Proceedings

The objective of this project was to measure the performance of mesophilic anaerobic digestion (MAD) of mixed primary and secondary sewage sludge at different temperatures to maximise the efficiency and performance, and describe the behaviour of MAD using numerical modelling. Three laboratory scale digesters were operated at temperatures of 35, 37 and 39°C. Biogas quantity and composition, temperature, and pH were recorded by continuous on-line monitoring. Additionally, dry solids (DS) and volatile solids (VS) were measured. Absolute and specific gas production increased by 22% and 7%, respectively, with increasing digestion temperature. However, there was no significant effect of temperature on VS destruction. An empirical numerical model based on the Gompertz function provided a statistically representative description of MAD behaviour at different temperatures. The study provides quantitative evidence of the benefits of increasing the mesophilic temperature regime on the rate of anaerobic digestion and gas production. The impact of optimising digester performance on renewable energy generation and carbon footprint of a sewage treatment works (STW) is illustrated.

Keywords: Anaerobic digestion, biogas, gas yield, mesophilic, sewage sludge,

Introduction: Sewage sludge is an unavoidable and essential by-product of urban wastewater treatment. The production of sewage sludge in the UK is approximately 1.5 M t/y DS, and represents 15% of the total sewage sludge production in the EU. Anaerobic digestion (AD) is the most common form of sewage sludge treatment process adopted in the UK and approximately 60% of sludge production is treated by this process (Defra, 2011). Anaerobic digestion of sewage sludge provides the appropriate treatment to avoid nuisance and mal-odour, and to reduce vector attraction as well as the pathogen content of sludge. Furthermore, AD can recover valuable nutrients and produces biogas from the treatment of sludge providing a source of renewable energy.

Thames Water is a major producer of renewable energy in the south of England with a self generation from AD of approximately 170,000 MWh annually. Thames Water operates the majority of its anaerobic digestion at mesophilic temperatures in the region of 37-39 °C. Recent evidence from full-scale operation suggests that an increase in operational temperature from 35o C to 38o C increases biogas production and provides better pathogen reduction (Figure 1). This suggested that increasing the temperature of MAD might improve the treatment efficiency and stabilisation of treated sewage sludge. However, these observations required substantiating and quantification by carefully controlled laboratory investigations of the AD process at different mesophilic temperature regimes.