Process Evolution of Dry Anaerobic Co-digestion of Cattle Manure with Kitchen Waste

Manure residues have been identified as a major source of environmental pollution. Traditionally, the waste has been disposed directly or after composting, as soil amendments in the agricultural industry. Since this practice has resulted in the degradation of air, soil, and water resources, new regulations for protecting the environment have been promulgated to control land application of animal manure.1 As such, livestock industries and regulatory agencies are seeking alternative technologies to manage manure residues in an en vironment-friendly manner.2 Anaerobic digestion is an attractive treatment strategy and shows great potential from an environmental point of view and for social benefit by providing a clean fuel.3–5 Anaerobic digestion of complex wastes in liquid form (<5 % total solids) is a mature technology that has been well studied and successfully implemented at full-scale.6 However, digestion in liquid form produces excessive fermented liquid, the disposal of which is difficult. Anaerobic digestion of high-solid substrates (≥20 % total solids), referred to as ‘dry digestion’, has been proven feasible.7–9 In addition, dry anaerobic digestion has become attractive because it requires relatively simple pre-treatment and addition of less water before digestion than in liquid form.10 High efficiency dry anaerobic digestion has attracted attention owing to the increasing generation of municipal solid wastes (MSW). The state-of-the-art of dry anaerobic digestion has been reviewed by De Baere10 for digestion of MSW and by Qi11 et al. for digestion of lignocellulosic substrate. Kitchen wastes (KW) contain a high concentration of biodegradable organic compounds.12 But the single-phase anaerobic digestion of KW is not straightforward for the accumulation of intermediary compounds occurs easily, giving rise to an unbalanced fermentation, and consequently, diminishing the stability of the process.13 On the other hand, the soluble organics are converted rapidly to volatile fatty acids (VFAs) at an early stage of the digestion process,14 resulting in a drastic pH drop if no sufficient buffering capacity is present.15 As a result, anaerobic treatment of KW is often performed at low organic loading rates (OLR) of 2–3 g COD L–1 d–1 to prevent process failure.16 Therefore, it is important to investigate the optimum conditions and efficiencies of digesters by examining VFAs.17 Moreover, the quantity of KW Process Evolution of Dry Anaerobic Co-digestion of Cattle Manure with Kitchen Waste