Phase I research will be a laboratory-scale demonstration of the technical merit of an innovative two-phase digestion process for the simultaneous biomethanation and stabilization of sludge and liquid effluents from kraft paper mills. Kraft mill waste was chosen because of the large number of kraft pulping plants. A process developed to convert kraft mill waste to methane is also expected to be applicable for bioconversion of other types of paper mill wastes. Bioconversion energy could displace up to 30% of the purchased energy in a paper mill. The planned thermophilic two-phase process provides for (1) detoxification, solids hydrolysis, and neutralization in a slowly-stirred upflow first-stage acid reactor and (2) optimization of coupled acetogenic-methanogenic fermentation of the acetogenic end products in a hybrid upflow second-stage reactor containing a stationary packed bed of inclined tubes. The reactors effect enhanced solids retention allowing high- efficiency conversion of feeds at lower hydraulic retention times (HRT) (relative to 20-40 days for conventional digestion of paper mill sludge). Phase I research will focus on demonstration of a stable and efficient operation at a system HRT of about 6 days. Alkali scrubbing of first-stage digester gas and its recirculation to the head space would detoxify the effluent (methane digester feed). Feed neutralization and nutrient retention within the overall system will be effected by recycling methane digester effluents to the first-stage reactor. The digestion system effluent will be treated in a trickling filter with the excess sludge returned to the acid digester. Phase II of the project will be concerned with process scale-up and pilot plant demonstration needed to finalize design and operating procedures for commercial plants.Anticipated Results/Potential Commercial Applications as described by the awardee:Phase I studies are designed to prove the technical feasibility of the planned process in terms of operability without toxicity effect and to provide biomethanation and stabilization efficiencies. At the end of Phase II, design details and commercialization plans will be available. There are about 500 pulp and paper mills in the United States. About 40% of these mills practice kraft pulping. The planned process would be potentially applicable to simultaneous biomethanation and stabilization of effluents and sludges from these mills. The U.S. paper and pulp industry purchases about 2.5 quads/yr. Application of the planned process could in theory displace about 30% of the purchased energy amounting to significant energy savings.
