September 22, 2005 | General

Extra Methane Production From Municipal Anaerobic Digesters

BioCycle September 2005, Vol. 46, No. 9, p. 64
Marquette University study evaluates high-strength wastes from breweries, fermentation companies and restaurants to gain extra biogas for renewable energy.
Daniel Zitomer and Prasoon Adhikari

Many municipal wastewater treatment plants have existing anaerobic digesters that may be used to codigest high-strength wastes with wastewater solids. When used for codigestion, these digesters in effect become regional renewable facilities. To explore specific feedstocks, four residuals were studied at Marquette University to determine appropriate operating conditions for codigestion and analyze the economics. The residuals were Miller Brewery beer filter, Lesaffre Yeast fermentation, Southeastern Wisconsin Products fermentation and Pandl’s Restaurant food. These high-strength wastes were mixed with biosolids at the South Shore Wastewater Treatment Plant (SSWWTP) in Oak Creek, Wisconsin. The food waste was pretreated using the Rothenberg Wet Waste Recovery System provided in the United States by Ecology, LLC (Glendale, Wisconsin).
South Shore Wastewater Treatment Plant
SSWWTP is located south of Milwaukee along the Lake Michigan shoreline and was put into operation in 1968. The plant treats an average of 90 million gallons of waste-water each day, most of it from the southern and western portions of the service area. The facility has a peak capacity to treat approximately 300 million gallons per day of municipal wastewater.
Raw wastewater flows through primary, secondary and disinfection treatment processes. Biosolids are conveyed to anaerobic digesters where microorganisms stabilize the solids and create biogas containing methane. This biogas is collected, and is typically used to continuously run blowers for the activated sludge process as well as produce electricity using an engine generator set for approximately eight hours per day. If more biogas was produced, then the engine generator set could be operated for more than eight hours per day. This may help reduce energy costs at the treatment plant. Stabilized biosolids are applied to farmland as a fertilizer and soil conditioner known as Agri-Life or dried to produce a commercial soil amendment called Milorganite.
SSWWTP has 12 single-stage, high-rate, anaerobic digesters. At the time of this study, Digesters 1 through 5 and 7 were being used to store digested biosolids, Digesters 6 and 8 were out of service, and the remaining four digesters (9 through 12) were active. Digesters 9 through 12 are 125 feet in diameter and have a side water depth of 38 feet. Recirculation pumps direct sludge through spiral heat exchangers at each digester to maintain sludge temperatures in the range of 90 to 95°F (32 to 35°C) for mesophilic digestion. Typically, waste heat from the blowers and engine generator set is used to heat the digesters. When the ambient temperature is very low, natural gas is also used to fire boilers providing additional heat for the digesters.
The digesters have fixed concrete covers with a gas dome, access manholes and sample ports. Each of the gas domes has a pressure relief valve and a flame trap. The gas domes collect the biogas produced in the anaerobic digestion process. Digester biogas is recirculated through compressors and forced back into draft tube mixers to mix digester contents. Excess gas is withdrawn from the digester cover through a gas header. In addition, some biogas can be stored in pressurized storage vessels for later use.
A one-year study was conducted to investigate increased energy production from codigestion of industrial feedstocks at the treatment plant.
As part of the three phase study, the wastes were first tested for biochemical methane potential (BMP) and also tested using anaerobic toxicity assays (ATAs). Second, bench-scale digesters were operated in the laboratory and finally, a full-scale demonstration was performed by feeding wastes to the treatment plant’s anaerobic digesters.
The average maximum BMP values were as follows (ml CH4 per gram COD): Miller Brewery – 413; Lesaffre Yeast – 2274; Southeastern Wisconsin Products – 943; and Pandl’s Restaurant – 488. The maximum theoretical BMP for any waste is 395 ml CH4 per gram COD assuming all the COD is converted to methane. The abnormally high values for Lasaffre Yeast and Southeastern Wisconsin Products wastes indicates that they stimulate methane production from background COD present in the biomass, which was digested sludge from the SSWWTP. The COD in Miller Brewery and Pandl’s Restaurant waste is essentially completely convertible to methane.
Fourteen bench scale, fill-and-draw anaerobic digesters were operated. Each 2-liter digester was fed a different blend of one high strength waste and municipal waste-water solids (70 percent primary sludge and 30 percent v/v thickened waste activated sludge from SSWWTP). Each digester was completely mixed and operated at a 15-day solids retention time (SRT) at 37±1°C. It was determined that Lasaffre Yeast and Southeastern Wisconsin Products wastewaters can be successfully codigested with municipal wastewater solids at all blend ratios tested (from 20 to 80 percent v/v wastewater in municipal wastewater sludge). Similarly, the food waste can be successfully codigested at all blend ratios tested (from 3 to 11 percent v/v food waste in municipal wastewater sludge).
