Performance and Stability of Two‐Stage Anaerobic Digestion

Abstract

The stability, capacity, and solids destruction efficiency of single versus two-stage anaerobic digestion was studied in bench-scale reactors using combined waste activated and primary sludge. Laboratory staged mesophilic digesters showed an improved volatile solids and volatile suspended solids destruction efficiency over a single-stage system (at the same total solids retention time [SRT]) of approximately 3.2 and 5.8 percentage points, respectively. To quantify stability and capacity, a new digester monitoring method was introduced that measured the digester maximum acetate utilization capacity, V max,ac , and was used to investigate the potential for digester instability at different transient loadings. The ratio of the V max,ac value to the estimated acetate production rate for a given digester loading was termed the acetate capacity number (ACN). Values greater than 1.0 indicate excess acetate utilization capacity. The first stage of the laboratory two-stage mesophilic system (10-day SRT for each stage) had an ACN number of 1.3 compared with a value of 1.8 for the single-stage 20-day SRT digester. Thus, while a staged mesophilic system can improve solids destruction efficiency, it demonstrates a lower capacity for metabolizing highly variable loads. Water Environ. Res., 79, 488 (2007).

Anaerobic Digester Operating Conditions Affects Acetate Utilization Capacity

“…The VS loadings to the 1 st stage are higher than the corresponding 2 nd stage, and the loadings to the 1 st stage are approximately double the control digester. The average V max value of the 1 st stage is 43% greater than the control digester, and this result is similar to findings by Jeff Zahler, where a 10 day HRT mesophilic digestion system had 40% greater V max values than a corresponding 20 day HRT digestion system (Zahller 2004). …”

“…Previous work has led to a test method termed V max , a direct measure of the acetate utilization capacity of the methanogenic population in the digester (Zahller 2004). Based on limited samples from full-scale digesters, the acetate utilization capacity measured by V max was found to be only slightly greater than the average acetate production rate estimated from digester loading and volatile solids (VS) destruction (Conklin 2005).…”

An Impact of Iron on the Digestion Efficiencies and Odor Generation of Multi-Stage Anaerobic Digestion System

“…Among those advanced digestion systems, the thermophilic anaerobic sludge digestion at 50-60 o C and its combination with other digestion systems have been the subjects of intensive researches due to greater pathogen and solids reduction capability associated with the high temperature digestion of sludge (Zahler et al, 2007). However, high temperature digestion systems have been reported to cause the process instability associated with pH suppression, emission of odorous compounds, high ammonia production, etc (Kim et al, 2002).…”

Avoiding Digester Upset

“…By improving volatile solids reduction, treatment plants may increase digester capacity. This can be done by using acid-phase digestion (Bhattacharya et al, 1996;Ghosh et al, 1995), temperature-phase anaerobic digestion (Oles et al, 1997) or series mesophilic digestion (McCarthy et al, 2004, Zahller et al, 2007. These operating conditions utilize high, first-stage acetate concentrations that require close monitoring to avoid digester upset.…”

“…Instead, digester stability can be monitored by using the acetate uptake bioassay (AUB). Developed over 25 years ago (Owen et al, 1979;Shelton and Tiedje, 1984), the AUB has been applied in the laboratory elsewhere and at treatment plants to estimate the remaining acetate transformation capacity for digesters (Conklin et al 2006;McCarthy et al 2004;Zahller et al, 2007).…”