Effect of Varying Solids Concentration and Organic Loading on the Performance of Temperature Phased Anaerobic Digestion Process

The effect of varying total solids (TS) and volatile solids (VS) concentrations and organic loading on the performance of a temperature phased anaerobic digestion (TPAD) system treating a mixture of primary and waste activated sludges was evaluated. An optimum volatile solids destruction of 61.5% occurred at a feed concentration of 4.9% (corresponding to 3.8% VS concentration) in the system operated at a total detention time of 20 days. At a total solids concentration of 7.9% (5.8% VS concentration), the volatile solids destruction efficiency dropped to 52.5%. At all conditions (4.4 to 7.9% TS) the TPAD system was able to meet the requirements for Class A biosolids, including those for fecal coliform and volatile solids destruction. The effluent fecal coliform concentration never exceeded 628 most probable number (MPN)/g TS. Thermophilic biomass activity tests were run on both the thermophilic (55 °C) and the mesophilic (35 °C) sludge. Biomass from the thermophilic reactor showed much greater activity at 55 °C than at 35 °C. However, significant activity was still present when the test was run at 35 °C. Activity tests completed on samples from the mesophilic reactor also had high activities at 55 °C, sometimes equal to the activity of the thermophilic biomass. These results suggest that the bacterial consortia in the TPAD system may be temperature-tolerant and not necessarily two distinct communities with two distinct temperature regimes as had been previously assumed.