Anaerobic digestion (AD) is a process by which microorganisms break down biodegradable material in the absence of oxygen. Anaerobic digestion can be used to treat various organic wastes and recover bio-energy in the form of biogas, which consists mainly of CH4 and CO2. A great option for improving yields of anaerobic digestion of solid wastes is the co-digestion of multiple substrates. Numerous studies demonstrate that using co-substrates in anaerobic digestion system improves the biogas yields due to positives synergisms established in the digestion medium and the supply of missing nutrients by the co-substrates. In addition, codigestion offers several possible ecological economical advantages. Recent research (published during the past three years) on this topic is reviewed in the current paper. Special attention is paid to anaerobic co-digestion of animal waste, crop and crop residues, municipal solid waste (MSW), as well as municipal sewage sludge.
KEYWORDS: Anaerobic; Co-digestion; Biomass; Methane; Animal Manure INTRODUCTION
Biomass has been defined as organic matter formed by photosynthetic capture of solar energy and stored as chemical energy (Gunaseelan, 1997), which includes agricultural crops and wastes, animal wastes, forest and mill residues, wood and wood wastes, livestock operation residues, aquatic plants, fast-growing trees and plants, and municipal and industrial wastes. Recently, oil crisis have brought great interest in the exploration of renewable energy, specially, bioenergy contained in biomass. The solar energy stored in biomass could be released as biogas, a mixture of methane (CH4), carbon dioxide (CO2), and some trace gases, through anaerobic digestion.
Anaerobic digestion (AD) is a process in which microorganisms break down biodegradable material in the absence of oxygen. Anaerobic digestion can be used to treat various organic wastes and recover bio-energy in the form of biogas, which contains mainly CH4 and CO2. Methane could be a source of renewable energy producing electricity in combined heat and power plants (Clemens et al., 2006). The organic loading rate (OLR) and hydraulic retention time are two major parameters used for sizing the digesters and their optimum values are specific to the substrate as well as the operating temperature of digesters (Romano and Zhang, 2007) Significant effort has been dedicated in recent years to find ways of improving the performance of digesters treating different biomass. One of options for improving yields of anaerobic digestion of organic matter is co-digestion.
What is co-digestion?
Co-digestion is the simultaneous digestion of a homogenous mixture of two or more substrates. Traditionally, anaerobic digestion was a single substrate, single purpose treatment. Recently, it has been realized that AD as such became more stable when the variety of substrates applied at the same time is increased.
The most common situation is when a major amount of a main basic substrate (e.g. manure or sewage sludge) is mixed and digested together with minor amounts of a single, or a variety of additional substrate (Braun, 2002). The use of co-substrates usually improves the biogas yields from anaerobic digester due to positive synergisms established in the digestion medium and the supply of missing nutrients by the co-substrates (Mata-Alvarez et al., 2000) Advantages and Limitations of co-digestion
Several possible ecological, technological and economical advantages and limitations was shown in Table 1
Table 1 Merits and Limits of co-digestion technology (Braun, 2002) Merits
Improved nutrient balance and digestion
Increased digester effluent COD
Additional pre-treatment requirements
Equalization of particulate, floating, settling,
acidifying, etc. wastes,...