An Assessment of Wastewater Inventory and its Energy Potential: Bangladesh 1 Perspective 2

5 Background: Everyday considerable amount of wastewater is produced by each one of us. 6 Developed country produce more wastewater than developing country. But disposing this 7 wastewater into the environment is an expensive task and need heavy initial investment for 8 that reason developing country only focus on water supply not on wastewater treatment 9 considering it’s a burden and threatening the human health environment and climate 10 change. Research shows that wastewater contain considerable amount kinetic and 11 biochemical energy. But tapping energy from wastewater, proper inventory of national 12 wastewater including type and characteristics of both industrial and municipal wastewater is 13 essential which is presently absent in Bangladesh. In this paper, efforts have been taken 14 firstly to estimate yearly total domestic as well as industrial wastewater production in 15 Bangladesh based on reliable secondary data and monthly per capita income. Secondly, 16 common, and emerging energy recovery technologies ideal for tapping energy from 17 wastewater have been reviewed systematically and identified which are anaerobic microbial electrolysis and microbial fuel Finally, energy potential has been estimated basing on previous 20 research outputs and with empirical formula. At the end barrier and overcoming strategy with 21 important recommendation has been proposed for researchers and decision makers. 22 Results: Estimated yearly domestic and industrial wastewater is 4.874 billion and 0.452 23 billion tons respectively. For energy estimation,10%-50% total wastewater has been 24 considered. Calculation shows that, 10% wastewater can produce 2.41, 1829.09 and 1.97 25 GW energy yearly through AD, MHP, MEC and MFC technologies, respectively whereas 26 50% total wastewater can generate 9145.94 GW energy yearly by MHP only. 1 Conclusion: Estimated quantity of produced wastewater and energy potentials from 2 wastewater is based on secondary data. For more reliable estimation feasibility study may 3 be conducted by the researchers supported by stakeholders. Both wastewater producers 4 and treatment plant owners should have noble desire backed by governmental 5 organizations will facilitate the process. Outcomes are very significant and optimistic and it 6 is expected that this findings not only inspired researcher, wastewater operators and policy 7 makers of Bangladesh but also other developing countries around the globe. 8


