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Converting various wastes into biodiesel, electricity and heat at wastewater treatment plants to minimize waste disposal and GHG emission



Wastewater streams contain various amounts of trap greases, nitrogenous (N) and phosphorus (P) compounds, and other impurities. The generation of trap greases in the US is approximately 13.37 lbs/person/year [1] and most of them merge in wastewater treatment plants (WWTPs) through grease hauler trucks and municipal sewer system [2]. The trap greases after primary clarification process are sent to landfill where they decompose and release significant amount of GHG every year [3]. N and P compounds have the potential of causing eutrophication in lakes and rivers. The activated sludge technology is effective in removing the N and P compounds; however, the disposal of resulting sludge in landfill incurs significant GHG emissions [4].   

Our team proposes the idea of implementing an integrated bioenergy production system at WWTPs to utilize the trap greases, N and P compounds in wastewater stream for producing various forms of energies. Trap greases are used for both biodiesel production and biogas generation (through an anaerobic digester (AD)). Wastewater from secondary treatment is fed to the algae pond where algae consume N and P compounds as the nutrients [5]. Also, the CO2 from both the AD and the combined heat and power (CHP) generation unit are used as the carbon source for algae photosynthesis [6]. The harvested algal oil is for biodiesel production and algal biomass is used for other valued-added purposes. The energy products from the integrated system are biodiesel, electricity and heat. For a WWTP receiving 11 million gallons of trap grease per year (MGPY) [2], the annual production of biodiesel, electricity and heat are approximately 310,000 gallons, 108,380 kWh, and 482 MMBtu, respectively. Accordingly, the reduction of GHG emission is approximately 3,581 tons CO2-eq. As compared with standalone anaerobic digestion or incineration options, the proposed solution has advantages in environmental, economic social aspects, which are detailed in the later section.

Category of the action

Reducing emissions from waste management

What actions do you propose?

The implementation of the integrated bioenergy production system at WWTPs will promote clean energy production, improve waste management and mitigate GHG emission. 

The physical actions to be adopted include the construction and operation of the integrated bioenergy production system at WWTPs. As shown in Figure 1, the system consists of a trap grease treatment facility, a biodiesel production facility, an aerobic digester (AD), an algae growth facility, an algae treatment facility and a combined heat and power generator (CHP). The working principle of the system is detailed below:

1. After reaching the WWTP, the wastewater stream goes through the primary clarification process where the trap grease is separated from the bulk part of the wastewater. The trap grease is sent to the treatment facility.

2. In the trap grease treatment facility, the mixture undergoes a three-phase separation process where the solids are separated from the fats, oils, and greases (FOGs) and the remaining wastewater is discharged to the bulk wastewater stream. The FOGs are sent to biodiesel production facility and the solids are fed to the AD. The wastewater is sent to the secondary treatment facility for removal of BOD and COD.

3. In the biodiesel production facility, the FOGs are refined into biodiesel, a renewable fuel that is direct replacement of petroleum diesel.

4. In the AD, the solids are decomposed and biogas is formed which typically consists of 65% methane and 35% of carbon dioxide (v/v). The resulting biogas passes through the algae growth facility where CO2 is consumed by algae photosynthesis. Meanwhile the wastewater coming out of the secondary treatment facility is fed into the algae growth facility as the growing media. The N and P compounds in the wastewater are consumed by algae as the nutrients. The wastewater coming out of the algae growth facility is sent to the secondary treatment of the WWTP.

5. The “scrubbed” biogas (rich in CH4) coming out of the algae growth facility is then sent to the CHP for energy generation. The digestate from the AD is a good material for composting. The CO2 formed from the combustion of CH4 is fed back to algae growth facility after cooling.

6. The harvested algae are treated in the algae treatment facility so that algal oil is sent to biodiesel production and the algal biomass is processed into value-added products, such as animal feed. 

