Analytical Study to Use the Excess Digester Gas of Wastewater Treatment Plants

Power consumption is increasing day by day around the world. It is necessary to reduce energy consumption and increase energy efficiency both in industries and in residential buildings. It is a challenge for the world to expand the usage of renewable energy. Climate change is also a concerning issue that encourages everyone to embrace renewable energy sources. Only 17% of U.S. electricity is generated from renewable energy sources [1].

The U.S. Department of Energy (DOE) promotes converting waste to energy, especially sludges from the municipal wastewater treatment plants (WWTPs). At many facilities, wastewater treatment plants use digester to stabilize the sludge that is removed from the wastewater during treatment. The purpose is to reduce the mass of the sludge and convert it into a non-hazardous form so that it may be handled or used with minimal health hazards. This stabilization can be performed by using anaerobic digestion. In this process, a significant fraction of the organic matter is broken down into hydrocarbons, such as carbon dioxide (CO₂), methane (CH₄), and traces of other contaminant gases, which is accomplished in the absence of oxygen. About half of the amount is then converted into biogases, while the remainder is dried and becomes a residual soil-like material. The biofuel can be used to meet some of the energy requirements of the wastewater treatment facility through combined heat and power (CHP) gas engines (cogeneration). Using micro gas turbine (MGT), a CHP technology can be introduced in WWTPs. The waste heat of MGTs can be further used to drive absorption chillers. The combination of MGTs and absorption chillers is a promising technology as it produces electricity, heating, and cooling simultaneously. It is also useful to other thermal energy users.

According to the U.S. Department of Energy CHP Installation Database, the microturbine market share has reached 25% of capacity from 100 kW to 5 MW [2] (Fig. 1). Several researchers reported on CHP, and the use of digester gas in CHP as fuel [3–7]. Tourlidakis et al. [8] investigated CHP generation for small municipality areas for heating purposes in Northern Greece. They examined the replacement conventional heating system with CHP. CHP with the combination of heating and cooling network was studied by Sdringola et al. [9]. Their case study shows the synergy between CHP and heating and cooling potential. Saidi et al. [10] reported a comparative study of CHP for the food industry basing on the system covers space heating and cooling and power. A supervisory control system is established by Cho et al. [11]. They used a feed-forward control system for CHP operation. The transient process of micro-CHP was examined by Ippolito and Venturini [12]. Hwang [13] investigated the energy benefits of the absorption chiller integrated with MGT. Abbas et al. [14] presented a study for net-zero-energy wastewater treatment plants and achieved the goal; they utilized the anaerobic digester gas into a CHP system.

The microturbine technology works based on the Brayton cycle (Fig. 2). The compressed ambient air enters the combustion chamber and then routed through a recuperator. Recuperator is a non-mixing exhaust to an air heat exchanger. Microturbine utilizes a portion of exhaust heat with the assist of the recuperator to preheat the combustion air. As a result, less fuel can be required to achieve the exhaust temperature. It also leads to increased efficiency in fuel to electric energy. The compressor, turbine, and generator are mounted on a single shaft, so they produce electrical power while drawing air to maintain the overall process. The faster the shaft spins in the magnetic field, and the more electrical output is obtained by the generator [15].

An absorption chiller can be installed with microturbine as it is a heat-operated refrigeration equipment. The cooling capacity of it is generated using waste heat. The WWTP has the benefit of using the excess digester gas (EDG) to implement CHP as the WWTP produces EDG during its process. Digester gas has the prospective to capture methane that would escape into the environment [17]. It is a form of biogas, which is a proven source of energy used in the U.S. and all over the world for decades. Nutrient-rich soil and pelletized also come as a byproduct, which can be used as fertilizer. In the study by Wisconsin, many cogeneration plants burn a combination of biomass with conventional fuels to generate electricity and heat. Figure 3 shows a typical cogeneration system.

