Abstract
The impact of two sludge treatment configurations on sludge biochemical composition, particularly sludge carbohydrates, proteins and associated biomass related extracellular polymeric substances (EPS) and soluble microbial products (SMPs) were investigated. The sludge treatment configurations used were: the Intermediate Thermal Hydrolysis Process (ITHP) which consists of conventional mesophilic anaerobic digestion (MAD) followed by thermal hydrolysis process (THP) and conventional THP configuration. In this study, semi-continuous digestion experiments were conducted over an 18 months period during which digester input and output samples were collected and analysed for EPS and SMP concentration. The data showed that the total carbohydrates, proteins and associated biomass related EPS and SMP were partitioned in a greater extent in the hydrolysed sludge feed and in the digestates produced from the ITHP and THP configurations. Furthermore, the results showed enhanced conversion of EPS to SMP in the ITHP configuration in comparison with the THP configuration. These findings reinforced the increased biogas yield and organic matter conversion trends from the long term experiments conducted as part of a wider investigation carried out; clearly indicating that enhanced anaerobic sludge digestion process efficiency due to the impact of the ITHP.
Keywords
Anaerobic digestion, thermal hydrolysis, EPS, SMP, Carbohydrates, Lipids, kinetics
1. Introduction The Extracellular Polymeric Substances (EPS) in sewage sludge, a part of sludge biochemical composition (carbohydrates and proteins) is believed to be a factor for poor anaerobic digestion of sewage sludge and its subsequent dewatering. There are two types of sludge biochemical composition related EPS involved in the sludge anaerobic digestion process. These are activated sludge proportion of digester feed input and the EPS released in the digesting sludge due to microbial metabolism, self-protective reaction and cell lysis (Grady et al., 1999 cited in Subramanian, 2004, Azami et al., 2012, Zuthi et al., 2013). Several authors have attributed the problem of sewage sludge digestion and dewatering to the existence of extracellular polymeric substances in the activated sludge and digested sludges (Urbain et al, 1993 cited in Alam, 2013, Laspidou and Rittmann, 2002a; Garnier et al., 2005; Le and Yang, 2007; Kepp et al., 2009).
According to Azami et al., (2012), Zuthi et al., (2013) EPS is composed of proteins and polysaccharides. Often the proteins contents of EPS remain unaffected during anaerobic digestion process and is one of the causes of odour from sludge (Subramanian, 2004), which is a sign of an incomplete digestion process. Laspidou and Rittmann, (2002b) further stated that the predominant component of activated sludge is proteins related EPS and that the hydrolysis of EPS produced biomass associated product.
Moreover, during biological wastewater treatment, the organic matter is consumed by bacteria and in this process both EPS and Soluble Microbial Products (SMP) are formed (Parkin and McCarty, 1981; Noguera et al. 1994, Barker and Stuckey, 2001 cited in Aquino and Stuckey, 2008). Often, EPS and SMP are excreted during bacteria growth, decay or in their response to changing environmental conditions (Janus and Ulanicki, 2010) and as a result of anaerobic digester organic shock load; sudden changes in digester temperature and HRT or toxicity (Aquino and Stuckey, 2008). Moreover, Frolund et al (1996) cited in Neyens et al., (2004) showed that about 80% of total mass of activated sludge is EPS. A number of research reported by Laspidou and Rittmann, (2002a), Aquino and Stuckey (2008), Aquino, (2004) cited in Zeng et al., 2010), identified that the free EPS surrounding the activated sludge flocs as SMP. For example, Reid et al, (2008) described SMP as soluble cellular components which were released during cell lysis. The studies of Ji et al, (2010) cited in Zhou et al. (2012) later confirmed this statement by Reid et al, (2008) when they used ultrasound as a sludge pre-treatment process and observed the conversion of EPS in to SMP.
According to Neyens et al., (2004) and Ni et al., (2011) activated sludge is an amalgamation of dead and live body of bacteria that is composed of gelatinous and slimy material, a substrate bridged with EPS and that provide adhesion of sludge flocs and maintain floc structure. For example, the EPS forming highly hydrated biofilm matrix in where the microorganisms are embedded (Figure 1a), and it is located at or outside the cell surface (Laspidou and Rittmann, 2002b). Figure 1b shows the impact of thermal hydrolysis pre-treatment of sewage sludge on the sludge EPS content where the cell contents were burst open and surrounding EPS layer was ruptured.
