Biosolids are the nutrient-rich organic product of wastewater treatment. A beneficial resource, biosolids contain essential plant nutrients and organic matter, and are recycled as a fertilizer and soil amendment. Biosolids from Dayton's wastewater treatment plant are produced as a result of microorganisms feeding upon the solids and dissolved materials that are in the wastewater coming into the plant.

The City of Dayton wastewater treatment plant receives an average of 55 million gallons per day of wastewater. The plant serves 340,000 people and industry in a 160 square mile service area within the City of Dayton and surrounding satellite communities. Wastewater entering the plant receives primary treatment, in which solids are settled out, followed by two stages of secondary biological treatment - trickling filters, followed by activated sludge for conversion of ammonia to nitrates.

The solids settled out in primary treatment, along with the biological solids from both stages of secondary treatment are pumped into large tanks called anaerobic digesters. In an anaerobic digester, bacteria are utilized to digest and stabilize solids generated during the wastewater treatment process. A daily average of 386,000 gallons of solids are processed in anaerobic digestion each day. Once the solids in the digesters have been properly stabilized by the anaerobic bacteria, the solids are then considered to be biosolids.

The term anaerobic means "without air". When a biological digestion process occurs without air, the bacteria used for the process must use oxygen that is bound-up in solids and liquids in the tanks. Instead of giving off carbon dioxide gas as a respiration by-product (as a person would), anaerobic bacteria give off methane gas. Therefore, two useful products are produced from anaerobic digestion: methane gas, which is used to produce heat and electricity, and biosolids, which is used as a fertilizer and soil amendment.

Biosolids are pumped to a biosolids dewatering facility following digestion. The City's private biosolids contractor, BioGro, owns and operates a 100 % privatized biosolids dewatering facility located on the City of Dayton WWTP grounds. The private contractor pumps solids to any of four three-meter belt presses for dewatering. The belt presses squeeze water out of the biosolids, changing the biosolids from a liquid, containing only 2% solids, to a solid product containing 25% solids. Dewatering biosolids greatly reduces the cost of transporting them to agricultural land application sites. The City of Dayton wastewater plant produces about eight truckloads of biosolids every single day of the year. The private contractor recycles these biosolids on over 7,000 acres of farmland in Southwest Ohio to help grow crops such as corn, soybeans, wheat and hay.

For additional information please contact Thomas Schommer or Phil Bennington at: The City of Dayton Advanced Wastewater Treatment Plant 2800 Guthrie Road Dayton, Ohio 45418-2704 Telephone (937) 268-9511 Fax (937) 268-5277

Technical Information

The Dayton solids digestion system is operated as a single-stage system, consisting of 8 primary digester tanks. Four digesters have an operational capacity of 1,459,000 gallons each and four digesters have an operational capacity of 1,273,000 gallons each. It is a standard rate system with loadings less than 0.10 pound of volatile suspended solids per day per cubic foot of digester capacity (1.6 kg/m3). The 1998 annual average solids retention time of the anaerobic digester process was 44 days.

The total digester feed is comprised of both primary settled sludge and thickened waste activated sludge. Mixing is accomplished with an automated multiple point sludge recirculation system sized to turn over the digester contents at least once every 24 hours. The Dayton digesters are operated at approx. 36.7oC at a pH 7.0 to 7.5. Digesters are monitored for pH, volatile acids and alkalinity once each week. The temperature and carbon dioxide of each digester is measured three times each day. The annual solids loading of the digesters average 53 pounds of volatile solids per 1,000 cubic feet per day of digester operating capacity. The gas production of the Dayton digesters averaged a total of 342,000 cu. ft. / day in 1998.

The digester gas is utilized as a fuel to operate a 2.2 MW power plant cogeneration facility as a second source of electric power for treatment and provide a means to reduce utility bills. The performance goal of this cogeneration facility is to produce 9600MWH per year of electric power and 45 billion BTU's of heat per year. The cogeneration facility is operated in a peak-shaving mode to accrue the best cost savings in avoided electric utility costs.

