These wastes are characterized by varying water contents, but high VS contents (>95%). However, these parameters vary considerably. Most food wastes have bal­anced nutrients and large amounts of readily fermentable carbohydrates and thus are among the most suitable feedstocks for AD. According to a recent study, 348 m3 of CH4 can be produced per dry ton of food wastes within only 10 days of AD [93]. Food wastes amount to approximately 43.6 million dry tons each year in the USA [81]. This represents a potential of 15.2 billion m3 of CH4 per year. During food processing, a significant portion of foodstuffs also ends up in wastes or wastewaters. For example, 20-40% of potatoes are discarded as wastes during pro­cessing. National data on the amount of food-processing wastes are not available.

The state of California generates more than 4 million dry tons of food-processing wastes each year [54], potentially producing 1,200 million m3 of CH4. This trans­lates into an annual potential of several billions m3 of CH4 in the USA. Except for the wastes from animal meat processors, most food-processing streams are rel­atively poor in nitrogen, but rich in readily fermentable carbohydrates. As such, food-processing wastes can be co-digested with other nitrogen-rich feedstocks (e. g., municipal sludge or animal manures) to enhance AD system stability and CH4 production [46].

Approximately 250 million dry tons of MSW are produced annually in the USA. The organic fraction, such as paper, yard trimmings, and food scraps, is biodegrad­able and can be converted to methane biogas. Although the composition of MSW varies dramatically depending on society, season, collection, and sorting, OFMSW accounts for more than 50% of the MSW in most societies. Most OFMSW has little moisture or readily fermentable carbohydrates and is relatively deficient in N or P, but has a relatively large BMP (300-550 m3 CH4/ton) if digested adequately [25]. The OFMSW generated annually in the USA has a CH4 potential of 37.5 billion m3.

Crop residues amount to an estimated 428 million dry tons each year in the USA. Although the majority of crop residues is typically left in the field, approximately 113 million dry tons are recoverable and available for conversion to methane biogas [69]. Crop residues typically have relatively low water contents, high VS contents, and variable contents of readily fermentable carbohydrates. Most crop residues are non-leguminous and are poor in available nitrogen. The BMP of crop residues varies from crop to crop (from 161 to 241 m3 CH4/ton) (124). If subjected to proper AD, at least 20 billion m3 of CH4 can be produced annually from the crop residues available for biogas production in the USA. Similarly for other nitrogen-poor biomass, co­digestion of crop residues with animal manures or municipal sludge substantially improves CH4 yield [50]. In theEU, 1,500 million dry tons of biomass are available each year for biomethanation within the agricultural sector, with half of this being crops intended for bioenergy production [5]. It should be noted that production of bioethanol and biodiesel from energy crops only utilizes a fraction of the biomass, and implementation of AD by the bioethanol industry can generate substantially more energy (up to 30% of the total energy of the initial biomass) [3, 74]. This also holds true for many other biomass-based processes producing non-food products.

All these types of feedstocks likely contain bulky materials, such as peeling, papers, stems and leaves. Pretreatment, especially reduction of particle size by grinding or milling, is typically required to enhance AD [40]. Other pretreatments such as alkaline pretreatment [53] have also been evaluated to further enhance the hydrolysis step in laboratories, but few of them have been implemented in full — scale AD plants. As mentioned earlier for the AD of livestock manures, co-digestion with other nitrogen-rich biomass (e. g., municipal sludge or animal manure) can also substantially stabilize the AD process and increase CH4 production [50, 94].

The above mentioned wastes have relatively low water contents. They can be digested using some wet AD processes (e. g., CSTR and CMCR) after dilution. The Lemvig Biogas plant in Denmark is one example of such wet AD. It is a centralized biogas plant consisting of three thermophilic CSTR with a total volume of 7,000 m3

that digests various types of organic industrial wastes, source-sorted MSW, and manures [7]. The biogas produced is used to generate electricity and heat.

Apparently, dry AD is advantageous for these low-moisture feedstocks because it eliminates the need to dilute the feedstocks to a fluid state and produces a low — moisture digestate, which is easier to transport and disperse [90]. The DRANCO technology is a dry AD technology successfully used to convert low-moisture organic wastes (e. g., OFMSW and crop residues) to methane biogas [21]. The DRANCO technology requires the feedstock to be shredded and milled first to reduce particle sizes (<4.0 mm in diameter). A digested sludge or digestate is then mixed with the feedstock in a 6:1 to 8:1 ratio in a mixing compartment. The mixture is heated by steam (to 30-40°C for mesophilic AD or 50-55°C for thermophilic AD) and then pumped into the digester at the top. The feedstock descends by gravity while digestate is withdrawn at the bottom. The biogas rises and exits the digester through the roof of the digester. The retention time in a DRANCO digester averages 20 days with a pass-through time of 2-4 days. The DRANCO technology is marketed by the Organic Waste System (OWS) in Belgium (http://www. ows. be/index. php). According to OWS, the DRANCO technology has a number of advantages including high solid digestion, high loading rates (10­20 kg COD/m3 of reactor/d), high biogas productivity (100-200 m3 of biogas/dry ton of feedstock), small digester volumes, no maintenance or failures inside the digester, less energy consumption, well controlled external inoculation, and kill — off of pathogens and seeds. The largest DRANCO digester started operation in 2006 in Vitoria, Spain. This digester has an effective volume of 1,770 m3 and a capacity of 120,750 tons/yr of primarily OFMSW. It produces 5,962 tons of biogas, which can generate 6,000 MWh of electricity, and 12,580 tons of compost per year. As of this writing, most of the DRANCO digesters in use are located in Europe, and the capacity of dry AD has exceeded that of wet AD of solid wastes [21]. The ECOCORP (www. ecocorp. com), BEKON (www. bekon-energy. de), Kompogas (www. kompogas. com), and Linde (http://www. anaerobic-digestion. com) processes are emerging dry AD technologies mostly used in Europe for dry AD of solid biomass wastes.

A new two-staged AD process was evaluated by Parawira et al. [67] in digesting solid potato wastes under mesophilic and thermophilic conditions. This process uses a solid leaching bed reactor for hydrolysis and acidification while an UASB reactor is used for methanogenesis. High loading rates (36 g COD/L/d), high methane yields (0.49 L/g COD removed), and stable operation were observed under mesophilic conditions. The utility of this new process remains to be validated for other types of feedstocks containing significant amounts of lignocellulose.