Introduction 13
Worldwide water demand is increasing but water availability is reducing whereas 14 greenhouse gas emission is also escalating with the increase of population growth and  Per capita per monthly average income varies from various cluster of people which is around 24 50 USD (HIES, 2016). As per Asian development report middle income group of people is 25 around 3.7 million and their per capita monthly income also 300 USD (Bank, 2010). All the citizen does not have access to electricity only 75.92 % population has the access to 1 electricity only and water hygiene and sanitation (WASH) though the installed capacity is 2 19570 MW and daily electricity production is an around 10264 MW. Out of this produced 3 electricity renewable source is only 0.19% solar, 1.18% is hydropower and rest from fossil 4 fuel based primary fuel( figure 1) which contribute to greenhouse gas emission (Board, 5 2018). In US nationwide, water and wastewater treatment plants needed around 3 to 4% of 6 total electricity utilization. This energy consumption is in similar range for other developed 7 countries (Gude, 2015).Oher estimate accounts that energy costs varies from 5% to 30% of 8 the total operating costs of water and wastewater utilities worldwide (Chae and Kang, 2013). 9 Most of the developed countries maintain the statistics of total water and wastewater 10 production, treatment and standard for safe discharge as well monitor the total water cycle 11 for enforcing the regulation. In case of developing country picture is totally different as do not 12 maintain any comprehensive record of water management cycle particularly wastewater 13 production. Some of the country do keep record of water supply but partially or even no 14 record of wastewater production and treatment. As per UN record Bangladesh only treat 2% 15 of wastewater and rest of the wastewater directly goes to the environment without proper 16 treatment(UNESCO, 2017). In Bangladesh, as wastewater treatment considered as energy 17 intensive process and initial investment is more believing that wastewater treatment does not 18 provide any visible output that's why both wastewater producers are not interested to invest 19 money on this sector though developed country and few developing countries are serious 20 about the environment and greenhouse emission issue and committed to treat their 21 wastewater before discharging to waterbody. They view wastewater as a carrier of energy 22 not a burden and optimise the energy balance of WWTPs to the point of energy self-23 sufficiency or even further to be "energy-positive" (Kollmann et al., 2017).To turn this sector 24 as an energy producer, worldwide extensive research is going on and recent results found 25 that wastewater contain around 5-10 times more energy than its energy requirement to treat 26 it (Heidrich et al., 2011), (Dai et al., 2019). Wastewater contain organic and inorganic 27 substance in terms of Biochemical oxygen demand (BOD) and chemical oxygen demand 28 k is a unit conversion factor and value is1.318 for(feet and seconds) and 0.85 for SI units 17 (meters and seconds). C is a Hazen-Williams Coefficient and value varies (within 100-150) 18 Where 20  Mostly, two hydropower generation system may be installed at WWTP which has been 5 shown at the following fig 4(a) and 4(b). In first case, hydro plant may be installed at the 6 upstream where turbine should be more corrosion resistant, and the diversion pipe entrance 7 must be equipped with a trash rack to control debris. For second case hydro system should 8 be installed at the downstream, turbine face cleaner water and corrosion is not much 9 influence on turbine. It can be noted that both possibilities can be technically implemented. 10 In Jordan,Samara project is a best example where upstream and downstream hydropower 11 scheme has been implemented and generating 12.5 Gwh/y and 8.6 gwh/y respectively 12 (Esha, 2005). Delhi Jal board( DJB) of India commissioned a hydropower plant at the 13 downstream of WWTP located at east Delhi, whose capacity is 9 million gallon per 14 day(MGD). Effluent falls from 4.5 m above the level of water receiving stream and expected 15 to generating around 2000 kWh electricity annually (Hydroreview, 2015   Hsinchu, taiwan DTE n/a n/a n/a 11 Taichung, Taiwan DTE n/a n/a n/a 68  Study shows that sludge production rate from WWTP is estimated as 0.04 kg dry matter per where: V bg is the estimated biogas production rate associated with wastewater(m 3 Table 3 shows the country wide biogas 12 production. 13

MFC 3
Microbial fuel cell (MFC) is an emerging and promising technology to produce electricity 4 catalysing the microbes present in the waste stream simultaneously this technology also 5 carryout wastewater treatment. MFC is like a battery cell having an anode which need to 6 anaerobic, but cathode must be in contact with air and separate by an ion passable 7 membrane (optional).Anode and cathode should be connected to some load to complete 8 the circuit and wastewater as electrolyte allows to flow through the cell. Microbes available 9 in wastewater will form biofilm over the anode surface, breakdown the organic matters and 10 produce electron and proton ion are produced at anode and cathode as per the following 11 formula respectively 12 [ Eq (12 )and (13 )] where organic matter in wastewater is represented as Glucose 13 The theoretical cell voltage of the overall reaction (the difference between the anode and 15 cathode potential) determines if the system is capable of electricity generation [Eq (11)].

MEC 2
An MEC is a form of MFC where both anode and cathode are anaerobic and have a 3 membrane in between is optional. Unlike MFC instead of produce electricity, in the cathode 4 chamber, e-and H+ ions are combined to generate H 2 gas. However, H 2 formation at 5 cathode chamber is not spontaneous i.e. it requires an external bias. A small amount of 6 electricity (with acetate this is in theory 0.114 V, in practice <0.25 V shown in Eq (14)), is 7 required to generate the H 2 gas (Call and Logan, 2008). This is substantially less energy 8 than is required to produce H 2 through water electrolysis, typically 1. Anode and cathode reactions are shown at the [Eq (15), (16) and (17)]. 16 As wastewater contain organic and inorganic substance strength is indicated by COD so H 2 9 yield production rate based on COD can be evaluated by the following equations [ Eq (18)  10 and (19)]. is calculated as 11 where, Y H is H 2 yield in mg-H 2 /mg-COD; nH 2 is the moles of recovered hydrogen recovered 14 calculated from the ideal gas law based on the volume of hydrogen recovered, M H2 is the 15 molecular weight of hydrogen, and V L is the volume of liquid in the anode chamber, reactor 16 influent COD is CODi and effluent COD is CODf for continuous process as well as batch 17 process; V H2 is the volume of H 2 measured at 1 atm pressure; P (atm) is the pressure and R

