Proceedings

Parallel Digestion of Secondary and Primary Sludge

Winter, P. and Pearce, P., Thames Water Research and Development

(free)

Abstract: 
It is common practice in the UK Water Industry to blend surplus activated sludge (SAS) and primary sludge prior to conventional digestion. Within Thames Water, the majority of the anaerobic digestion plants operate with a blended feed sludge. The problematic of processing these two very different types of sludge is understood however little scientific information is available considering parallel digestion of SAS and primary sludge. The paper is a continuation of research, which suggested that digestion of SAS and primary together potentially compromise biogas production. The paper discusses digester performance criteria and reviews recent literature. Data derived from an improved research digestion facility that was operated with a pumped feed and on-line monitoring of operational parameters are reviewed. Two anaerobic digesters (60 litre volume) were operated in parallel and fed with SAS and primary sludge respectively. The results show that a SAS only digester could produce stable digestion performance with a relatively low biogas yield. Digestion of primary sludge may produce more biogas in comparison; however a primary digester may require close monitoring for early recognition of acidification.
Keywords: Surplus Activated Sludge, Primary Sludge, Anaerobic Digestion
Introduction: 
It is common practice in the UK Water Industry to blend surplus activated sludge (SAS) and primary sludge prior to conventional digestion. Within Thames Water, approximately 60% of the 180,000tds of SAS produced in 2007 were processed at anaerobic digestion plants that operate with a blended feed sludge of SAS and primary. Recent research suggests that digestion of SAS and primary together potentially compromises gas production and that gas production could be increased if SAS and primary were digested separately (Winter et al., 2009). Mininni et al. (2009) suggested that separate treatment of primary and secondary sludge could facilitate sustainable sludge management. Primary sludge accumulates pollutants and could therefore be considered for thermal treatment post digestion, whereas SAS contains the majority of the nitrogen and could be considered for land application.
Surplus Activated Sludge (SAS)
Surplus activated sludge has relatively low degradability, especially that resulting from the operation of activated sludge plants at long sludge ages (Carrrere et al., 2010). Winter et al. (2009) demonstrated this through the potential to produce Volatile Fatty Acids (VFA) from SAS derived from various sludge ages. Bolzonella et al. (2004) suggested a clear relationship between gas production during anaerobic digestion and solid retention time in the activated sludge process. The composition of SAS is fundamentally different to that of primary sludge because the activated sludge process results in biomass composed of microbial and extracellular polymeric substances (EPS). These are a complex mixture of biopolymers comprising polysaccharides, proteins, nucleic acids, uronic acids, humic substances, and lipids, amongst others. EPS is relatively recalcitrant to anaerobic digestion by nature (Carrrere et al., 2010). SAS has a low C:N ratio and may contain 40% protein (Gonzales, 2006). The theoretical gas production from SAS can be assessed based on elemental constituents. Various authors (Mininni et al., 2004; Horan and Lowe, 2008) showed that the biogas potential from SAS is relatively high, between 0.767 – 0.868 Nm3/kg VS, considering that the digestibility of SAS is commonly perceived as poor.
Primary Sludge
Primary sludge comprises of settleable solids derived from primary settlement tanks. It is mostly organic matter that is highly putrescible. Typically, primary sludge has a higher C:N ratio than secondary sludge. Gonzales (2006) showed that a primary sludge studied contained 17% protein but 27% carbohydrates. Biogas production from primary sludge could be between 0.842 – 0.968 Nm3/kg VS (Mininni et al., 2004; Horan & Lowe, 2008). However, Horan & Lowe (2008) suggest that sewage sludges are in general a poor feedstock for anaerobic digestion because it contains insufficient carbon and too much nitrogen. Barber & Lancaster (2009) calculated a theoretical gas yield from sewage sludge at 35oC as 1.07 m3/kg VS destroyed.

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