Szega and Zymelka [19] studied the thermodynamic and economic analysis of CHP with absorption chillers. The study was conducted for real back-pressure CHP unit and single effect absorption refrigerators using water and lithium bromide as the working fluids. The research shows that after 25 years, the net present value of the combined production of cooling, heat, and power unit is a little over one million USD. The internal rate of return of capital is 14.5%. They concluded that the deterioration of the economic situation would not significantly affect the profitability of the investment as the payback period (PBP) is 13 years, and the invested amount would be returned at around half of the planned operating time. Ghaebi et al. [20] applied a dynamic and economic analysis to evaluate the CHP plant with lithium bromide (LiBr)/water absorption chiller to produce cold. Low-pressure steam (3 bar and 1 bar) powers the absorption chiller. The primary energy-savings in this system are 21,000 MWh/year. Zhang et al. [21] also investigated a trigeneration system with LiBr/water absorption chiller. The combined cooling, heating, and power system can recover the heat of the refrigeration cooling water for reheating the boiler water, which increases the energy conversion efficiency. The results show that the trigeneration system can bring a 26.6% decrease in primary energy rate consumption compared with the CHP plant and separate generation of cold.

Yun et al. [22] studied an analytic approach for defining optimal operation decisions for a power generation unit in CHP for facilities in Chicago, IL and Philadelphia, PA. Bruno et al. [23] studied a case study on a sewage treatment plant with a trigeneration system using biogas. They concluded that the best trigeneration systems are entirely replacing the existing system with a trigeneration plant that uses all the available biogas and additional natural gas to meet the heating demands of the sewage treatment plant completely. Co-trigenerative arrangements were also attempted by several researchers. [24,25].

The authors’ team performed energy assessments at several facilities in Wisconsin. The study contains the breakdown of the annual total energy cost in terms of electricity, gas, and water, facility site survey, and list of energy efficiency measures and a group of interactive measures of the facility. Before implementing the CHP system in a WWTP, multiple issues should be considered. Such issues include as to if they are with adequate concurrent thermal loads and availability of the natural gas, the compatibility of the existing system with the recoverable heat available from the prime mover, the availability of space for the CHP system [26]. Picardo et al. [27] analyzed the potential of district heating with the biogas generated from the WWTPs. They claimed that the development of this system saves energy, and 1.8 × 10³ tons of CO₂ can be saved. A case study is completed in Japanese WWTP by Nakatsuka et al. [28]. This study shows that, by introducing CHP in WWTP, the CHP reduces annual costs by 35% and CO₂ emission by 1%. Another case study on adding the CHP in WWTP shows that it can save up to 46.63% of the operational cost of the plant [29]. Trendewicz and Braun [30] studied a large-scale (15.5 MW) CHP system in a large WWTP. They completed an exergy analysis and concluded that the highest contributor to exergy destruction is a waste heat recovery unit (25%). The study of Basrawi et al. [31] represents the optimization of biogas fueled cogeneration systems in WWTP.

Mosayeb Nezhad et al. [32] reported on the integrated system of both solid oxide fuel cells and MGT for wastewater treatment plants. In this integrated system, a heat exchanger is installed to transfer thermal energy to the sludge. The biogas that is not fed into solid oxide fuel cell can be used in MGT. If the produced biogas is not enough to operate the system, then natural gas should be applied. According to the authors, even though natural gas may need to use, the plant efficiency is increased by 7%. Another research group of Basrawi et al. [33] studied the techno-economic performance of biogas-fed MGT in sewage treatment plants. They suggested where heat demand fluctuates, the smaller size of MGT should use for better technical and economic performance. Somehsaraei et al. [34] developed a steady-state thermodynamic model for CHP system. The model also reported that MGT system should use on a small scale. Ge et al. [35] described the trigeneration system with MGT and absorption chiller. Their module is divided into a power and heat section, a heat diversion and refrigeration section, and a supermarket module. The module shows that with higher fuel mass flowrates the efficiency is increased. A trigeneration system was studied in the subtropical area [36]. In the subtropical area, summer is scorching. So, there is almost no heating demand. But the cooling demand is significantly high. Thus, the study shows how to maintain effective use of digester gas in summer for space cooling. The researchers concluded that a double effect absorption chiller driven by bypass of 450 °C flue gas is the best choice in this case.

