Anaerobic digestion involves a consortium of bacteria, with the degradation of complex particulate sludge solids being described as a multi-step process of serial and parallel reactions. Acid phase digestion (APD) or Enzymic Hydrolysis (EH) separates out the hydrolysis and acidogenesis stages from the methanogenic stage, providing optimal conditions for hydrolysis and acidification. The different microbial groups have different environmental and nutritional requirements, and this is the fundamental premise for enzymic hydrolysis or two stage acid phase digestion. Hydrolysis, acidogenesis, and acetogenesis proceed faster in an acidic environment, and methanogenesis proceeds faster in a neutral environment. Under these conditions, hydrolysis is no longer the rate-limiting reaction, and digestion becomes more efficient.
In 2007, Wessex Water installed an APD plant upstream of six existing conventional Mesophilic Anaerobic Digesters at their wastewater treatment works at Avonmouth, Bristol to maximise the generation of renewable energy at the site. At the time of construction it was the largest advanced digestion plant in the UK.
This paper discusses the integration and optimisation of the APD plant with the methane phase plant at Avonmouth.
Key Words: Acid Phase Digestion, Methane Phase Digestion, Advanced Anaerobic Digestion, Biological Hydrolysis, Enzymic Hydrolysis, Two-Phase Digestion.
The Sludge Treatment Centre (STC) at Avonmouth treats a mixture of indigenous primary and secondary sludge, imported liquid municipal sludge, and imported liquid commercial waste. The STC treats the sludge using Mesophilic Anaerobic Digestion (MAD), before recycling to agriculture. The STC has undergone a number of expansions over the last few years. In 2007, Wessex Water installed an Acid Phase Digestion (APD) plant upstream of six existing conventional Mesophilic Anaerobic Digesters at the STC at Avonmouth, Bristol to maximise the generation of renewable energy at the site, and to produce a conventionally treated sludge. At the time of construction it was the largest advanced digestion plant in the UK.
Anaerobic digestion is unique amongst current treatment technologies in that it stabilises sludge, reduces volume and odour, and generates biogas that can be used as a renewable energy source. The flow sheet for a conventional anaerobic digestion plant is shown in Figure 1 below, and is a fair representative of the STC at Avonmouth before the acid phase pre-treatment stage was installed.