Enhancing biogas production with combined ultrasound and enzymatic hydrolysis treatment

Negral, L., Marañón E.*, Fernández-Nava Y. and Castrillón L., University Institute of Industrial Technology of Asturias, University of Oviedo, Spain



One of the aims of the “END-O-SLUDG” EU FP7 project focuses on the pre-treatment of sludge to enhance biogas production in anaerobic digestion. Different pre-treatments were studied within this context. This paper presents the results of applying ultrasound cell disruption (24 kHz, 3500 – 20000 kJ/kg TS) and Enzymatic Hydrolysis (42°C, 48 hours) as single treatments and in combination. Tests were performed on primary, secondary and combined sludge from two wastewater treatment plants. Each pre-treatment was followed by 24 hours of fermentation at 37°C to monitor the short-term improvement in performance. The results are discussed in terms of the variations in soluble chemical oxygen demand (sCOD) and ammonia. An increase in sCOD is interpreted as enhanced conversion of the organic load and hence greater energy recovery since the sCOD provides the carbon source for the microorganisms that produce biogas. As this parameter increases the level of recalcitrant compounds is also reduced accordingly as a consequence of the pre-treatments prior to sludge digestion. Ammonia is another parameter considered an indicator of biomass degradation, particularly of proteins.

Key words

Sewage sludge, ultrasound, enzymatic hydrolysis, COD, ammonium, anaerobic digestion


Methane production occurs in nature as a result of the degradation of organic matter by anaerobic microorganisms. This process may be intensified and biogas collected when it takes place in chemical reactors (Duda and Oliveira 2009; Sperling and Oliveira 2009; Castrillón et al. 2011). Depending on the operational conditions of the reactor, there is a list of parameters such as temperature, acidity, alkalinity or the substrate type that need to be controlled to favour the bacterial metabolism enhancing biogas production (Barbosa Correa et al. 2003). The theoretical methane yield reaches its maximum with 0.35 l per gram of degraded COD (Wu et al. 1993). A variety of strategies may thus be developed to improve the biodegradability of a substrate and achieve this target value. An upgrade in the methane yield may be achieved either by selecting a culture of microorganisms (e.g. operating under thermophilic rather than mesophilic conditions) or by pre-treating the substrate prior introducing it into the reactor. Chemical, biochemical (e.g. enzymatic hydrolysis intensification) and mechanical (e.g. ultrasound disruption) techniques are used to condition the feed (Bougrier et al. 2006; Bruni et al. 2010).

An idea of the rapid availability of a substrate to microorganisms is given by the sCOD. The sCOD reports about organic matter which can be easily withdrawn from the environment by bacterial blankets. Nevertheless, many raw substrates present total COD values much higher than sCOD. This situation would seem to indicate that these substrates have a practical limitation. This particular case refers to the sludge from wastewater treatment plants (WTP). Sludge is a biological waste mostly made up of bacteria whose cell walls avoid the release of nutrients to the environment and, consequently, the feed cannot be withdrawn by biogas producers in reactors. This approach is managed by quick tests that check the increase in sCOD after applying a pre-treatment to the feed. The sCOD is measured just after the pre-treatment and one day later so as to monitor the short-term evolution under anaerobic conditions and the same temperature as in the potential reactor. Although the principal concern of the pre-treatments applied to substrates rich in bacterial walls is to break up these structures, ammonium nitrogen (NH4-N) gives an idea of the degree of protein degradation (Broderick 1987; APHA 1998). These tests are not the traditional biodegradability tests monitoring biogas production; however, parameters related to biogas production are controlled, while saving both time and costs. This concept has been employed here and the results of pre-treated sludge from two WTP are reported. The tested pre-treatments comprise ultrasound disruption, enzymatic hydrolysis (EH) through the novel technique of Inverted Phase Fermentation® (IPF) and combinations of these techniques.

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