Introducing biogas-fed microturbines is one of the sources to develop the renewable energy sector. This study investigated a trigeneration system in three WWTPs and discussed the feasibility of it.

From 2017 to 2019, the authors’ team conducted research through the onsite visits at nine WWTPs in the state of Wisconsin. Authors complete pre-assessment analysis before the field-assessment, after the completion of the onsite assessment of energy-savings recommendation data. The annual energy consumption and annual energy cost collected from the utility bills of the nine plants are shown in Fig. 4.

In this analytical study, an absorption cooling system is integrated into MGT. The excess biogas from the digester performs as a fuel of MGT, and the waste heat from MGT was used for an absorption chiller and a boiler. The chilled water was used to condense water in the biogas pre-treatment before its combustion in the MGT. It was also used to cool the combustion air entering into MGT. Besides, the chilled water can also be used for air-conditioning purposes. This trigeneration system leads to power generation, space heating, and cooling.

Three WWTPs (WWTP-7, WWTP-8, and WWTP-9) were considered in this study by using excess digester gas for analyzing annual energy and cost-saving. WWTP-7 is a massive plant with a capacity of 32 × 10³ gallons per day (MGD), WWTP-8 is a small plant with a capacity of 3.16 MGD, and WWTP-9 is the smallest one with a capacity of 1.5 MGD. According to the plant personnel, the average digester gas of WWTP-7, WWTP-8, and WWTP-9 is 392,535 ft³/day, 38,800 ft³/day, and 28,500 ft³/day, respectively. The lower heating value for digester gas was considered 600 Btu/ft³ [37]. As listed in Table 1, the energy content of the digester for WWTP-7, WWTP-8, and WWTP-9 is 236 MMBtu/day, 23 MMBtu/day, and 17 MMBtu/day, respectively. The study assumes 50% of the summer and winter operation. The excess digester gas achieved from these plants is 190 MMBtu/day, 19 MMBtu/day, and 6 MMBtu/day, respectively. According to the U.S. Environmental Protection Agency (EPA) CHP Partnership, the electricity efficiency and power to heat ratio are considered 26% and 0.88, respectively [38].

According to the U.S. Department of Energy Office of Energy Efficiency and Renewable Energy, CHP is suitable where digester gas is available in the process of anaerobic digestions. Biogas is a product of anaerobic digestion, which is formed by the anaerobic microorganisms. The microbes feed off carbohydrates and fats. It produces methane as a byproduct and carbon dioxides as metabolic waste products. Biogas helps to generate green energy from waste and reduces the volume of waste disposed of in landfills. It also promotes greenhouse gas emission and maximizes the biogas production. Figures 5 and 6 represent the annual electricity-savings and cost-savings as a function of space heating, cooling, and CHP application. With regard to the energy-savings and cost-savings with additional heat available (AHA), WWTP-7 shows the highest value as it the largest plant considered than WWTP-8 and WWTP-9. The lowest annual cost-savings (ACS) and energy-savings for WWTP 9 are due to the most economical digester gas. In terms of CHP application for WWTP-7, the annual cost-savings are lower than the annual energy-savings (AES) due to the small unit price of electricity of this plant.

