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Volatile solids reduction is used to measure the efficiency of an anaerobic digestion system and provides a measure for methane gas production. Higher volatile solids reduction also relates to less solids production and reduces recycling/disposal costs. Use first order kinetics for volatile solids reduction (i.e., hydrolysis) to evaluate the following digestion alternatives.A TPAD digestion system is being considered as well as single stage mesophilic digestion for a municipality. The following data applies to the design concept:

Influent Total solids (TS) loading concentration: 5.0% Influent Volatile solids (VS) concentration: 4.0% Non-degradable VS in feed 20% of VS (1st order kinetic calculation applies only to the biodegradable VS) Solids feed flow to digester: 473.5 M3/day Kh_1 for mesophlic digestion VS reduction: 0.326 day-1 Kh_2 for thermophilic digestion VS reduction: 0.345 day-1 Thickened solids concentration for TPAD 6.0% TS Thickened solids concentration for mesophilic: 5.5% TS Gas Production 0.93 M3/KG VS destroyed Polymer cost per feed dry ton for mesophilic: $0.027/dry KG Polymer cost per feed dry ton for TPAD: $0.035/dry KG Disposal cost per wet volume hauled: $6.08/ M3 Energy cost recovery from gas utilization $0.07/M3 gas produced

Evaluate the operating cost difference for the two options at the following total detention times: 15 days for TPAD and 25 days for single stage mesophilic. For the TPAD system, the volume of the thermophilic first stage will be 50% of the volume of the mesophilic second stage. If the construction cost of the two options is approximately the same, which might seem to be the most cost-effective?

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