Benefits of thermophlic digestion at Christchurch wastewater treatment plant

In 2010, two 7,000m³ anaerobic digesters were commissioned at the Christchurch Wastewater Treatment Plant (CWTP), which now operates a Temperature Phased Anaerobic Digestion (TPAD) process – thermophilic and mesophilic in series. This paper outlines the thermophilic commissioning process including difficulties, solutions and contingency plans, and the improvement in digester performance.

From the existing digester arrangement (four off 5,000m³ mesophilic digesters in parallel) to the new TPAD digester arrangement (two off 7,000m³ thermophilic digesters in parallel followed by four off 5,000m³ mesophilic digesters in parallel), the methane production was seen to increase by 67%. From the interim mesophilic operation of the new digesters, it is estimated that two thirds of this increase is due to increased solids retention time, with the remainder due to the increased activity of the thermophilic digesters. About 85% of the biogas is produced in the thermophilic digesters at a typical concentration of 61% methane, compared to 65% from the mesophilic digesters.

Similarly, the volatile solids destruction increased 19% from an average of 65% to 77%, which reduced the dry solids content of the digested sludge from 1.9% to 1.4% and the volatile content of the digested solids from 69% to 62%.