Figure 1. Illustration of the integrated bioenergy production system

Figure 1. Illustration of the integrated bioenergy production system

As an example, for a WWTP that receives 11 million gallons of trap grease every year, the estimated production of biodiesel, electricity and heat is summarized below:

· Biodiesel production

The concentration of FOG in the trap grease is typically 3% by volume [3] and hence the amount of FOG is 0.33 MGPY. By assuming an 89% biodiesel yield [7], the biodiesel production from trap grease is 0.3 MGPY. It is reported that 1.8 ton CO2 is needed to produce 1 ton of algae [8], so the algae production is estimated based on the amount of CO2 generated from biogas and burning of CH4. As a result, the amount of the harvested algae is 53.69 ton/yr. By assuming a 40% algal oil concentration [9], the total algal oil production is 3,976.28 ton/yr and the corresponding biodiesel production is 0.01 MGPY. Therefore the total annual biodiesel production is 310,000 gallons.

· Electricity and heat generation

The solid concentration in the trap grease is typically 2% [3] by volume and therefore the amount of solids is 0.22 MGPY. By assuming the density of solids being 8.4 lbs/gal and a 40% volatile solid (VS) concentration [10], the amount of VS is 335.6 million grams. The CH4 generation rate is 0.1 L/g VS [11], so the corresponding CH4 generation is 22.15 tons. As the efficiencies of electricity and heating generation from CHP are 33% and 43% respectively [11], the production of electricity and heat from biogas are 108,380 kWh, and 482 MMBtu. The energy generation from biogas is able to displace the use of 32,304 m3 natural gas.

The following social actions will be taken to gain public support for the implementation of these physical actions.

· Campaign will be conduced to educate the public about the social and environmental benefits of this project.

· The environmental agencies will release the monitoring reports to ensure the public the environmental safety of this project.

· Certain forms of policy support (e.g. sales tax exempt) will be provided for the private investors, such as biodiesel companies, to encourage their involvement.

Who will take these actions?

The implementation of the proposal will be undertaken by a collaborative force. Depending on the negotiation, the construction and operation of the integrated bioenergy production system can be categorized into three scenarios:

A. the WWTP invests in and operates the entire system. In this case, the WWTP will be the sole owner of the biodiesel and the Renewable Identification Numbers (RINs) that are generated with biodiesel production.

B. the WWTP invests in and operates the AD and the trap grease treatment facility. The biodiesel company invests in and operates the algae production facilities. The capital and operation costs of the biodiesel production facility are shared by these two parties. The two parties share the ownership of biodiesel and RINs. The company will lease space from the WWTP.

C. the biodiesel company invests in and operates the entire system. The company will be the sole owner of the biodiesel and RINs. The biodiesel company will lease space from the WWTP.

The roles of other key actors are:

· Our team will be involved in the initial phase of the implementation to: 1) conduct the feasibility study, and 2) collaborate with other key actors to prepare the strategic plan for project implementation.

· The city will collaborate with the WWTP and other key actors to facilitate the implementation of the proposal (e.g. funding and policy supports)  

· Environmental agencies will issue the permits and oversee the environmental impacts from the implementation of this proposal.

· National Biodiesel Board will be involved as the partner to provide consultative services

In addition, the WWTP and the biodiesel company will work together to apply for funding support from various federal and state-level funding sources, such as the Office of Energy Efficiency and Renewable Energy (EERE) and Ohio Third Frontier Fund. 

Where will these actions be taken?

