Significant advances have been made in the knowledge base and practical options for improving thermophilic anaerobic digestion over the past 10 to 15 years. Today, more options exist that promise to enhance the degree of stabilization achieved and finished product quality in new and retrofitted existing processes. In parallel, the body of knowledge regarding pathogen inactivation in these systems has also been advanced, well beyond that available at the time that the 40 CFR Part 503 regulations1were promulgated. This paper summarizes recent work with thermophilic anaerobic systems. This paper is a literature review and comparison of recent publications and work from a number of sources.
DEVELOPMENT OF THERMOPHILIC ANAEROBIC DIGESTION
Thermophilic anaerobic digestion of municipal wastewater sludge has been investigated from the 1920s onward, initial work being completed at laboratory scale. However, full-scale work began by the 1940s and 1950s.
- Garber’s work at the City of Los Angeles in the 1950s and 1970s (Garber et al, 1975)2 was aimed primarily at improved dewatering, but the work indicated process concerns and instability at temperatures above about 49 degrees C.
- Popova and Bolotina (1963) 3identified thermophilic digestion for Moscow (Russia). The primary purpose was better pathogen reduction and operation at lower Solids Retention Time (SRT) than mesophilic digestion.
- Research by Goluecke (1958)4 and Malina (1961)5 was instrumental.
- Chicago work (Rimkus et al, 1982)6 at the Stickney Plant showed thermophilic digestion success at limited SRTs and the ability to reduce digester foaming.
By the late 1980s, thermophilic anaerobic digestion was generally thought to achieve greater Volatile Solids Reductions (VSRs) than comparable mesophilic digestion, and perhaps be operated at reduced SRTs. However, there was major concern for process stability and concern for product odor. Dewatering recycle characteristics were known to be strong. Pathogen reduction benefits were recognized; however the US EPA had not recognized thermophilic anaerobic digestion as a PFRP – Process to Further Reduce Pathogens.
By the 1990s, the situation began to change significantly. Work in Germany showed that thermophilic digestion worked well, especially in combination with mesophilic digestion (i.e., thermophilic, then mesophilic, staged digestion), and that thermophilic temperatures of 55 degrees C provided stable operation. At Vancouver, Canada, the Greater Vancouver Regional District (GVRD) operated its Lions Gate Plant first with single-stage thermophilic digestion, then with 2-stage thermophilic digestion and showed the significant disinfection improvement (and disinfection reliability) with a 2-stage configuration (Krugel et al, 1998). 7 Work by Professor Richard Dague at Iowa State University in the mid-1990s showed that thermophilic operation at 55 degrees C was quite reliable and, in combination with a second stage mesophilic digester, produced a high-quality, well-stabilized product with low odor (Han and Dague, 1996).8
Engineers working for GVRD then designed, in the mid-1990s, the Annacis Island Plant digestion facility using 4-stage thermophilic digestion and modeled the pathogen reductions based on the US EPA’s research work for Class A biosolids that had evolved from development of the Part 503 regulations. By 1999, this facility proved that it could produce a continuous, Class A digested (and dewatered) product that was not odorous and operated in a stable and reliable manner at 55 to 56 degrees C (Schafer et al, 2002). 9 Total system SRT was about 25 days.
With the publication of the US EPA’s Part 503 rules in 1993 and especially the time/temperature equations that could be used to produce Class A biosolids, engineers developed various process configurations to meet the requirements. There was some confusion initially about the time/temperature equations until it was recognized that they were developed with the intention that every particle of sludge was to be subjected to the time/temperature requirements (i.e.,
operated in a batch or plug-flow arrangement).
The thermophilic process options and configurations for wastewater sludge in the US are largely driven at this time by the desire to achieve Class A digestion and Class A biosolids. The most common approaches that have evolved are summarized in Figure 1, although there are several variations on these concepts. However, some agencies have implemented thermophilic digestion, or more normally the thermo-meso staged arrangement, to achieve greater VSR, more gas production, and a more stable digested product (i.e., not necessarily a Class A objective).
Pre-pasteurization of sludge prior to anaerobic digestion, is a proven PFRP process, but is not discussed in this paper because the pre-pasteurization process is usually conducted at higher temperatures (65 to 70 degrees C) which are not conducive to a biological anaerobic digestion process. This paper evaluates only thermophilic anaerobic digestion processes that are successfully operated with a viable thermophilic anaerobic biological population.