According to Figs. 5 and 6, the total energy-savings from AHA and CHP application for WWTP-7 are 10,389,633 kWh, and cost-saving of $367,508. Annual energy and cost-savings for WWTP-8 is 1,038,523 kWh/year, and $50,532, whereas the WWTP-9 can save 570,352 kWh/year, which is $25,564. For WWTP-7, WWTP-8, and WWTP-9, 6 × 120 kW MGT, 2 × 60 kW, and 1 × 60 kW need to be installed, respectively. According to the U.S. Environmental Protection Agency (EPA) CHP Partnership [38] guide, the average cost per kW is $4000. The simple PBP for WWTP-7, WWTP-8, and WWTP-9 is 7.84 years, 9.5 years, and 9.39 years, respectively (see Table 2).

Hybrid Optimization Model for Multiple Energy Resources (HOMER) Pro software [39] was used for analyzing the capacity of the turbine and biogas consumption. It is developed by the National Renewable Energy Laboratory (NREL) in the United States. Figure 7 shows the simulation results of the output capacity of the microturbine of these three WWTPs for every month. The power output for WWTP-7 is around 1200 kW in every month. In the cases of WWTP-8 and WWTP-9, it is around 120 kW and 90 kW, respectively. The electricity production (EP) depends on the biogas consumption of the plants. Thus, it is required to keep tracking of the biogas consumption. According to Fig. 8, for WWTP-7, biogas consumption is the highest in August, which relates to the EP. For WWTP-8 and WWTP-9, the highest biogas consumption is in January for both the plants, and it is relatable with the data shown in Fig. 7. Considering that turbines work for the year-round, the electrical summary is given in Tables 3 and 4.

Table 5 shows the gas emission rate for installing a CHP system in the plants. All the pollutants show the highest value for WWTP7, as it is the largest plant considered. The highest carbon dioxide and carbon monoxides are 0.000931 × 10³ metric tons/year and 0.000024 × 10³ metric tons/year 931 megagrams/year, and 24 megagrams/year, respectively. According to the report of the U.S. Energy Information Administration [40], the coal-powered plant emits 1127 × 10³ metric tons of carbon dioxide every year. The Department of Energy reports that 46 giga watt (GW) of CHP capacity is achievable. Kaarsberg et al. [41] concluded that the CHP would result in net energy-savings of 2.6 quads and carbon emissions reductions of 74 × 10³ metric tons of carbon equivalent in 2010. The district energy system and industries together can avoid the emissions of 1.4 × 10³ tons of SO₂ and 0.6 × 10³ tons of NOx emissions. According to the report of the U.S. Environmental Protection Agency, from June 2011, the CHP system was installed at 104 WWTPs, which represents 190 MW of capacity in 30 states of U.S. [38]. The database of U.S. DOE CHP Installation 2016 shows that the WWTPs can generate 2.41 GW of electricity at 11,676 sites [18].

The electricity and thermal potential for introducing CHP in WWTP are discussed in this paper. The analysis achieved was gained using a reliable source of renewable energy as well as reduces the emission of greenhouse gas. Implementation cost and PBPs were calculated, which represents a high cost of implementing such a system, and it leads to a PBP of 7–10 years. For smaller plants, the implementation cost is in the order of $200,000–$500,000, whereas it requires about $3,000,000 for a plant of 30 MGD. Even though this system is expensive to install, it is beneficial in the long run. From an environmental point of view, the system offers less pollution than the coal-fired system. The system is feasible, environmentally, and technically. This study provides economic research in terms of MGT price, electricity, and gas savings. Before planning to adopt the technology, it is advisable for the facility to request an incentive from the federal and state government agencies since there are several incentive programs available in each municipal and state.

Finally, it can be mentioned that the study gives an idea to energy engineers and plant personnel about the benefits of installation and the impact of CHP using MGT combining absorption chiller and space heating.

U.S. Department of Energy funds this project under grant DE-EE0007716.

CCHP=

combined cooling, heat, and power

CHP=

combined heat and power

EPG=

energy potential of gas (Btu/day)

HR=

heat recovery (Btu/day)

IAC=

industrial assessment center

Rg=

average gas cost ($/therm)

RE=

average electricity cost ($/kWh)

η_B=

the efficiency of the boiler (%)