BioCycle September 2008, Vol. 49, No. 9, p. 40
To find a way to stabilize costs and minimize odors, the City of Akron has installed an anaerobic digester to process a portion of its biosolids.
Annette M. Berger and Brian Gresser
FOR THE last 20 years, the City of Akron, Ohio has been managing its wastewater solids by composting. While the operations of the facility and the marketing of the end product have been a success, it has not come without its challenges. Odors still are present and the cost of operations is steadily increasing. In a continual effort to minimize odors and find a way to stabilize costs associated with handling the wastewater solids, the City has turned to an anaerobic digestion process.
Phase I of the digester project began operating in November 2007. It is designed to process up to 5,000 dry tons/year of sludge, approximately one-third of the solids presently being composted. The city and its contractor, KB Compost Services, selected the Schmack Biogas digester system, which uses a two-phase, mesophilic, high solids processing technology.
The Akron Water Pollution Control Station (WPCS) is a single stage nitrification, activated sludge treatment facility with an average design flow of 90 million gallons/day (mgd). Raw primary and waste activated sludges are individually thickened before being blended and pumped to the Akron Composting Facility (ACF). Built in 1984, the ACF was designed to process 73 dry tons/day (on a 7 day/week basis) using the Paygro enclosed mechanical composting system. There are four composting bays that are 720 feet long by 20 feet wide and 10 feet deep, allowing 4,500 cubic yards of material to be placed per bay. Due to the nature of the solids (raw) being processed at the facility, the ACF has one of the largest odor control systems in the U.S. It is a two-stage Pepcon system that scrubs the process air collected from the compost bays.
The Akron WPCS pumps the raw, thickened solids through a 10-inch force main located under the river to the ACF. The ACF, which sits on 15 acres with approximately 5 acres under roof, also houses the dewatering equipment. The plant has five Arus Andritz belt filter presses that take the liquid solids from approximately 5 percent to 30 percent solids.
Approximately 70 million gallons of sludge is dewatered annually, producing roughly 45,000 wet tons or 15,000 dry tons of solids. Dewatered solids are stored in two surge bins (70 wet tons capacity each) equipped with live bottom feeders that are all automatically controlled.
Compost produced at the facility is classified as Class A Exceptional Quality (EQ) material (per the USEPA 40CFR Part 503 rule and requirements of Ohio EPA). Compost is bagged under the trade name of EarthPro™ Premium Compost. With a production of approximately 100,000 cubic yards annually there still is a large need to move the material through the traditional markets of topsoil blenders, landscapers and nurseries.
The ACF has always been operated by a private entity, with the City funding the arrangement through reimbursement of expenses for labor and purchasing of chemicals, equipment and materials. To reduce costs and increase efficiencies, in 2000 the City of Akron solicited proposals for long-term operation and maintenance services of the composting facility. The long-term agreement that resulted is for a period of 10 years. The present operator is KB Compost Services Inc., who has overseen the operations since 1990.
ADDING ANAEROBIC DIGESTION
While the Akron Composting Facility has been able to manage 100 percent of the wastewater solids produced on a consistent basis for the last 15 years, the city and its contractor saw a need to address some operational challenges and to diversify its management alternatives. Odors continue to be a significant problem, especially for parts of the operation that occur outside of the enclosed building, such as curing and product storage. In addition, the facility is 20 years old and is showing its age. The enclosed composting operation has been corrosive to the building structure and equipment. Upgrades have been made to both, including coating the underside of the entire roof line with Preferred Solutions Inc.’s foam product that encases the members, followed by a hard coat product sprayed over the foam for moisture and fire protection.
Simultaneously, many of the day-to-day operational costs have been increasing slowly. Fuel costs have had an impact not only on the increased dollars spent for that commodity but the ability to receive necessary commodities for the composting process (i.e., polymer, sawdust, sulfuric acid and hypochlorite solution). The annual operation uses approximately 80,000 cubic yards of sawdust that must be transported to the facility. Sawdust, once perceived as a waste, is now an alternative fuel source and the facility finds itself competing with other markets for this commodity. This has resulted in an increased cost to secure the necessary volume.
These factors led City of Akron officials to look at other potential processes that could help stabilize the costs associated with managing the wastewater solids for the future. In doing so, certain criteria had to be met. These include: The ability to use some of the existing compost facility infrastructure with the new process; finding an alternative that was less dependent on the economic climate of the world, creating more self-reliance at the plant; having the ability to better control odors; and processed solids still had to be managed such that they would still be environmentally sound and preferably meet EPA’s Class A standard for EQ material.
AD FACILITY DESIGN
In late 2004, KB Compost Services hired Applied Technologies from Milwaukee, Wisconsin to review the anaerobic digestion (AD) process from Schmack Biogas AG (Schmack) from Germany. The system is designed to process high solids feedstocks. There are about 200 AD facilities using this technology, mainly in Europe with an installed capacity of 100 MW. The company, which had expanded to Japan, was interested in the North American market. Schmack BioEnergy LLC, located in Independence, Ohio, was licensed to sell the technology.
Analyses were performed on the wastewater solids by Schmack’s laboratory, along with an independent study by Marquette University. Preliminary results indicated that Akron’s solids were very suitable for the AD process with the biogas production value being 19.4 ft3/lb of volatile solids destroyed with approximately 50 percent volatile solids destruction. Applied Technologies proceeded with a feasibility study of the Schmack AD system for Akron, knowing it had to be integrated into the composting operations as much as possible.
