Several years ago I had the opportunity to visit the Southerly wastewater treatment plant in Cleveland, Ohio, where I learned that the plant, which serves a population of more than 601,000 in the Greater Cleveland area, is one of the largest activated sludge plants in the U.S.. Southerly has an average daily flow of 125 million gallons per day (mgd). The process involves separating the solids from the liquids and using a sequence of tretements to reduce the amount to water in the solids to the point where it can be incinerated. The fluid effluent is processed and then flows into the river. The ash from the incineration process goes into ash lagoons. The plant's electricity requirements, mostly for pumping, are about 7.8 million kwh per month which costs $492,000/month. The natural gas requirements for the incinerators is 37,000 million cubic feet (mcf) per month, which costs them $417,000/month. That is about $900,000 per month for energy, and this is just one of three plants in the Greater Cleveland area.
I blogged about a research project using microbial reverse-electrodialysis cells (MRC) to digest organic solids to produce electricity that suggested that the energy flow in treating wastewater could be reversed. It was estimated that with this process there is ~9× more energy available in domestic wastewater than is required to treat it using conventional methods.
Generating power from waste in Milwaukie
So I was really interested today when In his presentation at the Canadian Water Network conference in Ottawa, Kevin Shafer, Executive Director of the Milwaukie Metropolitan Sewerage District(MMSD), said that he thinks of water treatment plants as power plants.
The MMSD has two water reclamation plants, Jones Island and South Shore.
The South Shore Wastewater Treatment Plant first went on line in 1968. At this treatment plant biosolids are sent to anaerobic digesters where microorganisms convert a large part of the biosolids into methane gas. The gas is collected and burned to produce electricity for the plant.
The Jones Island plant has been in operation for almost 90 years. It is also located on Lake Michigan. Jones Island is unique because this is where Milorganite, an organic fertilizer, is made. Wastewater is treated with microbes to digest nutrients. The resulting solid materials are then dried at temperatures ranging from 900⁰-1200⁰F which removes any pathogens. The treatment plant is designated as a National Historic Civil Engineering Landmark by the American Society of Civil Engineers. In June the water treatment plant and the Milorganite factory will begin to be powered by electricity generated at the Jones Island plant from methane gas produced at a nearby landfill and piped to Jones Island.
Kevin thinks of the water treatment plants as energy production plants. Jones Island will bring three turbines on-line in June with about 9 MW capacity. After that the plan is to add two more turbines and the plant will be able to export power and actually generate revenue.
The South Shore plant can generate power directly from biosolids extracted from wastewater so there is a program with a local company Insinkerator that manufactures waste food disposal units to encourage food waste in the home to be ground up in the sink and added to wastewater and ultimately to generate electricity at South Shore or in the case of Jones island to be used to manufacture fertilizer.