Our team will implement the first pilot system at Metropolitan Sewer District of Greater Cincinnati (MSD) in Cincinnati, OH. This implementation is favored by several advantages: 1) the team has collaborated with MSD to upgrade their FOG into biodiesel feedstock collaboration includes the potential opportunity of implementing a pilot production system on site; 2) MSD has a large wastewater processing capacity (90 MGPD), enabling the team to demonstrate the scalability of the system; and 3) local algae biodiesel companies (e.g. Algaeventure Inc.) are available. The pilot system is designed to process 1 million gallons of trap grease per year. The timeline of the implementation is shown in Fig. 2 and the detailed description of each major stage is listed below:

Figure 2. Timeline for implementation of a pilot system at MSD, Cincinnati, OH

Low-income countries

The proposed system is scalable based on the capacity of the WWTP. It is adaptable to different waste generation and financial situations in the low-income countries by offering the option to implement one or more of the three waste-to-energy processes.

The implementation of the integrated bioenergy production system in low-income countries may face one or several of the following challenges:

·         Waste-to-energy technologies are not available

·         The market for the products is not mature, e.g. no use of biodiesel in the fleets

·         Funding supports are limited

·         Policy support is limited, e.g. no incentives or mandate for biodiesel production and use

·         Public acceptance may be low

To address these challenges, the following solutions should be considered by the government in the low-income countries:

What are other key benefits?

The key benefits include reducing GHG emissions and other advantages in environmental, economical and social aspects.

1. Utilize both solid (trap grease) and liquid wastes (wastewater) to minimize the waste disposal.

2. The trap grease treatment step separates FOGs from solids, reducing the risk of inhibitive effect on AD [14,15]. This increases the efficiency and reduces the potential maintenance cost for the AD.

3. Using algae for N and P removal lowers the usage of the activated sludge, reducing the operation cost and avoiding the associated GHG emission from sludge disposal.

4. Minimizing the CO2 emission from AD and CHP by using it for algae growth

5. Diversified end product profile.

6. Revenue for the WWTPs from the sales of biodiesel ($3.68/gal), RINs ($0.5/gal biodiesel) and glycerin ($ [3].

7. Reduce pressure on landfill. The amount of trap grease sent to landfill can be reduced by 80%, due to the production of biodiesel and biogas.

How much will emissions be reduced or sequestered vs. business as usual levels?

The same WWTP (with 11 MGPY trap grease incoming flow) is used as an example when calculating and comparing the GHG emissions of the “business as usual” scenario and the “proposed” scenario. The GHG emission from the “business as usual” scenario is 430 tons CO2 eq./yr, mainly from the transportation of dewatered trap grease to landfill and the decomposition of these materials. The disposal of waste sludge is not included in the calculation due to the lack of data.

On the other hand, the total net reduction of GHG emission by the operation of the proposed system is 3,581 tons CO2 eq./yr from the “business as usual” scenario. The details of the reduction are shown in the table below:

Figure 2. Calculation of GHG reduction by implementing the proposed system· Displacing diesel with biodiesel: 3,129 tons CO2 eq./yr

· Displacing NG: 176 tons CO2 eq./yr

· Reduced landfill disposal: 276 tons CO2 eq./yr.

By assuming the same GHG reduction rate at 21,594 wastewater treatment plants in the US [16], the total GHG reduction is over 77 million tons of CO2 eq. every year.

What are the proposal’s costs?

As an example, the cost analysis is performed based on the implementation of the system at the same WWTP (with 11 MGPY trap grease incoming flow). It is assumed that the CHP has already been installed and its operation is not influenced by the implementation of the proposed system at the WWTP. Total capital cost is $2.37 million and the total annual operational and maintenance (O&M) cost is $1.30 million/yr. On the other hand, the annual revenue and cost saving is $1.91 million/yr. The major cost and revenue sources are listed below. The capital costs of the facilities are estimated based on the existing case studies by applying the economy-of-scale rule.