The digester system is made up of a minimum of two tanks, the Euco and Coccus. The Euco is a plug flow digester that can accommodate high solid wastes (up to 18% dry solids [DS]) since the agitation is done by a heavy duty horizontal mixer that does not allow the sink or swim layers to develop. The Euco comes in sizes ranging from 400 to 1,000 m3. The Coccus is a common circular tank that provides a complete mix and incorporates the same style mixer as found in the Euco. In addition, a desulphurization process occurs in the wooden members found in the flexible roof membrane of the unit. The Coccus can be from 2,000 to 4,000 m3 in size.
Schmack BioEnergy developed a unit referred to as the all-in-one (AIO), which is attached directly to the Coccus. This provides a short run for the biogas and maximizes process efficiency. The AIO unit houses the generator, feed dispensing system (a single pump that delivers solids to the various tanks by a common head arrangement) and a gas analyzer. The parasitic load of the digesters is between 10 and 15 percent of the electricity produced and thus the surplus can be used for auxiliary electric requirements. Excess heat from the combined heat and power (CHP) unit is used to heat the digesters.
The system installed in Akron consists of one 600 m3 Euco, one 2,000 m3 Coccus, an AIO unit equipped with a 335 kW Jenbacher genset, two centrifuges and the necessary auxiliary equipment. Plans include installing an 8,000 gallon pasteurization tank to bring digested solids up to 70°C for 30 minutes to ensure that the biosolids meet EQ requirements. The digesters are situated outside to the east of the actual composting building, off of the mixing area. This area in the past was used to store amendment and cure compost. While the composting operation was losing valuable real estate to the digesters, it was determined that after the construction and startup phase it was the best use of the area. With one-third of the solids being digested, the need for as much curing space would diminish. Likewise, amendment usage would decrease accordingly, lending itself to less space necessary to store that commodity.
Applied Technologies’ design took advantage of existing solids dewatering and storage infrastructure. Two of the six sludge wells at the ACF were taken out of service to accommodate holding wells once solids had been digested and pasteurized. Raw solids continue to be dewatered using the belt presses and the cake is conveyed to either of the two surge bins. Cake sludge can be metered into the pugmill mixer where amendment is added for composting, or it can be directed to a set of screw conveyors used to accommodate the AD process. Cake sludge being metered out of the surge bins cannot be directed to both composting and AD simultaneously, therefore a new live bottom feed bin that holds a day’s volume of cake sludge for the digestion process has been incorporated into the system.
Cake sludge is metered out of the bin and into the Euco feed hopper once every hour over the 24-hour period. To address the high percent solids obtained during the dewatering process (30%), a volume of liquid biosolids is pumped from the sludge wells holding the raw solids. Enough liquid solids are added to reach the 18 percent solids consistency. Once biosolids are in the Euco, the retention time is approximately 8 days. The material is kept at 95°F by heating water with heat from the generator and running it down the specially designed shaft of the mixer. The biosolids are mixed slowly by a set of paddles that are positioned on a shaft that runs parallel to the length of the Euco. Capacity of the Euco is 160,000 gallons.
Both the partially digested solids and the biogas generated in the Euco are sent to the Coccus for further processing. Capacity of the Coccus is 550,000 gallons. Material in this digester is maintained at 95°F. Heat tubes are placed around the perimeter of the tank for circulation of hot water, which helps in maintaining temperature. Additional heating is supplied by the two mixing assemblies found in the Coccus, which are identical to the unit in the Euco but shorter in length. The two mixer assemblies are staggered in height from one another to provide uniform mixing of the tank contents. The retention of the solids in the Coccus is roughly 15 days.
The digested solids are then pumped to a designated sludge well where they will be held briefly, prior to pasteurization After pasteurization, the solids will be dewatered using two D5LL centrifuges from Arus Andritz. The two centrifuges were added to the dewatering room that houses the belt filter presses. The pasteurization unit has not been installed. Once it is operating, KB Compost will have the option of taking the dewatered, digested and pasteurized biosolids to a soil manufacturing facility. Currently dewatered digested solids are being stockpiled, or are being fed into the composting operation.
To start up the Coccus last November, 150,000 gallons of anaerobically digested seed sludge from the Kent (Ohio) wastewater treatment plant (at 4.35% DS) were fed into the unit, along with 275,000 gallons of Akron’s raw sludge (at 5.5% DS). A temporary boiler unit was used to bring the digester to 90 to 95°F. To start up the Euco, 100,000 gallons of material digested in the Coccus were fed in, along with 40 wet tons of raw cake solids (at 28 to 30% solids). The raw cake was added on a daily basis. The temporary boiler unit was used to achieve the desired temperature in the digester.
Current feed rate into the digester is 13 dry tons/day. The digestion process had no problem meeting the necessary vector attraction reduction requirement of a minimum of 38 percent volatile solids destruction (Figure 1). Methane content in the biogas in the Euco has been averaging 50 to 55 percent. Generation of electricity reached 335 kW by March 11, 2008, and a month later, the engine was running at peak with excess biogas (Figure 2). The 350 kW genset can produce about 8,000 kW/day of electricity. It is being utilized by the WPCS.
Once the digesters are operating in a steady-state, KB Compost Services will install the pasteurization unit. Capital costs for Phase I are about $6 million, and have been financed by KB Compost.
Annette M. Berger is Vice-President of Operations with KB Compost Services, Inc. Brian Gresser, P.E., is with the City of Akron, Ohio.