Abstract:
Pressurised carbon dioxide (P CO2) is a useful technique for non-thermal food sterilisation and has also been reported to enhance biogas production in anaerobic digestion. In the current work, co-settled sewage sludge was pressurised with CO2 at 2800 kPa for 23 hours before daily feeding to a pair of semi-continuous mesophilic digesters over a period of 70 days. The results were compared to those obtained from an identical pair of control digesters fed on the same material but without the pressure treatment. Both cases were compared to the results taken over a 90-day period where all 4 digesters were run on the same feed without any pre-treatment, to establish a baseline for performance and stability. As test parameters the digesters were monitored for specific biogas and methane production, volatile solids destruction and destruction of Escherichia coli. The results showed that the pre-treatment had no enhancement effect on any of these parameters.
Key words:
Pathogen, biogas, cellruptor, faecal indicator organism
Introduction:
Anaerobic digestion (AD) is one of the most favoured technologies for retrieving renewable energy from organic wastes (DEFRA, 2007). Sewage sludge (SS) is a huge bio-resource whose production has been consistently increasing in the UK and elsewhere. Average annual production of SS in the UK during 2006-07 was 1.73 million tonnes (dry weight) (Water U.K, 2008), 82.6% of which was either recycled to agricultural land or used for land reclamation (Water U.K, 2007). AD is the method most frequently used by wastewater treatment plants for the stabilisation of SS (Pérez-Elvira et al., 2006) while also generating renewable energy (biogas) and organic soil conditioner (digestate) (Dohanyos and Zabranska, 2001). However, SS contains a significant proportion of pathogenic microorganisms (Strauch, 1991) that can survive the digestion process (Sahlström, 2006) and therefore potentially pose serious threats to human and animal health when digestate is applied to arable land (Sahlström, 2003). Increased public awareness due to the outbreak of food-borne diseases has intensified the demand for improved biosecurity and food safety. This became the driving factor for an agreement between the British Retail Consortium and UK Water in February 1998, known as ‘The Safe Sludge Matrix’ (SSM), which determines the criteria for safe land recycling of SS. As a result of this agreement, agricultural utilisation of untreated SS has been banned in the UK since 31 December 1999. Moreover, proper sanitisation of SS for the removal of pathogens in line with the quality criteria described in SSM is mandatory for making it safe for land recycling (ADAS, 2001).
There are various techniques, such as thermal, chemical, thermo-chemical, sonication, biological, enzymatic and mechanical methods, for pre-treatment of SS prior to AD (Appels et al., 2008). These are aimed at making the organic matter easily accessible to anaerobic bacteria, and thereby enhancing the digestibility and biogas production as well as decreasing the quantity of sludge for final disposal (Pérez-Elvira et al., 2006). Additional benefits could potentially include improved sludge dewatering, reduction in foaming and enhanced pathogen kill (Pérez-Elvira et al., 2006); but to be considered an ideal method the pre-treatment must also be economic both to install and to operate (Tyagi and Lo, 2011).
The detrimental effect of carbon dioxide (CO2) on bacterial growth was first reported by (Valley and Rettger, 1927), but it is only over the last two decades that P CO2 has become established as a technique for non thermal sterilisation (Garcia-Gonzalez et al., 2007). Its application has, however, been confined to the food industry until recently, when a novel process called ‘Cellruptor’ (initially Bug Buster) was developed by Eco-Solids International (Spooner et al., 2007) and was reported to enhance biogas production by around 30-40 % by using pressurised biogas (35-45% CO2) to pre-treat secondary sludge biosolids before anaerobic digestion. The principle of pressurised CO2 treatment as applied elsewhere is that CO2 dissolves in the aqueous phase of a liquid to form carbonic acid (H2CO3) which diffuses into any microbial cells present. This causes a drop in intracellular pH (Daniels et al., 1985) which affects the enzymatic activities (Jones and Greenfield, 1982 ) and triggers a series of complex interrelated processes leading to the loss of cell viability (Garcia-Gonzalez et al., 2007). Upon decompression, CO2 dissolved in the cytoplasm expands causing the microbial cells to rupture and release their contents (Nakamura et al., 1994). It has been assumed in the case of anaerobic digestion that this cell rupturing process will make the biomass more readily available to anaerobic bacteria, thereby helping to overcome the rate-limiting hydrolysis step of AD (Spooner et al., 2007). The process could therefore boost biogas production and an additional benefit would be the enhanced sanitation of digestate as pathogenic bacteria might also be expected to be inactivated. The present study was carried out to test whether P CO2 pre-treatment has these benefits when applied to co-settled sewage sludge before mesophilic digestion, by testing for improved biogas production and enhanced removal of Escherichia coli as a pathogen indicator organism.
