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Riverside, CA Turns Restaurant Grease into Electricity
by: Chris Graves

In a variation on spinning straw into gold, the city of Riverside, California is turning restaurant grease into inexpensive electricity at its wholly owned wastewater treatment plant. The plant, a publicly owned treatment works (POTW), is designed to handle 40 million gallons per day (MGD). Currently it treats an average of 33 MGD using a fully tertiary treatment process utilizing anaerobic digestion. Starting in April 2005, the city began adding collected grease wastewater to the existing anaerobic digesters to generate methane gas. The methane gas is then fed into an on-site cogeneration facility that produces electricity for the plant. The “grease-to-gas” program has been a resounding success, creating more than enough cost savings to pay for itself in the first year of operation and provide additional revenue for the city.

Using grease wastewater to generate gas actually grew out of a need to solve two pressing problems. First, grease discharged into the sewer lines from restaurants had long been an issue for Riverside. Grease clogged sewer laterals and frequently lead to sanitary sewer overflows (SSOs). Over 850 restaurants operate in Riverside, about 500 of which have grease interceptors ranging in size from 750 to 13,000 gallons. All told the city has a grease interceptor capacity totaling 450,130 gallons. That it was restaurants that either lacked grease interceptors or failed to maintain them properly who created the preponderance of problems only underscored the need of restaurants for a cost effective disposal option for grease interceptor wastewater.

Second, the treatment plant was facing an urgent need for gas. It was about to lose a landfill gas supply, which was used to fuel the plant’s cogeneration facility. Three internal combustion engines constitute the prime movers for the on-site cogeneration plant, which has a capacity of 3 MWs. Additionally, the gas requirements at the facility were set to increase, with the proposed installation of a 1 MW fuel cell plant . The twin needs for an economically viable way of disposing of restaurant grease wastewater and for a cost effective supply of gas drove the thinking to collect grease wastewater and inject it into the anaerobic digesters.

The ability of grease wastewater to produce gas comes from its composition. Grease wastewater contains organic material in the form of fats, oils and greases (FOGs), carbohydrates, sugars and other organic solids. Added to the wastewater in an airless environment, bacteria will metabolize the organic solids into methane. This methane can then be collected and compressed for use in a combustion engine.

Under the program the grease wastewater is collected and pretreated by a third party under contract to the city. The wastewater treatment plant charges the third party a tipping fee, calculated to recover the plant’s full costs of processing the grease wastewater. The third party is able to pass this tipping fee onto restaurants in the form of a marked-up collection fee. The pretreated grease wastewater is injected directly into the digester, adding only marginally to the plant’s cost structure.

The program has yielded phenomenal benefits. Both the volume of gas produced and the energy content of the gas have risen by double-digit percentages. So much so in fact, the city now operates two of the three cogeneration engines 24 hours a day, seven days a week. This on-site source of inexpensive energy production has made the city’s goal of energy independence more attainable.

Before the program began, natural gas represented 17 – 25 % of the total gas used in the cogeneration plant. Looking at the period of January 2005 to April 2006 one can see the dramatic cost savings the grease-to-gas program is providing the City. The plant’s peak natural gas usage occurred in January, totaling $90k in cost of fuel and transmission charges. Post grease-to-gas those same costs fell to approximately $10k in April 2006. That onetime savings alone of roughly $80k was more than enough to pay for the entire project. Pre-project the city’s wastewater division spent about $1,000,000 per year on natural gas and electricity. The grease-to-gas project has the potential to eliminate those costs completely.

Riverside is also finding cost savings in the reduction of blockages in the sewer system attributed to restaurant grease. Over the past three years, grease blockages due to restaurant wastewater discharges have fallen from 15-20% of all overflows to less than 1%. Both the affordable disposal option created by the grease-to-gas program and the efforts of the Environmental Compliance Section of Riverside can be credited with this reduction. As the cost of responding to a grease blockage overflow ranges from $589 to $1,624, the reduction in SSOs caused by restaurant grease blockages has reduced overall response costs by $14K – $39K.

The project has yielded environmental benefits as well. NOx emissions from the cogeneration engines have fallen dramatically. When burning 100% digester gas, the engines have generated 26% less NOx emissions, 40.2% below air quality permit limits.

The project’s resounding success brings into light how much more there is to be done nationally to make full and efficient use of our energy resources. According to the U.S. E.P.A., only 2% of all publicly owned treatment works (POTW) in the US that have digesters actually use the gas generated to create electricity. With municipal governments searching for cost savings in existing budgets and new means of insulating themselves from fluctuating energy costs, Riverside’s innovative grease to gas program can be considered a model for achieving those goals with existing infrastructure.

Thanks to the City of Riverside and especially to Regan Bailey for sharing information with us on this project.

PROJECT DETAILS Capital Costs: Total cost of materials, supplies, analyses, and labor (design and labor supplied in house) ≈ 40-50K. Riverside Public Utilities Department provided a grant of $16,237 to cover the costs of laboratory services and analyses and equipment purchases and installation. Wastewater composition: Laboratory analyses found a range of 92-98% volatile solids in the wastewater grease. Because of the high percentage of volatile solids no increase of biosolids resulted from digestion. An increase in biosolids dewatering activities did result from the digestion. Wastewater volumes: Riverside currently has a contract with a third party to deliver between 19,400 and 24,000 gallons of wastewater grease per day to storage tanks, for timed delivery into the digester. Riverside expects to receive about 35-45 K gallons per day once additional capacity is added. Revenue: Once additional capacity is added riverside expects is will generate about $125 - $150K of revenue each year. The cost for the storage tank rental is about $80K per year. This yields a surplus revenue of about $45-70K per year to pay for periodic maintenance of the equipment. Trucking/tipping fees: Riverside charges the third party contractor $0.01 per gallon. The third party contractor charges restaurants $0.10/gallon to service their grease interceptors. Typically, restaurant interceptors are pumped 2-4 times per year. The majority of grease interceptors in the Riverside have a capacity of 750-1,500 gallons. Tipping fees for other disposal locations in the Riverside area range from $0.05-$0.20 per gallon. Septic waste haulers, which use POTW’s facilities for 100% of their wastewater treatment, pay $0.03/gallon for disposal. Equipment: Each digester has a 1.5 million gallon capacity, and both receive 60 – 70 K gallons of plant solids per day. One is used as a control and the other as the experimental digester to monitor the effects of the grease wastewater dosing. In March 2006, a delivery system was that injects grease wastewater into the digester uniformly, over a 24 hour period, was installed to improve gas production. The delivery system has a capacity of 20,000 gallons. All the materials for the delivery system were obtained on site except for two 200-gpm pumps which were purchased for $15,000 each. The internal combustion engines require a small percentage of natural gas for optimum performance with the current fuel mapping system. The amount of supplemental natural gas needed is about 7%. Gas requirements: Each megawatt of electricity takes about 250,000 cubic feet of gas. An increase of 1.6 megawatts of electrical power in one day is enough electricity to power 1,203 homes for one month, or a month’s production of 1.5 megawatts would provide enough power for 36,090 homes for one month. Methane gas production: With the exception of the starting month, average monthly increases in gas production resulting from adding grease wastewater have ranged from a low of 22% to a high of 87%. Maximum increases during any given month have ranged from a low of 20% to a high of 129%. In August 2005, gas samples were taken from the control digester and the experimental digester. The BTU value for the control digester was 536 and the BTU value for experimental was 626.