Experimental and Modeling Study of a Two-Stage Pilot Scale High Solid Anaerobic Digester System

E-mail: chens@wsu.edu

An increasing attention has been paid to anaerobic digestion (AD) for converting the organic fraction of municipal solid wastes (OFMSW) to renewable energy and reducing environmental impact. Since OFMSW is easily degradable but tends to acidify quickly inhibiting the digestion process in conventional digester designs, a new two-stage AD technology was developed to stabilize this process and enhance bio-methane productivity. Bench-scale and pilot-scale experiments have been done to treat food wastes operated within a total solids range of 15-20%. Effluent recirculation is critical to realize this process for providing mixing, pH control, and seeding of anaerobic microorganisms to the high solid digester. Therefore, a model involving transport and reaction kinetics is needed to understand and assess the effect of the effluent recirculation on the digester performance.

In this study, the model concept was developed from the IWA Anaerobic Digestion Model No.1 (ADM1). So far, the ADM1 model contains the most complex biochemical reaction kinetics in anaerobic digestion such as hydrolysis, equilibrium, substrate uptake and inhibition. We put this model into the two-stage AD system which includes a high solid digester and a liquid digester. The effluent was recirculated from liquid digester to high solid digester. The high solid digester was assumed to be a combination of continuous stirred-tank reactor (CSTR) and plug flow reactor (PFR). The liquid digester was assumed to be a PFR where upflow anaerobic sludge blanket reactor (UASB), a high rate digester, was used to retain more methanogens and maintain pH at the range of 6.0-8.0 in the whole system. The model was verified by the data from the pilot scale experiments and parameters in the kinetics models were corrected. Results show that the optimized effluent recirculation rate could be predicted in which the whole system was under well operation. The balances among the volume ratio of the solid digester and the liquid digester, bacteria wash out velocity in the liquid digester and pH maintenance capacity by methanogens could be determined to lower the operational cost and enhance bio-methane productivity. Overall, these results will facilitate the understanding, design and optimization to promote the commercialization of the presented high solid anaerobic digester system in the next step.