Anaerobic digestion of slaughterhouse waste: Main process limitations and microbial community interactions
Introduction
In previous decades, solid slaughterhouse wastes were usually treated by rendering process, providing the slaughterhouses with a valuable source of income. In recent years, because of BSE (bovine spongiform encephalopathy) the economic value of these materials has been substantially reduced and in many cases they have to be disposed off as waste (EC no 1774/2002). Further modifications to European Parliament and of Council Regulations regarding disposal and uses of animal by-products (EC no 92/2005) allow biogas transformation if approved pre-treatments are applied, depending on the by-product category (according to the sanitary risk: pasteurization, high pressure and temperature or alkaline pre-treatment).
Slaughterhouse wastes are characterized by a high organic content, mainly composed of proteins and fats. In fact, little information is available on quantification, characteristics and treatment options of animal by-products and wastes from slaughterhouses. Tritt and Schuchardt (1992) and Edström et al. (2003) reported the first reviews of material flows and treatment strategies in German and Swedish pig and cattle slaughterhouses, respectively. Recently, Hejnfelt and Angelidaki (2009) have characterized individual fractions of Danish piggery by-products and determined their potential methane yields. Contrary, more reports in the literature concern the anaerobic treatment of slaughterhouse wastewaters. The increased automation in carcasses dressing together with the incorporation of washing at every stage (scalding, bleeding, evisceration and tripe treatment) have increased water consumption in slaughterhouse facilities. Wastewaters are normally subjected to a primary treatment which generally includes the use of screens, settlers and fat separators. Some slaughterhouse wastewater treatment plants (WWTP) have a secondary anaerobic reactor, usually based on upflow anaerobic sludge blanket (UASB) or expanded granular sludge bed (EGSB) reactor systems, due to the high organic content of the intake. Several successful experiments at laboratory, pilot and industrial scale with such wastewater and reactor configurations are reported in the literature (Kim and Shin, 2010).
The high fat and protein content mean that slaughterhouse waste can be considered a good substrate for the anaerobic digestion process, due to its expected high methane yields. However, slow hydrolysis rates and inhibitory process have been reported. In particulate materials that are difficult to degradable, such as animal by-products, hydrolysis must be coupled with the growth of hydrolytic bacteria, and this factor can limit the overall degradation rate (Vavilin et al., 2008). Furthermore, lipids can cause biomass flotation and wash-out; and during lipid hydrolysis by extracellular lipases, long-chain fatty acids (LCFA) are produced. These intermediate products have been described as inhibitory species (Rinzema et al., 1994). Also, ammonia is generated during protein degradation and its inhibitory effects on anaerobic digestion (in the form of NH3) has been reported elsewhere (Hansen et al., 1998). For all those reasons, and due to the difficulties of its digestion as a unique substrate, large-scale anaerobic digestion experiences with animal by-products consists of their co-digestion with other industrial, agricultural or domestic waste in centralized biogas plants (Angelidaki and Ellegard, 2003).
A compound may be judged inhibitory when it causes inhibition of microbial growth or affects microbial activity, often resulting in a shift in the microbial population (Plugge et al., 2010). Nevertheless, the toxicity of a given substance to microorganisms can be reduced significantly by promoting biomass adaptation. Some notable strategies have been developed in view of LCFA or NH3 inhibition. The addition of easily degradable co-substrates (Kuang et al., 2006) or the adoption of recurrent pulse feeding strategies (Sousa et al., 2007, Schnürer and Norberg, 2008) demonstrated the possibility of a higher population tolerance to these inhibitors. Currently, data regarding microbial populations and potential syntrophic associations for different compositions of complex organic waste, such as slaughterhouse waste, are scarce; but they are essential to gaining a better understanding of inhibition mechanisms. Although recent advances in molecular microbial ecology allow a better understanding of the specific microorganisms that are involved in syntrophic acetogenesis–methanogenesis (Hatamoto et al., 2007, Schnürer and Norberg, 2008, Sousa et al., 2009), there are still few studies focused on eubacterial and archaeal population, or on diversity and evolution in reactors fed with real complex organic waste.
The aim of this work is to study the anaerobic biodegradability and the methane potential of representative real fresh mixtures of pig/cattle slaughterhouse waste, in order to identify the main process bottlenecks and the potential inhibition mechanisms, as well as to study the microbial community structure under successive waste treatments at increasing concentrations.
Section snippets
Analytical methods
Total solids (TS), volatile solids (VS), suspended volatile solids (VSS), total Kjeldhal nitrogen (TKN), ammonia nitrogen (NH4+–N), fat content (Soxtec), pH and chemical oxygen demand (COD) were determined according to standard methods (APHA, AWA, WEF, 1995). Methane (CH4) content in the biogas produced was determined using a gas chromatograph CP-3800 (Varian, Palo Alto, CA, USA) fitted with Hayesep Q 80/100 Mesh (2 m × 1/8” × 2.0 mm SS) packed column (Varian, Palo Alto, CA, USA) and TCD detection,
Slaughterhouse waste characterization
The characterization of individual solid animal by-products is summarized in Table 1. Minced mixtures (SW) containing all the individual solid waste fractions, as described in the Section 2, and with different lipid–protein (L/P) ratios (SW1 and SW2), are also detailed in Table 1. The SW mixtures can be defined as high organic content substrates (870–1360 gCOD kg−1) composed mainly by lipids (68–82% fats/VS). Hejnfelt and Angelidaki (2009) reported mixtures of pig slaughterhouse by-products with
Conclusions
Batch tests performed on characterized slaughterhouse waste mixtures showed high anaerobic biodegradability and methane potentials, but also that lipids had a limiting effect on the global process kinetics. Despite the reported severe inhibition at the highest substrate doses, the system was able to recover methanogenic activity and finally to degrade the substrate by an adaptation phenomenon, related to an enrichment of both specific eubacterial (proteolitic and β-oxidative) and archaeal
Acknowledgements
The authors would like to thank Laura Tey from GIRO (Spain) for help with the experimental set-up. This work was supported by the Spanish Ministry of Science and Innovation Projects ENE2004-00724/ALT and ENE2007-65850.
References (36)
- et al.
Basic local alignment search tool
J. Mol. Biol.
(1990) - et al.
Anaerobic digestion of slaughterhouse by-products
Biomass Bioenergy
(2009) - et al.
Physiological and molecular characterization of an anaerobic thermophilic oleate-degrading enrichment culture
Anaerobe
(2001) - et al.
Long-chain fatty acids inhibition and adaptation process in anaerobic thermophilic digestión: batch test, microbial community structure and mathematical modelling
Bioresour. Technol.
(2010) - et al.
Methanogenic and non-methanogenic activity tests. Theoretical basis and experimental set up
Water Res.
(1993) - et al.
Materials flow and possibilities of treating liquid and solid wastes from slaughterhouses in Germany. A review
Bioresour. Technol.
(1992) - et al.
Hydrolysis kinetics in anaerobic degradation of particulate organic material
Waste Manag.
(2008) - et al.
Codigestion of manure and organic wastes in centralized biogas plants
Appl. Biochem. Biotechnol.
(2003) - et al.
Defining the biomethane potential (BMP) of solid organic wastes and energy crops: a proposed protocol for batch assays
Water Sci. Technol.
(2009) Standard Methods for the Examination of Water and Wastewater
(1995)