A Tale of Two Sludges – Trialling Operational Temperature Control of Anaerobic Digester
Annual Conference
❄
It was the best of slimes, it was the worst of slimes…
Anaerobic digestion is a commonly used method of solids stabilization in municipal wastewater treatment plants. The Christchurch Wastewater Treatment Plant (CWTP), owned by Christchurch City Council (CCC), operates a Temperature-Phased Anaerobic Digestion (TPAD) system, wherein primary sludge and secondary sludge are combined and then digested in a 55°C thermophilic stage followed by a 35°C mesophilic stage.
This paper describes a trial investigating diverting cold secondary sludge directly to the mesophilic digesters to provide cooling. The full scale trial was conducted on a single mesophilic digester (Digester 4), to determine the viability of this approach before full implementation. The trial involved monitoring the performance of Digester 4 to assess whether the change in feedstock negatively impacted digestion or downstream processes. Parameters monitored to assess performance included: biogas production, methane concentration, hydrogen sulfide concentration, volatile and total solids, pH, and various volatile acids.
The trial results show that supplementary cooling of the mesophilic digesters using secondary sludge did not negatively affect the performance of Digester 4. A general increase in the rate of volatile solids destruction from 17% up to 30 – 45% was observed for Digester 4, indicating that the microbial population responded positively to the adjusted feedstock.
Planning is now underway to convert all four mesophilic digesters over to secondary sludge cooling. As a result, the aging spiral heat exchangers will not be replaced providing a CAPEX saving in the order of $1 million and increased on-site bore water availability.