Understanding the impact of thermal treatment of sewage sludge on the efficiency of anaerobic digestion process

Shana, A.1, Ouki, S.2, Asaadi, M.3, Pearce, P.1, Mancini, G.2, 1Thames Water, Innovation & Development, UK, 2Surrey University, 3AD Technologies Ltd



The use of Thermal Hydrolysis Process (THP) as an intermediate treatment configuration rather than as a pre-treatment process has the potential to enhance the sludge digestion process efficiency. The purpose of this work was to investigate the impact of intermediate thermal hydrolysis (ITHP) process on sludge digestion process efficiency in comparison with conventional THP process configuration. In this study, two thermal hydrolysis configurations, one with THP as a sludge pre-treatment arrangement and the other with THP as a post sludge digestion thermal hydrolysis and re-digestion process were evaluated. The effect of both sludge hydrolysis configurations on organic matter degradation process kinetics was investigated. The degradation of sludge constituents such as carbohydrate, protein and fat as a result of thermal pre-treatment and ITHP followed the first order kinetics. The sludge organic matter constituents’ degradation kinetics constants indicated that the use of THP as an intermediate configuration can enhance already digested sludge organic matter degradation process, further reducing the sludge mass and increasing its conversion to biogas.

Keywords Sludge, degradation, Kinetics, composition, pre-treatment, post-digestion THP, ITHP, HRT

Introduction Anaerobic digestion of complex, particulate organic material has been described as a multi-step process of series and parallel reactions (Gujer and Zehnder, 1983; Pavlostathis and Giraldo-Gomez, 1991). The anaerobic digestion process steps are catalysed by consortium of bacterial communities. These steps are: (i) Hydrolysis of complex organic materials such as Carbohydrates, proteins, and fats, (ii) Acidogenesis or fermentation of the hydrolysed products to volatile fatty acids (VFA), (iii) Actogenesis, the conversion of long chain VFA to short chain VFAs, and (iv) Methanogenesis processes, the conversion of acetic acid, carbon dioxide and hydrogen into methane (Gujer and Zehnder, 1983; and Sanders, 2001). These steps are interdependent and the performance of one step affects the next. For example, the performance of acetogenic bacteria relies on the performance of the hydrogen scavenging bacteria, (CIWEM, 1996). If Acidogenic bacteria convert the organic mater to volatile fatty acids at a higher rate than the methanogenic bacteria can utilize them, the acidogenesis step itself is inhibited due to toxic waste accumulation in their environment. This in turn reduces the rate of organic matter degradation (CIWEM, 1996).

Preliminary conversion mechanisms such as cell lyses, non-enzymatic decay, and physical breakdown precede the more complex (chemical) hydrolytic steps when a composite organic matter in the sewage sludge is degraded (Batstone et al., 2002). Sludge as a digester feed is often composed of complex biodegradable matter which must be solubilised and broken down into smaller monomers before being assimilated by bacterial cells (Gunnerson and Stuckey, 1986). Furthermore, during anaerobic digestion process, the sludge constituents are solubilised by the bacterial action into chemical oxygen demand (COD). The soluble COD is in turn fermented into volatile fatty acids (Gallert and Winter, 2005; Shana et al., 2012). The volatile fatty acids (VFA) are ultimately converted to biogas by Methanogenic bacteria.

In most studies the hydrolysis of sludge constituents has been considered as an experimental expression which combines the degradation of different sludge constituents under a generic substrate called sewage sludge constituents (Gonzalez, 2006). Several authors noted that during the process of anaerobic digestion of sewage sludge, the methanogenic process is limited by the hydrolysis rate of organic matter; although methanogenesis could in some cases be the rate limiting process, particularly when the digester hydraulic retention time (HRT) is low and there is a risk that methanogenic bacteria could be washed out (Pavlostathis and Giraldo-Gomez, 1991; Vavilin et al., 2008; Batstone et al., 2009; and Donoso-Bravo et al., 2010).

Panter, (2008) stated that the acknowledgement of this problem has led the application of sludge pretreatment technologies in an anaerobic digestion process and this speeded up the sludge hydrolysis process and thus intensification of the process. The THP process makes the sludge digestion process more tolerant to organic matter shock load and improves sludge volatile solid reduction from 30 -45%, to 50 – 60%.

However a recent study reported by Shana et al, (2011) indicated that when two separate mesophilic anaerobic digesters were fed with thermally hydrolysed sludges containing equal amount of volatile solids content (76 – 78% VS), they almost showed similar performance, resulting in similar volatile solids content in their respective digestate (60 – 65% VS). This indicates that 35 – 40% of the potential energy was destroyed during the digestion process 60 – 65% VS remained in the Digestate. Therefore, a substantial amount of the initial biodegradable organic matter is recycled to land, particularly when the digestion process is highly loaded in terms of organic matter loading.

Karia and Chritian, (2006) stated that the first order reaction constant for organic matter (substrate, S) degradation during anaerobic digestion process could be obtained from the rate equation assuming that the rate of reaction proceeds directly proportional to a single reactant concentration.

However, the overall sludge organic matter degradation path could not always be described by single first order kinetics constant as described by Monod, (1949). Krylow and Tal-Figiel, (2003) studied and found that a high organic solid loaded anaerobic sewage sludge process followed stepwise degradation kinetics.

Although Michitsch, (2004), studied the impact of steam injection and digestion process under the name of Supper blue box recycling (SUBBR), the use of THP as a post sludge anaerobic digestion configuration for improved sludge organic matter destruction and the associated biogas production has not been fully investigated. Therefore, the objective of this study was to assess the impact of the ITHP configuration on the digestion efficiency in comparison with conventional THP configuration.

Experimental work was carried out where the biodegradation of several sludge constituents was studied. However, in this paper, specifically, the data on carbohydrates biodegradation kinetics is reported.

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