Municipal sludge includes primary sludge and waste activated sludge derived from centralized wastewater treatment plants that employ biological treatment of sewage. It is probably the first type of feedstock subjected to AD. It has very high contents (95-99%) of water, low contents (15-20%) of volatile solid (VS, representing the biodegradable portion of total solid, TS), and low contents of readily fermentable carbohydrates [8, 94]. However, most municipal sludge has rich and balanced nutri­ents (nitrogen: 3-6%; phosphorus: 1.0-1.2%; of TS). The biochemical methane potential (BMP) of municipal sludge is relatively small, ranging from 85 to 390 m3 CH4/dry ton. Municipal sludge contains a high density of bacterial cells (mostly aerobic and facultative anaerobic bacteria), some of which may be pathogenic to humans and/or animals. Toxic compounds may also be present in some munici­pal sludge, especially those derived from large metropolitan areas. Approximately

6.2 million dry tons of municipal sludge are produced annually in the USA (based on 1999 data [39]), representing an annual potential of at least 6 billion m3 of methane biogas. At present, however, only a portion of the municipal sludge is digested and the methane biogas yields are relatively low. This is largely attributable to the relatively small net amounts of energy that can be produced. However, when municipal sludge is co-digested with carbohydrate-rich yet nitrogen-poor biomass wastes (e. g., OFMSW and food-processing wastes), the energy yields can increase substantially [4]. For example, in a full-scale two-staged AD system, a 25% increase in organic load rate (OLR) with OFMSW resulted in an increase in biogas yield by 80% and overall degradation efficiency by 10%, which resulted in an increase in electrical energy production by 130% and heat production by 55% [94]. Additionally, when co-digested with carbohydrate-rich yet nitrogen-poor biomass wastes, municipal sludge can stabilize the AD process of the former [46].

Municipal sludge is among the most studied feedstocks in AD. Numerous books and reviews have been published on AD of municipal sludge (e. g. [79]). In gen­eral, because of the presence of high levels of suspended solid (SS), most AD technologies are not suitable for the AD of municipal sludge. Continuously stirred tank reactors (CSTR) and completely mixed contact reactors (CMCR) are most commonly used in AD of municipal sludge [79]. For example, the CSTR with a total volume of 1,350 m3 in Karlsruhe, Germany digests municipal sludge at 37°C and produces approximately 3,800 m3 of biogas of 62-70% methane daily [33]. More recent research efforts have been directed at pretreatment to enhance degra­dation of the solid found in municipal sludge and production of methane biogas (see [28, 45] for reviews). Thermophilic AD, in single — or two-staged systems, is also being increasingly used to enhance biogas production and sanitation [92]. Additionally, because of the low solid contents (1-5%) and low BMP, large digesters are required for the conventional “wet” AD. Currently, “dry” AD technology is being evaluated to produce methane biogas from dewatered biosolids, which have significantly reduced water contents (70-85%) and thus reduced digester volumes [64]. Dewatered biosolids are also ideal feedstocks to be co-digested with other solid feedstocks, such as OFMSW and crop residues.