3.
Water and Wastewater cycle management in Bangladesh 2

General overview of water supply system in Bangladesh 3
Three types of water utilities ensure the smooth supply of water to the consumers of urban, 4 peri urban population as well as industry and service organizations, these are Water Supply

Wastewater discharge regulation 19
According to DoE, Environment Conservation Rules, 1997 of Bangladesh, there are 20 standard value for different effluents parameters based on industry which need to follow 21 before discharging into the environment. Following table summarizing the parameters with 22 limit for compliance(The Environment Conservation Rules, 1997 ). 23 Table 6. DoE standard of municipal and industrial effluent of Bangladesh 24 Type and source of ETPs, in the various industries either centrally or individually are in place. But there is no 5 central statistics or data base of wastewater production generation and discharge. As there 6 is no database, so for assessing total domestic wastewater production, income base 7 economical estimation approach has been adopted where total population, citizens monthly 8 earning both middle class group of people's monthly income (Bank, 2010) and average per 9 capita per month income has been considered according to equation 20, supply water also 10 be calculated by equation 21 (Campos and Von Sperling, 1996). Produced wastewater, Where, X is Number of minimum salaries per month (0.5 and 3.0 based on average and 1 middle class groups salary respectively considering 100 USD a unit ) 2 Total estimated wastewater inventory is presented on the following table 7. On the other 3 hand for calculating industrial wastewater, major industries operating at the major cities, 4 BEPZAs industries situated at different EPZs, textile and leather industries as a whole, 5 pharmaceutical industries, vegetable oil industries, dairy mills , pulp and paper industries wastewater production that's why quantification of Industrial wastewater has been carried 10 out by consulting various literatures, industrial association reports, annual report of major 11 industries which tabulated at table 8. 12

4.1
Selecting suitable technology for harnessing energy from wastewater 5 In the previous paragraph 2, both common and emerging technologies which are already 6 implemented and potential to implement have been discussed in depth. Integrating energy 7 generation technology with wastewater treatment process is not straight forward-need 8 details study for technical as well as financial point of view. Integration is most suitable for 9 newly planned WWTPs if consider during planning stage. As already mentioned before that 10 due to high cost of treatment and initial heavy investment is required that's why developing 11 countries ignore this issue and indirectly damaging the environment, health and total 12 ecosystem. Adequate data, analysis of previous data on wastewater collection, treatment 13 and discharge, cost analysis of action and no action in terms of wastewater treatment will 14 instigate to plan, think and consider for harnessing energy from this unexplored sector. Well 15 composed plan, careful prefeasibility and feasibility study, engineering, and development 16 strategy will lead to integrate energy harvesting technology to the WWTP. Figure 8 illustrate 17 the probable step to be followed for deciding energy tapping steps from any WWTPs in 18 general. In this figure both common and emerging technologies have been considered for 19 energy generation from wastewater. Initially prefeasibility study needs to be carried out to 20 find out which technology match better based on the detail discussion on the section 2. If no 21 technology is suitable, then decision may be taken to drop the plan for integrating energy 22 generation option from that WWTP. If single or two technologies suits for integration then 23 carry out feasibility study, based on feasibility study if it is found that implementing single 24 technology AD or MHP then engineering and development step may be considered before 25 installation and commissioning.