· Capital cost for the FOG treatment facility is estimated based on the cost of a similar facility at Oceanside WWTP in San Francisco, CA [3]: $0.50 million

· Capital costs for the algae pond and the algae treatment facility is estimated based on the cast study conducted by NREL [17]: $0.65 million

· Capital cost for the biodiesel production facility is estimated from Haas et al. (2006) [18]: $ 0.98 million

· Capital cost for the AD is estimated by using US EPA CoEAT model [19]: $0.24 million

· O&M cost for trap grease treatment and biodiesel production [3,18]: $1.25 million

· O&M cost for AD is estimated from Moriarty (2013) [20]: $0.04 million

· O&M cost for algae pond and treatment facility [17]: $0.01 million

· Revenue from tipping fee, sales of biodiesel, glycerin and RIN [2,3]: $1.82 million

· Cost saving from displacing NG [21] and reduced landfill loads: $0.09 million 

Time line

Depending on the size of the system, the acquisition of the permits and construction of the facilities may take up to 7 years [3]. 

The general time line for the implementation of the proposed system at individual WWTPs is shown below:

· Year 1 to 2: conduct feasibility study and form the project consortium

· Year 2 to 4: fund raising and obtain the permits for the installation and operation of the system

· Year 4 to 7: construct the facilities. The time for construction may vary, depending on the size of the system.

· Year 7 to 27: 1) test operation and full-scale production; 2) continue operation until the life time of the facility is reached (typically 20 years after the beginning of the full-scale operation)

· Year 27 to 30: retire and salvage the old facilities

So the WWTP can start the installation and operation of a new system every 30 years.

The timeline for the implementation of the proposed system in the entire US and other countries in the world:

Short term (5-15 years): implement the system at the WWTPs where one or more of the following facilities already exist: trap grease treatment facility, algae growth facility, algae treatment facility, biodiesel production facility and AD.

Medium term (15-50 years): implement the system at all the WWTPs in the US

Long term (50-100 years): implement the system in major cities in other countries.

Related proposals


[1] Wiltsee G. 1998. Urban waste grease resource assessment.

[2] Tu Q, Wang J, Lu M, Chai M, Lu T. 2012. Feasibility and practices of making biodiesel out of low quality greases. EM. January Issue, 26-29.

[3] Biofuels, B., Engineers, C., San Francisco Public Utilities Commission. 2013. Brown Grease to Biodiesel Demonstration Project Report (No. DOE000621).

[4] US EPA. 2010. Greenhouse gas emissions estimation methodologies for biogenic emissions from selected source categories: Solid waste disposal, Wastewater treatment, Ethanol Fermentation. EP-D-06-118.

[5] Pate R, Klise G, Wu B. 2011. Resource demand implications for US algae biofuels production scale-up. Appl. Energ. 88(10), 3377-3388.

[6] Stephens E, Ross I L, Hankamer B. 2013. Expanding the microalgal industry–continuing controversy or compelling case? Curr. Opin Chem Biol. 17(3), 444-452.

[7] United Soybean Board (USB). Life cycle impact of soybean production and soy industrial products;

[8] Algae cultivation inputs.

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[12] “FOGFUELS™ Announces Partnership with the City of Atlanta”,

[13] “BlackGold Biofuels opens Charlotte trap grease recycling facility”,

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[15] Pereira MA, Pires OC, Mota M, Alves MM. 2005. Anaerobic Biodegradation of oleic and palmitic acids: evidence of mass transfer limitations caused by long chain fatty acid accumulation onto the anaerobic sludge. Biotechnol. Bioeng. 92(1), 15-23.

[16] US EPA. 2008. Clean Watersheds Needs Survey 2008

[17] Davis R, Aden A, Pienkos PT. 2011. Techno-economic analysis of autotrophic microalgae for fuel production. Appl. Energ. 88(10), 3524-3531.

[18] Haas MJ, McAloon AJ, Yee WC, Foglia TA. 2006. A process model to estimate biodiesel production costs. Bioresource Technol. 97(4), 671-678.

[19] US EPA. Co-digestion Economic Analysis Tool (CoEAT).

[20] Moriarty K. 2013. Feasibility Study of Anaerobic Digestion of Food Waste in St Bernard, Louisiana.

[21] Ohio natural gas prices.