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Performance and Stability of Two-Stage Anaerobic Digestion

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The performance of mesophilic staged digestion and thermophilic-mesophilic staged digestion was studied in bench-scale reactors and pilot plant reactors, respectively, using combined waste activated and primary sludge. The objective of this research was to determine the enhanced solids destruction efficiency from a two-stage mesophilic digestion system compared to a single-stage mesophilic digester operated at the same total SRT of 20 days. A new digester monitoring method that measured the digester maximum acetate utilization capacity, Vmax,ac was investigated to evaluate the potential for digester stability at different transient loadings. The ratio of the Vmax,ac value to the estimated acetate production rate for a given digester loading was termed the acetate capacity number (ACN). Values greater than 1.0 indicate excess acetate utilization capacity. The laboratory staged digester had an improved VS and VSS destruction efficiency over the single-stage system by approximately 3.2 and 5.8 percentage points, respectively, which was statistically significant. Biogas quality also improved in both stages of the two-stage system over the single-stage digester, with 67-73% methane content from the two-stage system versus 62% from the single-stage. Pilot plant testing of a TPAD process indicated similar improvement in performance due to using staged digesters. The first stage of the 20-d SRT equally staged system had an ACN number of about 1.3 compared to a value of 1.8 for the single-staged 20-d SRT digester. Thus, while the staged system operation can improve solids destruction efficiency, it has a lower capacity for higher variable loads.
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Document Type: Research Article

Publication date: 2005-01-01

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