| Summary: | The anaerobic digestion (AD) process is an established treatment for sewage sludge produced at municipal wastewater treatment plants. However, water utilities lack guidelines on the effect of upstream processes and AD operational conditions on the overall digestion performance. This is the first research that investigates the interactive effects of different process conditions to determine their overall impact on sludge AD performance. The research aims at determining the impact of physico-chemical properties on the biodegradability of feed sludge; quantifying the impact of multiple AD operational conditions on process performance and digestate quality and integrating the findings into a steady-state model to design, monitor and optimise the AD process of sewage sludge. The Box Behnken method was selected to design the experiment on multiple operational parameters in order to improve the estimation of the regression coefficients. Novel automatic pilot scale digesters were used to conduct uninterrupted digestion experiments for approximately 1,000 days. Together with these an array of lab scale and batch digestion systems were used to provide reference and monitor feed sludge biodegradability throughout the experimental programme. The results showed that measured sludge physico-chemical characteristics did not control bio-methane yield of primary sludge (PS), and PS produced similar biogas yield irrespective of the sludge source. By contrast, physico-chemical and operational parameters impacted the bio-methane yield of surplus activated sludge (SAS). These included sludge physico-chemical conditions (volatile solids content and elemental content) and upstream process conditions (solids retention time in the activated sludge plant). The operational parameters tested were PS:SAS ratio, digestion temperature and feed dry solids concentration (DS%). The results indicated a positive and significant interaction of high temperature and high DS% on specific biogas yield and pathogen removal, which could have significant implications in terms of process optimisation. In terms of reactor stability, the concentration of alkalinity and ammoniacal nitrogen were found primarily increasing with increasing DS% and SAS content, suggesting that operating the digester at relatively high load produces a more stable bio-chemical environment. It was concluded that AD of sewage sludge should be operated between 6-8% DS% at a temperature of 45oC in order to simultaneously produce a stable bio-chemical environment in the AD reactor, maximize biogas production and produce a conventionally treated sludge within a 16 day retention time. The produced steady-state model was used to determine that investments to upgrade an existing large scale sludge AD plant in the UK operating at sub-optimal conditions, could have a payback time of six years due to the benefits arising from displacement of imported electricity and fossil fuels.
|
|---|
