From an economic, social and environmental perspective, lignocellulosic biomass wastes are good feedstocks for methane production through AD. Due to the slow hydrolysis of lignocellulose, however, methane production is slow, and a long reten­tion time and large digester volumes are required to produce enough methane biogas for cost-effective recovery. In the case of livestock manure, 40-50% of the solid passes through mesophilic AD undigested [8]. Two-stage AD processes can improve solid reduction and stability by separating the more robust hydrolysis and acidogenesis from the less robust syntrophic acetogenesis and methanogenesis [13]. TPAD digesters are promising two-stage designs, with the hydrolysis being enhanced in the first digester operated at an elevated temperature (typically at 55°C) and syntrophic acetogenesis and methanogenesis being enhanced and stabilized in the second digester operated at a mesophilic temperature (typically 35°C) [46, 78]. Indeed, significant increases in hydrolysis, TS reduction, and methane production resulted from the co-digestion of a primary sludge and OFMSW in a TPAD sys­tem [76]. Additionally, TPAD enhances sanitation of waste streams [73], reducing potential risks associated with certain types of feedstocks (e. g., municipal sludge and animal manures). Furthermore, TPAD processes eliminate the AD inhibition caused by the self-heating of mesophilic AD of high-energy feedstocks (e. g., energy crop and OFMSW) [49, 52]. The higher energy input required to operate TPAD is more than offset by the increased biogas and heat produced therefrom [22]. The TAPD technology will probably be applied more commonly in the near future when more lignocellulosic feedstocks (e. g., energy crops, animal manure, crop residues, and OFMSW) are subjected to AD.

Size reduction can dramatically enhance the AD of certain feedstocks, such as crop residues, OFMSW, and energy crops. Physical and chemical pretreatments can further enhance AD of these feedstocks [45, 53], but currently they may not be cost — effective, especially for those feedstocks that contain high water contents and for wastewaters. Low cost and efficient pretreatments need to be developed.

The entire AD process is often limited by three of the four steps of the AD process: hydrolysis, syntrophic acetogenesis, and methanogenesis. Hydrolysis of biomass polymers is typically the rate-limiting step of the entire AD process of lignocellulolytic feedstocks. Single or mixed cultures of lignocellulolytic microbes may be used to augment the capability of hydrolysis in digesters as exemplified by enhanced AD of cattle manure [62] and municipal sludge [30]. Methanogenesis can become the rate-limiting step when feedstocks containing large amounts of read­ily fermentable substrates (e. g., starch) are digested. In this scenario, acid-tolerant methanogen (e. g., Methanobrevibacter acididurans) cultures may be prepared and used to enhance the entire AD process or remediate upset AD operation. Bioaugmentation can also enhance the AD of feedstocks containing high concen­trations of particular substances, such as lipids [19]. As more and more digesters are put into operation, there will be increasing needs for such specialty cultures to enhance existing digesters, start up new digesters, and prevent AD failures.