Feasibility study of the anaerobic digestion of dewatered pig slurry by means of polyacrylamide E. Campos a , M. Almirall a , J. Mtnez-Almela b , J. Palatsi c , X. Flotats c, * a Laboratory of Environmental Engineering, Centre UdL-IRTA, Rovira Roure 191, E-25198 Lleida, Spain b SELCO MC SL, Pza. Tetua ´ n 16, E-12001 Castello ´ n, Spain c GIRO Technological Centre, Rambla Pompeu Fabra 1, E-08100 Mollet del Valle ` s, Barcelona, Spain Received 15 January 2004; received in revised form 20 September 2006; accepted 6 December 2006 Available online 5 February 2007 Abstract Liquid livestock waste can be managed by separating liquid and solid fractions then treating each separately by applying best avail- able technology, such as anaerobic digestion for the solid fraction. There is an increasing use of polyacrylamide (PAM) as a flocculant agent to improve solid–liquid separation. In the present work, the anaerobic toxicity of PAM residues and the optimal range of total solids concentration for maximum methane production were studied as a function of PAM dosage. Results showed that dry matter and its volatile solids content increased significantly with increasing PAM dosage. Batch anaerobic tests showed that methane yield decreased linearly with increasing total solids, while the methane production per unit of raw substrate reached a maximum at 16.4% total solids. No PAM toxicity was measured for PAM concentrations below 415 g/kg total solids, but some indirect inhibitory phenomena were observed, such as a limited hydrolysis rate due to particle aggregation, and inhibition of methanogenesis by high ammonia concentration. Ó 2007 Elsevier Ltd. All rights reserved. Keywords: Anaerobic digestion; Pig slurry; PAM; Polyacrylamide; Solid–liquid phase separation 1. Introduction Modern pig production, which has a very intensive and concentrated character, generates a large pig slurry surplus that often cannot be used as an agricultural fertiliser in the same geographical area, thus making its transport a limit- ing factor. One management strategy consists of separating the solid and liquid fractions, then treating the liquid frac- tion prior to using it for irrigation on nearby land, while treating the solid fraction in order to stabilise it and to reduce volume before transporting it to areas with nutrient and/or organic matter demand. Stabilisation of the solid fraction, prior to land application, can be achieved by aer- obic composting and/or anaerobic digestion. The second option provides a better energy balance and can be comple- mented by further aerobic composting in order to produce a higher quality end product. The efficiency of anaerobic digestion of this solid fraction can be negatively affected by high total solids concentration (Itodo and Awulu, 1999; Bujoczek et al., 2000). The main fraction of organic matter found in pig slurry takes the form of small suspended particles, mainly in col- loidal form, which are not easily separated by applying a simple mechanical system (Hill and Tollner, 1980). The effi- ciency of suspended solids separation using filters and presses is limited, and for colloids agglutination a chemical coagulation process is required (Sievers et al., 1994). Treat- ment with polyacrylamide (PAM) polymers, prior to mechanical removal or gravity settling, has the potential to enhance solid–liquid separation, thus concentrating nitrogen, phosphorous and organic carbon (Vanotti and Hunt, 1999). Since most suspended particles in wastewaters and aqueous solutions, such as livestock and poultry 0960-8524/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.biortech.2006.12.008 * Corresponding author. Fax: +34 935796785. E-mail address: xavier.flotats@giroct.irta.es (X. Flotats). Available online at www.sciencedirect.com Bioresource Technology 99 (2008) 387–395