September 20, 2006 | General

Anaerobic Digester Services Industrial Generators

BioCycle September 2006, Vol. 47, No. 9, p. 48
Waste Management’s new Anaerobic Pretreatment Center receives liquid waste streams with high COD and fats, oils and grease.

THE CID Recycling and Disposal Facility, operated by Waste Management, Inc., is located at its closed MSW landfill in Calumet City, Illinois, about 20 miles from Chicago. Waste Management is creating an environmental complex that houses a single stream materials recovery facility (MRF), a soil remediation operation, a biological liquid treatment center using the sequencing batch reactor (SBR) technology, and landfill gas recovery with a power generation station. The most recent addition to the CID facility is an anaerobic digester – the Anaerobic Pretreatment Center (APC) – that began operating in late spring. Methane gas produced by the digester is fed into the same landfill gas recovery line that feeds two CAT reciprocating gas engine generators. Enough electricity is created to power 8,000 homes.
The APC is designed to process high strength liquid waste streams, primarily from commercial and industrial customers. It is permitted to accept wastewaters and off-spec material from the food, dairy, beverage, pharmaceutical and personal care industries; grease trap wastes from the food, dairy, beverage and personal care industries; portable sanitation wastes; and septage.
The digester was designed by a consultant to Waste Management, and built using commercially available pumps, a tank and related components. It has a working volume of 1.5 million gallons. “The digester can handle a wide range of liquid residual streams,” says William Schubert, Director of Environmental Engineering for Waste Management’s Midwest Group. “If we can pump it and it is organic, it is acceptable to this technology. Then it becomes an issue of residence time.”
Jean LaPlanche, Manager of the APC, adds that the digester has 60,000 mg/L/day (50,000 lbs/day) of chemical oxygen demand (COD) capacity in its original design. “As long as the mixture isn’t over that COD level, our standard operating procedures and residence time will be fine. If it is more than that, we need to increase the residence time.” In general, waste streams with the following characteristics can be handled: Low to high soluble levels of chemical oxygen demand (COD) or biochemical oxygen demand (BOD) with concentrations ranging from 500 to 300,000 mg/L; Low to moderate levels of particulate matter (suspended solids) that are biodegradable, with concentrations ranging from 100 to 100,000 mg/L; and Low to moderate levels of biodegradable fats, oils and greases with concentrations ranging from 100 to 50,000 mg/L.
Waste Management decided to build the digester at the CID facility to service customers with high strength liquid waste streams, who otherwise have to pay costly sewer surcharges for high levels of BOD, COD and solids. It found that it was turning away wastewater and wash water loads too high in BOD content for the SBR. The other disposal alternative – which many generators in the Chicago area opt for – is to land apply the residuals in neighboring states. “That is cheaper than paying the surcharge to put it down the sewers, but there are pretty high transport costs involved, and the regulations are changing in those states, which will make it more difficult to land apply,” says Scott Combis, District Engineer at CID. “In addition, bringing these materials to the APC creates less environmental liability concerns for the generators.” The tipping fee at the APC is based on the type of waste stream and the degree of variability between loads from the same generator. “If the waste stream has higher concentrations of COD, it will take longer to degrade the material,” he adds. “Pricing is based on that variable and what additives are needed to condition the feedstock for the digester.”
Wastes generally are received via tanker trucks. They are unloaded into a tank with a macerating screen and transfer pumps. The screened materials are conveyed to a self-dewatering lugger box and then landfilled on-site. “The landfill only receives waste generated on site, such as the solids from the APC and the sequencing batch reactor,” says Combis. Loads are prescreened for ammonia, pH, alkalinity, cyanide, sulfides, reactivity and flammability in an effort to detect possible problems prior to off-loading. The presence of ammonia and alkalinity in the waste load, to a certain extent, would decrease the amounts of nutrients (i.e. nitrogen and magnesium) that would have to be added externally. “We also test the mixed liquor – the mixed contents of the digester – for volatile acids and alkalinity,” adds LaPlanche. “The ratio between the two concentrations helps to determine the overall health of the process.” Mixed liquor not in the desired range may sometimes be buffered with magnesium hydroxide in an effort to achieve the correct ratio.
When it began operating, the digester was receiving primarily sugar waters and grease trap waste. During the initial start-up phase, the microorganisms needed to get used to the primary substrate they can feed on, says Schubert. “The key was to get an adequate volume of one type of waste feedstock then blend other types of feedstocks gradually. Once that primary substrate is established, and there is a consistent and appreciable volume, a wider variety of customers can bring in loads. It was challenging to get it running on a commercial basis, but we achieved that after several months. The sheer size of the reactor acts as a buffer to varying residual streams.”
The digester reactor is fabricated from precoated carbon steel. Feedstocks are hydraulically blended with a jet recirculation system, thus there are few moving parts, and the digester can handle a lot of different feedstock consistencies. “The constant recirculation provides contact of all the waste with the microbes,” explains Schubert. “There isn’t any settling so all the materials are homogenized.” The outfeed pipe is in the middle of the tank and the refeeding pipe is manifolded at the bottom. This creates a swirling motion within the tank.
The entire tank recirculates (is fully mixed) in a day; there is a 30-day retention time. The digester operates in the mesophilic range, averaging about 95 °F. A portion of the mixed liquor passes through a heat exchanger that can run on either digester gas or propane (as a back-up). The heat exchanger has its own pump that helps in returning the mixed liquor to the reactor.
The digester tank has a gas-tight, floating, insulated membrane roof. A gas collection system transfers the biogas produced from the reactor through a condensate knockout system and transfers it into the site’s landfill gas collection pipes to be processed through the existing electrical generation facility. Condensate collected in the knockout tank is discharged back to the anaerobic reactor. The design gas flow is 350 to 450 cfm; during a tour of the digester in June, gas production was about 65 to 75 cfm. The gas is about 60 to 75 percent methane.
Material is discharged from the digester to a pair of rotary screens. Each screen has a design flow rate of approximately 150 gal/min. Typically, the process will discharge a volume that is equal to the volume of waste taken in. An automated feed pump adds floculant to facilitate separation of the solids. The effluent is discharged to the POTW; thickened solids, at about eight percent, are either fed back into the digester or into a centrifuge where they are dewatered up to 25 percent and disposed in the landfill. The quantity of solids produced is fairly low. “This anaerobic digestion technology achieves extraordinary solids destruction efficiency,” says Combis. “Therefore, we don’t anticipate generating enough solids to add a composting element to this facility.”
While the facility is currently designed for liquid organic residuals, it could be upgraded to process solid organic wastes such as feedstocks from grocery stores and restaurants, he adds. “We would need to get it into a slurry form. Materials with a higher cellulosic content would need to be preconditioned in some fashion prior to anaerobic digestion.” – N.G.

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