Conversion Strategies and Evaluation of the Transition from Mesophilic to Thermophilic Anaerobic Digestion of Municipal Sludge

The objectives of this study were to assess, at the laboratory scale, several conversion strategies for the transition from mesophilic to thermophilic sludge digestion, compare the performance of mesophilic and thermophilic digesters, and evaluate the impact of thermophilic digestion on sludge dewatering and centrate (filtrate) quality. Primary (PS) and thickened waste activated sludge (TWAS) samples were collected at the F. Wayne Hill Water Resources Center, Gwinnett County, Georgia. A PS/TWAS sludge mixture (20/80% on TS basis) was used throughout this study. Two digesters (R1 and R2) were operated at 36oC and a solids retention time (SRT) of 30 days for 25 days, and the R2 digester temperature was then increased at a rate of 3°C/day to 53.3°C. The daily gas production of R2, after an initial increase and then decrease, increased again and reached a steady state value about 25 days after the first temperature increase. The decrease in gas production of R2 was accompanied by a sharp increase in volatile fatty acids (VFAs), which reached 3,200 mg COD/L and then declined to a steady state value of below 100 mg COD/L. At steady state, the VS destruction in R1 (36°C) and R2 (53.3°C), both at a SRT of 30 days, was 32.4 and 35.5%, and the fecal coliform destruction was 97.9 and 100%, respectively. As the R1 and R2 digesters were operated at SRT values of 20 and then 8 days, the difference in VS destruction between the mesophilic and thermophilic reactor was less significant and the thermophilic (53.3°C) had a much higher concentration of VFAs and soluble COD than the mesophilic (36°C) reactor. Four strategies were used to transition to 60oC digestion. In all these attempts, a relatively stable reactor performance was achieved in terms of gas production, but the VFAs remained at very high levels (in excess of 5,000 mg COD/L) and thus methane production was lower than that of the mesophilic reactor. All reactors operated at 53.3 and 60°C achieved 100% fecal coliform reduction. Thermophilic anaerobic digestion at both 53.3 and 60°C resulted in a significant increase of the time-to-filter relative to mesophilic digestion, which implies a lower dewaterability of the thermophically digested sludge. Therefore, in order to achieve a high degree of VS and COD destruction and methane production, digester temperature should not exceed 58°C. For single-stage digestion, as tested in this study, the only significant benefit of thermophilic digestion was the high extent of coliform reduction, which is justified if production of Class A biosolids is the goal. In order to achieve both high levels of pathogen destruction and methane production resulting in a high digester effluent quality, two-stage, temperature-phased anaerobic digestion is recommended.