Innovative Solutions For Sewer Sludge

Every day, millions of gallons of sewer grease are processed in the United States, alone. This mixture of fatty solids from restaurants, home kitchens and other sources poses a double threat to the wastewater industry. The sewer grease, or sludge, is energy intensive and thus costly to process and it is also corrosive and can damage the sewer infrastructure including sewer pipes and fittings. New technology has the potential to turn this nuisance into a benefit for the industry, however. By looking at sewer grease as a potential energy source, wastewater treatment facilities may be able to offset not only the cost of treating the sludge, they may also be able to reduce their overall operating costs in the process.



The Magnitude of the Problem
The United States’ National Renewable Energy Lab (NREL) estimates that over 500 million gallons of sewer grease are processed every year. For the most part, wastewater utilities have had three options for dealing with this waste:

  • Landfill (which is expensive and prohibited in some locales)
  • Burning (which can produce toxic ash if the sludge is not fully treated)
  • Dispersion over agricultural lands (which is proving to be a toxic choice – see H2Bid’s article
    titled Food Processing Wastewater Problems in Western Michigan, from November, 2009)

Beyond dealing with the large, collected quantities of solid waste, there is also the residual sewer grease left in the pipes. Again from NREL sources, the City of San Francisco, California, spends an average of 3.5 million USD each year to remove the grease from the sewer system so that the grease does not react with or otherwise compromise the pipes and fittings of the sewer system. All of these costs, when summed, typically run between 25%-50% of the total operating costs of a wastewater treatment provider.

Turning Problems into Opportunities
Many private companies, universities and some sewer districts are collaborating to find novel solutions to the sewer grease problem. Instead of looking at sludge as a treatment and disposal problem, these innovative trailblazers are looking at the sludge as an energy source. On the whole, sewer grease has about 10 times as much chemical potential energy in it as it does to treat it; harnessing this energy is at the core of current endeavors in the field.
One potential opportunity comes from the methane produced when bacteria digest the sludge. Of the approximately 2,000 sludge processing facilities in the Unites States, approximately 650, or roughly 1/3, use anaerobic bacteria to digest the sewer grease. This process naturally produces methane gas in varying quantities; if sufficient, reliable quantities of methane are produced, the gas can be burned and harnessed for power much in the same way that traditional gas fired power plants work. As part of a broader water conservation and recycling effort, the Pajaro Valley Water Management Agency (PVWMA) partnered with the City of Watsonville, California, to implement a cogeneration electrical production facility at the wastewater treatment station. Initially, the amount of methane was not sufficient, but when the city began to add unprocessed restaurant deep-fryer grease to the sludge, the methane gas generation was increased by over 50%. Today, the cogeneration plant produces 90% of the electricity needed for primary and secondary wastewater processing.
In a second, independent effort, the City of San Francisco has teamed with BlackGold Biofuels to develop and build a sewage grease to diesel facility. The installation, the first of its kind in the United States, will be able to process approximately 100,000 gallons of sewer grease per year. With incentives to encourage green energy, the city will be able to sell the biodiesel on the open market for a premium versus raw petroleum-distilled diesel. City officials believe that the facility will have paid for itself and will begin to generate a profit in only 2 to 3 years. This revenue stream will go toward offsetting rising utility costs and fund needed capital improvements without having to demand a rate hike from customers.
Another use for the sewage grease is as a solid, combustible material. To facilitate this use, the sewer greases and fats are dried to produce biomass solids. These materials can be burned directly or can be “gasified” in a manner similar to coal-gasification. The gas can be used in place of natural gas in power plants. The University of Nevada is actively pursuing research in this area and Chuck Coronella, an associate professor of Chemical Engineering at the university, has estimated that for every 140 tons of wet sludge, 1 megawatt of power could be generated.
Far from being mature, these solutions do appear to be headed in the right direction for the industry. Reducing costs and dealing with sewage grease in a positive way rather than as a disposal problem will benefit the wastewater community, the customers and the environment. The industry should pay close attention to the results of these pioneering efforts and learn from them so that one day, the recycling of sludge will become a common practice.