Temperature phased anaerobic digestion increases apparent hydrolysis rate for waste activated sludge

It is well established that waste activated sludge with an extended sludge age is inherently slow to degrade with a low extent of degradation. Pre-treatment methods can be used prior to anaerobic digestion to improve the efficiency of activated sludge digestion. Among these pre-treatment methods, temperature phased anaerobic digestion (TPAD) is one promising method with a relatively low energy input and capital cost. In this study, an experimental thermophilic (50–70 °C)–mesophilic system was compared against a control mesophilic–mesophilic system. The thermophilic–mesophilic system achieved 41% and 48% volatile solids (VS) destruction during pre-treatment of 60 °C and 65 °C (or 70 °C) respectively, compared to 37% in the mesophilic–mesophilic TPAD system. Solubilisation in the first stage was enhanced during thermophilic pre-treatment (15% at 50 °C and 27% at 60 °C, 65 °C and 70 °C) over mesophilic pre-treatment (7%) according to a COD balance. This was supported by ammonia–nitrogen measurements. Model based analysis indicated that the mechanism for increased performance was due to an increase in hydrolysis coefficient under thermophilic pre-treatment of 60 °C (0.5 ± 0.1 d−1), 65 °C (0.7 ± 0.2 d−1) and 70 °C (0.8 ± 0.2 d−1) over mesophilic pre-treatment (0.2 ± 0.1 d−1), and thermophilic pre-treatment at 50 °C (0.12 ± 0.06 d−1).

Research highlights
► Temperature phased anaerobic digestion (TPAD) is a leading process for biosolids pre-treatment, and is a highly active research area, but has not been systematically assessed. This paper has included the study of this area.
► The enhanced VS destruction and methane production were demonstrated in a continuous thermophilic (60–70 °C)–mesophilic (35 °C) TPAD system against a control for waste activated sludge.
► Model based analysis indicated that the increased performances were due to an increased apparent hydrolysis rate, rather than an increase in degradability.