j.scitotenv.2016.12.186. EPA, 2012. Guidelines for Water Reuse.

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