The Miller brewery wastewater was not successfully digested by itself in the fill-and-draw digester utilized; however, it was successfully digested when blended with municipal wastewater solids. Although the brewery wastewater was not successfully digested alone, it is amenable to anaerobic treatment. If it is treated alone, then it is recommended that reactor configurations other than a fill-and-draw digester should be considered.
All the wastes utilized in bench-scale testing had metals concentrations below the Wisconsin Department of Natural Resources high quality limits for biosolids to be land applied and will not limit the land application of digested biosolids.
Full-Scale Demonstration Testing
A full-scale demonstration test was performed by feeding Southeastern Wisconsin Products and Pandl’s Restaurant wastes to the anaerobic digesters at the SSWWTP at the same time municipal wastewater solids were also being fed. There was a 70 percent increase in biogas production when Southeastern Wisconsin Products wastewater was codigested. The waste constituted one percent of the total COD loading to the digesters. Therefore, the biogas production increase was not due to additional COD, but may have been due to a synergistic effect resulting from the presence of bioavailable nutrients (e.g., iron) required for microbial growth. The additional biogas can be employed to produce 16,300 kw-hr/day of electricity worth over $200,000 per year using an existing biogas-powered electric generator set at the treatment plant. It is recommended that treatment plant personnel consider periodic or continuous codigestion of Southeastern Wisconsin Products wastewater and municipal wastewater solids; this may lead to a sustained increase in biogas production. In addition, investigations regarding the use of Southeastern Wisconsin Products and other similar yeast production/fermentation wastes as supplements to increase biogas production at other anaerobic digestion facilities are recommended.
Pandl’s Restaurant food waste was also delivered to the digesters at the SSWWTP. However, due to the relatively large solid particles (approximately 20 percent of solids greater than 4.76-mm nominal diameter), the food waste solids could have damaged pumps and appurtenances. Therefore, the Pandl’s food waste was discharged to the primary clarifiers at the plant. The solids that settle in the primary clarifier are screened and pumped to the anaerobic digesters. Laboratory settleability testing and sieve analyses data were used to estimate that 67 percent of the food waste COD would be conveyed to the anaerobic digesters at SSWWTP. It is estimated that the waste produced by Pandl’s Restaurant can be converted to 780 standard cubic feet per day of methane. The methane can be used to generate $992 per year of electricity or can be used to off set the purchase of $1,620 per year of natural gas.
It should be noted that Pandl’s Restaurant food waste is currently disposed of in a sanitary landfill. It is possible that anaerobic digestion with biogas utilization and safe application of the stabilized biosolids would prove to be a more economical approach than landfilling if a detailed lifecycle cost comparison of the two options was performed. In addition, if more restaurants began to practice waste shredding and storage, then it is probable that economy-of- scale savings could be accrued.
Industry Feedstocks And Economics
Industries with organic waste streams that use or could use anaerobic digestion include: Food process such as vegetable canning, milk and cheese manufacture, slaughterhouse and potato processing wastes; Drink industry – breweries, soft drinks, distilleries, coffee and fruit juices production; and Industrial products – paper and board, rubber, chemicals, starch and pharmaceuticals. For the Marquette University study, wastes were selected based upon their high COD, probable anaerobic degradability, and generation close to the treatment plant.
The additional biogas potentially produced from yeast production waste alone can be employed to produce 16,300 kw-hr/day of electricity worth over $200,000 per year using an existing biogas-powered electric generator set at the treatment plant.

Daniel Zitomer,Ph.D., P.E. is Associate Professor and, at the time of the study, Prasoon Adhikari was a Research Assistant in the Department of Civil and Environmental Engineering at Marquette University. The authors thank Larry Krom and the Wisconsin Focus on Energy Program for financial support, as well as Linda Ivarson and Cary Van Aacken (Ecology, LLC), James Surfus (Miller Brewing), Michael Malencore (Southeastern Wisconsin Products), Karl Curda (Lasaffre Yeast Corporation), and James Pandl (Pandl’s Restaurant) for their participation. Questions or comments should be e-mailed to Daniel Zitomer at

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