Laboratory studies on the temperature-phased anaerobic digestion of mixtures of primary and waste activated sludge

The temperature-phased anaerobic digestion (TPAD) involves the operation of two reactors in series, the first maintained at a thermophilic temperature (commonly 55° C) and the second maintained at a mesophilic temperature (commonly 35° C).;The purpose of the research was to compare the performance of the temperature-phased system with the conventional single-stage mesophilic system for treating mixtures of primary sludge (PS) and waste activated sludge (WAS). The TPAD systems used in the study consisted of two, completely-mixed reactors operated in series. The first and the second stages were operated at 55° C and 35° C, respectively. The single-stage system was operated at 35° C.;The reactors were fed 50-50 volumetric mixtures of PS and WAS. The WAS had an approximate total solids (TS) content of four percent. The TS content of the PS varied in a range from 3 to 5%, but was adjusted to 4% by either dewatering or dilution. The systems were operated as completely-mixed reactors over a range of solids retention times (SRT) from 11 to 40 days. The minimum and maximum SRTs in the thermophilic first-stage was one to eight days, respectively;The single-stage mesophilic unit achieved volatile solids (VS) destructions ranging from 32% at the 24-day SRT to 47% at the 40-day SRT. VS destruction for two-stage System A varied from 45% at the 14-day SRT to 50% at the 28-day SRT. For two-stage system B, VS destruction varied from 34% at the 11-day SRT to 45% at the 24-day SRT;For the same VS removal, the SRT required for the two-stage system was only 40% of that require for the single-stage system. In other words, the capacity was more than doubled by using the TPAD system without any deterioration in VS destruction. The increased capacity of the TPAD system was due to the much higher reaction rate achieved at the higher temperature in the first thermophilic stage. The biogas production per unit volume of the TPAD system was higher in proportion to the increased VS destruction in the TPAD system, as compared with the single-stage mesophilic system;The TPAD systems greatly outperformed the single-stage system in terms of both total and fecal coliform destruction. The single-stage mesophilic system achieved only a one log or less reduction in fecal coliforms. In contrast, the TPAD system achieved an average six log reduction in fecal coliforms. The effluent fecal coliform content was always far below 1000 MPN/g TS. Serious foaming in the single-stage was observed in an SRT range from 24 to 34 days. Nearly no foaming existed in both TPAD systems at all SRTs studied;It is concluded that the TPAD system for sludge digestion combines the advantages of thermophilic digestion (higher rates of VS destruction, high coliform destruction, and reduced foaming) with the advantages of mesophilic digestion (lower volatile fatty acids and a less odorous digested sludge) while eliminating the disadvantages of both the thermophilic and mesophilic anaerobic digestion