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Contact Digesters (High Rate) The contact reactor is a high rate process that retains bacterial biomass by separating and concentrating the solids in a separate reactor and returning the solids to the influent (Ettinger, Witherow et al. 1957; Eckenfelder 1966; Loehr 1974). More of the degradable waste can be converted to gas since a substantial portion of the bacterial mass is conserved. The contact digester can be either completely mixed or plug flow. It can be operated in the thermophilic or mesophilic range. The contact reactor can treat both dilute and concentrated waste provided the separator can concentrate the digester effluent solids sufficiently to enhance the process. A wide variety of separators have been tested over the past 30 years. Initially gravity separators (settling tanks), or solids thickeners were used. It was soon discovered that the solids could not be sufficiently concentrated in a gravity separator without degassing to remove the gas bubbles attached to the solids. Actively fermenting digester effluent containing gas bubbles floated rather than settled in the separator. Lamella or plate separators have also been used to concentrate the biomass after degassing. Both of these gravity-settling techniques are not effective for concentrated digester solids. Gravity separation techniques are effective with dilute waste following a completely mixed reactor. Separation requires several days of detention(Duke Engineering & Services 2001). The completely mixed digester will prevent stratification. The effluent is then allowed to separate by gravity in the separation reactor. The digester solids concentration should be less than 2.5% for gravity separation to be used. Long separator detention times are required. Mechanical separation devices have been tested to reduce the detention time required by gravity separation. Centrifuges, gravity belts, membranes, and other mechanical separators have been used with limited success. These disruptive devices have been shown to inhibit the bacterial consortia and thus limit the effectiveness of the contact process. Burke used gas flotation to separate and concentrate the digester effluent for the efficient and tranquil recovery of the anaerobic consortia. The process has been used for the digestion of dairy manure(Burke 1998), sewage sludge (Burke and Yokers 1999), and potato waste(Burke 1997). It has been shown to be effective for concentrating the biomass from actively fermenting digester liquors without the need for degassing. The process has been referred to as the AGF or "anoxic gas flotation" process. Gas flotation can achieve significantly greater biomass concentrations than gravity separation without the adverse consequences associated with disruptive separation techniques. © Environmental Energy Company 31 www.makingenergy.com 360-923-2000PDF Image | Dairy Waste Anaerobic Digestion Handbook Options for Recovering Beneficial Products From Dairy Manure
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