This study was undertaken to evaluate the performance of spherical Chinese digesters receiving dairy waste. Sixteen laboratory-scale (3L) reactors were operated in the mesophilic temperature range (35°C±2°C) and were subjected to hydraulic retention times (HRT) of 8, 10, 15, and 20 days and influent substrate concentrations (S₀) of 40.1, 57.8, 62.3, and 71.2 g BVS/L. Preliminary tracer experiments were performed to identify digester flow characteristics. A two-microbial culture mathematical model incorporating incomplete mixing and intermittent feeding was developed to predict effluent quality and gas production.

Stimulus-response data indicated that by positioning the outlet at the bottom of the digester, flow characteristics could be improved, leading to reductions in dead space and bypassing volumes by 47% and 91%, respectively. Digesters operated at HRT's of 10, 15, and 20 days performed normally. In these units, for a given S₀, as the HRT was increased the volumetric methane production decreased while the unit methane production increased. The improvement in effluent quality was substantial when the HRT was increased from 8 to 10 days, and was modest when the HRT was increased beyond 10 y days. The 8-day digesters were inhibited due to overloading. pH drops in these digesters were concurrent with alkalinity deficit. The optimum values for operational parameters were found to be an HRT of 10 days, and a loading rate of 7.12 g BVS/L-d.

The two-culture mathematical model predicted gas yield and effluent concentration under retarded and normal digester operations. The three-step feed model best described digester activity following feed addition. The sensitivity analysis revealed that the digester model was decreasingly sensitive to the inhibition coefficient of methane-formers (KI2), the inhibition coefficient of acid-formers (KI1), the half-velocity concentrations (KSl, KS2), and the flow-parameters (A, B).