The cogeneration facility has produced 1319 MWH and 6.9 billion BTU's of heat in the first quarter of 1999. This cogeneration operation has saved $52,491 of natural gas cost avoided through heat recovery. Over ten billion (10.2) BTU's of heat were produced in boilers from digester gas resulting in $ 77,932 of avoided natural gas cost. Total natural gas cost avoidance savings was $134,423 for the first quarter of 1999. This type of cost savings is typical of the savings from use of the digester gas.

A USEPA inspection of the City of Dayton was conducted on March 11, 1997 by Ms. Yvonne Ciccone, Senior Environmental Engineer, and Mr. Henry Willis, Staff Engineer, of SAIC. Rico Gonzales of the Ohio Environmental Protection Agency (OEPA) observed the inspection. The inspection concluded the City of Dayton was in compliance with all 503 regulations.

Biosolids are currently meeting Table III- 503 requirements for land application. The land application program as administered by Bio Gro has never been cited for any violations of Ohio's Land Application of Sludge Rules.

Intergovernmental Agreement for Biosolids Recycling

The City of Dayton is located in Montgomery County and is the largest city. The City of Dayton and Montgomery County governments joined together in 1981 to work cooperatively towards providing an efficient biosolids recycling system for the City's Wastewater Treatment Plant (WWTP) and two Montgomery County WWTP's (Eastern & Western). Biosolids are a primarily organic product, resulting from the wastewater treatment process, that can be beneficially recycled under applicable regulations.

After several years of research efforts a decision was made to privatize biosolids recycling. A Request For Proposal was written to solicit proposals from private firms to design, build, maintain and operate biosolids recycling facilities at a set fee per dry ton of biosolids for both the City and County. The private contractor was awarded a 20 year contract with five year renewable options to land apply biosolids for the City and Montgomery County. The contractor was to design, build, operate and maintain facilities, equipment and administrate personnel to completely operate biosolids recycling programs for the City and County. The private contractor executed contracts in April 1986 and began operations eight months later in a completely new dewatering facility in December 1986. The project is now in the third renewal period of the contract.

The City of Dayton, Montgomery County and the private contract cooperates daily to address operational concerns and requirements. The continued cooperative efforts have been demonstrated by holding a combined monthly meeting with Montgomery County, the City of Dayton and the private contract. The combined monthly meetings assist all parties in addressing specific concerns and allows the private contractor to update both the City of Dayton and Montgomery County in project activities. Special meetings to focus on single items are held as necessary.

For the reduction of biosolids odors and increased storage capacity, the private contractor has increased the storage pad dimensions at the Dayton facility to 140 feet by 300 feet and covered the pad with a 33 foot high York/Chief Steel Frame building. The increase in footprint and decrease in stockpile weathering increases the available capacity from 2700 to 6000 cubic yards. The surge tank was also covered to reduce odors.

The continued cooperative efforts have been demonstrated by Montgomery County Eastern Regional WWTP purchasing an existing lime conveyance system and truck scale from the private contractor. A reduction in the unit price/ton was made to reflect this purchase, to Montgomery County's advantage. The private contractor and the County agreed to /include the weight of lime when determining the dry tonnage breakpoint for excess unit price reductions. The contractor has equipped all trailers hauling Eastern WWTP biosolids with tarps to help prevent odors when hauling the lime stabilized biosolids.

The second Montgomery County WWTP (Western) entered the project in October 26, 1995. The private contractor initially utilized two mobile presses to reduce inventories at Western Regional WWTP and then reduced to one mobile press until May 1996, when permanent facilities were completed. The project is now in its third 5-year contract term with the Bio Gro Division (the private contractor), and will expire January 1, 2002. The City and County have renewed several Intergovernmental Agreements since May 27, 1986. The present Intergovernmental Agreement with the City of Dayton and Montgomery County for full service biosolids recycling will extend through December 31, 2001.