Calculation of energy potential from wastewater 3
All the estimated wastewater will not be possible to utilize for energy generation, may be 4 certain percentage can consider for harnessing energy. Accordingly, 10 scenarios both 5 single and integration of two technologies have been considered for energy estimation. 6 Table 9 describe the 10 scenarios starting from 10% domestic wastewater (DWW) and 10% 7 Industrial wastewater (IWW) corresponding to implementable technologies for harvesting 8 energy. 9 Table 9 Possible scenarios for energy potential calculation for hydropower generating systems can vary from 50 to 70%. Therefore, to determine a 4 realistic power output, the theoretical power must be multiplied by an efficiency factor of 0.5 5 to 0.7 (Saket, 2008). In scenarios 2, 3,4 and 5, for calculating biogas generation by 6 anaerobic digestion (AD) from sewage sludge of wastewater has been consider.Biogas is 7 produced from two type of wastewater sludge, primary sludge and activated sludge and 8 production rates are 380 ml/g VS and 612 ml/g VS respectively (Hanjie, 2010).Two key 9 factors are important when assessing the biogas potential from sewage sludge (1) the 10 amount of sewage sludge available to be digested, and (2)

Results and Discussion 4
Estimated energy potential is based on 10-50% of generated domestic and industrial total 5 wastewater. Wastewater inventory is based on limited archived secondary data, unavailable 6 information and statistics have been reasonably assumed. About the proposed technologies, 7 MHP at WWTP, effluent is a well-recognised means of recovering electricity by taking 8 advantage of constant discharge from WWTPs and some head though depend on the site. 9 Archimedes screw, water wheels and other turbines deliver reliable performance when 10 applied to downstream scheme. However, if hydro scheme is applied to untreated 11 wastewater, then percussion must be taken against corrosion. During pre-feasibility study, in 12 addition to flow rate, gross head can be estimated through geographical information system(GIS) and power potential can be estimated (Almaliki, 2019). Large-scale applications 1 in Australia, UK and Ireland, Jordan, Austria have proven the economic viability of 2 hydropower technologies in WWTPs. For assessing hydropower potential in this paper, as 3 there is no data on gross head and not consider any head other that 1 m, so assuming a 4 range of head from 0.25 to 10 m, and considering only 10% of total wastewater, a big 5 difference can be noticed which shown at fig 11. Sometime head is hard to find out for 6 WWTP at plane land but following the fig 3, slope can be increased to get an enhanced flow 7 velocity according the Hazen Williams and manning equations ( Eq 3 and 6) by storing 8 effluent in a separate reservoir and that effluent can be channelled for urban or peri urban 9 agriculture. According to equation (2) exponential power potential can be calculated if flow 10 velocity can be increased by double then power will be increased by quadruple times. As 11 mentioned before that either upstream or downstream scheme of hydropower plant may be 12 planned though corrosion is a significant barrier for upstream scheme. Regarding AD Technology, using WWTP sludge as feedstock to generate biogas is kind of 18 similar technology to produce biogas from municipal solid waste, cattle manure which is 19 practicing long before in Bangladesh and other part of the developing country. The 20 production of biogas by AD from sludge is currently the most widely used energy recovery 21 method worldwide. About 80% of the biodegradable COD fraction in the sludge can be 22 converted into biogas. Traditional AD technology is common but harnessing additional 23 biogas from wastewater need advance technology and developed country are adapted and 24 major part of energy requirement of treating process is meeting up by energy generation 25 from biogas. Fig 10 and 12, shows that, in terms of equivalent energy potential AD's 26 contribution is not significant but can be considering after feasibility study. Regarding MEC 27 and MFC, both technologies still not commercialized but carry much potential as 28 simultaneous wastewater treatment is possible with these processes though bioremediation 1 is not consistent. A typical WWTP with aerobic activated sludge and anaerobic sludge 2 digestion process consumes approximately 0.6 kWh of energy per m 3  Potential barrier, overcoming strategy and recommendation to tap energy from 4 wastewater sector 5

5.1
Barrier and overcoming strategy 6 There are several hurdles to explore the wastewater sector is a potential source of 7 renewable energy, it is a kind of fixed source as every day wastewater will produce, need 8 careful planning focus, intention, consensus, technical know-how and expertise. Among the 9 potential barriers regulatory, policy, lack of awareness, lack of knowledge and lack of 10 interest, technical, financial, are the most significant.  treatment, effluent quality and all other related matters then majority of the problem will be 1 solved. Data and information on wastewater generation, treatment and use is essential for 2 policymakers, researchers, practitioners, and public institutions in order to develop national 3 and local action plans for protecting environment and productive use of wastewater. 4 Regarding lack of awareness, interest, knowledge can be nurture, enriched and grow by 5 changing view perspective. If most of the citizen know the consequences of releasing 6 untreated or inadequately treated wastewater into the environment, then will be careful about 7 discharging untreated wastewater. Knowing the harmful effects on human health, negative 8 environmental impacts, and adverse implications on economic activities will lead all concern 9 to enhance their awareness, interest and knowledge. Bangladesh is a developing densely 10 populated country, human development index (HDI)is one of the lowest compare the other 11 nation which greatly depend on per capita energy consumption. If energy production can be 12 harnessed from renewable source like wastewater then not only energy generation will be 13 increased but also overall countries position will be elevated. In this connection motivation, 14 publicity by various institution is essential to encourage strong will and keen interested to 15 explore this untapped potential energy source. In regards of technical and financial analysis, All these bottlenecks somehow related with previously mentioned barriers and possible to 9 address by following above mentioned strategy. However, the successful implementation of 10 RRR in wastewater sector heavily depend on policy and legislative frameworks, market 11 values and the competitive situation, as well as user acceptance of a recovered energy and 12 willingness to pay for that. 13

Recommendation 14
In line with sustainable development, for exploring the untapped potential of wastewater in 15 the context of developing country particularly for Bangladesh in addition to following 16 overcoming strategy discussed above following suggestions are recommended: 17  A proper and consolidated inventory of wastewater is essential-that can be developed by 18 holistic approach by all stakeholders like water producers, consumers and policy and 19 regulatory body.  In terms of energy generation system integrating into the STP/CETP/WWTP local 26 resource may be explored or developed collaborating with the local university or national public/ private research institution for capacity building and attaining self-dependency on 1 technical expertise. 2  As hydropower potentiality is outperform other common and emerging technology, so it is 3 strongly recommended to plan and execute a pilot project anywhere in the country where 4 wastewater flow rate and gross head are optimum to get a hands on experience to scale 5 up further at the other WWTPs as well. Conclusion 8 According to government plan 10% renewable energy is expected but till time only 0.19% 9 solar, 1.18% is hydropower has been integration with total production system. Only 75.92 % 10 citizen has access to electricity. To increase the electricity generation and increase the share 11 of renewable energy sources is the top priority of Bangladesh. In this situation, energy 12 investment for wastewater treatment is not the priority for the WWTPs stakeholder. But if 13 wastewater can produce a reasonable percent of energy utilizing its potential then all 14 concern will be interested to treat their wastewater and discharge the treated water to the 15 environment. As wastewater treatment is an expensive and energy intensive process that's 16 why most of the wastewater disposed to the nearby waterbody causing environment 17 pollution, greenhouse gas emission. Developing country like Bangladesh need to focus also 18 for treating her wastewater for better good and harnessing energy from this untapped source 19 of renewable energy like developed world. There are few emerging technologies have been 20 discussed in detail to understand how these technologies would be integrated with 21 wastewater treatment process. A methodology has been also suggested to evaluate 22 implementable technology at the WWTPs. Based on the previous empirical formula and 23 experimental data of other literatures, an approximate estimation has been completed 24 considering 10 scenarios. Out of these, scenario 1, using only 10% of total generated 25 wastewater produce 2.41, 1829,18.9 and 1.97 Gwe/yr energy through AD, MHP, MEC and outperformed other process. This energetic study focussing on ignored and untapped 1 resource-wastewater which is otherwise viewed as burden to society can be source of 2 potential source of renewable energy. Findings of the study will aid to decide by the 3 stakeholders of industries, energy producers, wastewater utilities, policy makers and 4 regulatory bodies to tap energy from this dirty and ignored sector. Study outcome also will be 5 an inspiration for the researchers and scientist to explore the potential renewable energy 6 source. Potential barriers, overcoming strategy also highlighted. Finally, few important 7 recommendations have been suggested to implement and integrate with the mainstream of 8 wastewater treatment, resource recovery and power generation. 9

Declaration 10
Availability of data and materials 11 All data generated or analysed during this study are included in this published article.