Some small dairy and swine farms do not use water to flush their barns so they produce manure with low water contents. These manures can be digested using dry AD [27]. Large dairy and swine farms, however, use water to flush the manure out of the barns and hog houses, respectively, generating manure slurries of mod­erate solid contents (>8%). Traditionally, both types of slurries are stored in waste lagoons built on the farms. By installing a flexible or floating gas-impermeable plas­tic cover, such lagoons can be easily converted to a unique type of digesters, covered lagoon digesters [68]. Covered lagoons typically have a long retention time (several months or longer) and high dilution rates [11]. Because of impracticality in temper­ature control, covered lagoons are left to operate at ambient temperatures and can produce biogas efficiently only in areas with moderate and elevated year round tem­peratures. Covered lagoons are simple and cheap to construct, operate, and maintain, which justifies their low AD efficiency. Another disadvantage is the slow but contin­uous accumulation of undigested solids at the bottom of the lagoons, which is costly to remove. One example of covered lagoons is located at Royal Farms in Tulare, California. It has three cells with a surface area of nearly 2,800 m2. Supported by the US EPA AgSTAR Program (http://www. epa. gov/agstar/index. html), it was started in 1982 and has been in operation ever since. The biogas produced has been enough to fuel two Waukesha engine-generators to generate electricity to meet all of the farm’s electricity needs with excess being sold to the local utility. The heat recovered from the generators is used as supplemental heat in the nursery barns, and the stabilized effluent is used as fertilizer. Barham Farm in North Carolina also operates a covered lagoon that has an effective volume of 24,500 m3. It digests the manure slurry generated from 4,000 sows. Baumgartner Environics, Inc. and MPC Containment Systems, LLC are two providers and installers of anaerobic lagoon covers.

Another type of digester that has been successfully and commonly used in AD of dairy manure slurry is non-mixing plug-flow reactors [15], which can successfully digest manure slurries with high solid contents (up to 11-14%). With a HRT of 21 to 40 days, methane biogas containing more than 60% CH4 can be produced at rates from 0.37 to 0.79 m3/m3 reactor volume/d. As estimated from the bio­gas yields of three such digesters, the daily biogas production ranged from 1.16 to 2.41 m3 per cow per day [88]. Although non-mixing plug-flow reactors are nearly maintenance-free, the gas production is rather slow due to poor mass transfer. Recently, MPFLR has been built at several dairy farms in the USA by GDH, Inc. Herrema Dairy located in Fair Oaks, Indiana operates a MPFLR, which receives more than 400 m3 of manure slurry of 8% solids that is generated by 3,800 heads of cattle daily. Operated mesophilically with a HRT of 17 days, this reactor pro­duces enough biogas to steadily fuel two Hess engine-generators of 375 kWh each. The separated solids from the effluent are dried and reused for bedding in the barns, while the heat recovered from the engine-generators is used to heat the digester, barns, and alleyways.

Both CSTR and CMCR have been used in AD of dairy manure slurry. The con­tinuous mixing significantly enhances biogas production and reduces HRT (from months to 10-20 days) [11, 15]. Thus, implementation of CSTR and CMCR signif­icantly reduces the digester volumes required to digest the manure derived from a given number of cows or hogs. Although these two types of digesters cost more to build and operate, the increased costs may be offset by the increased biogas produc­tion and TS reduction. Other types of reactors that have been tried on AD of manure slurries include hybrid reactors [26] and anaerobic filter reactors [87, 88]. However, to prevent clogging of the filter media of these two types of reactors, the SS has to be separated prior to feeding to these biofilm-based digesters, resulting in reduced biogas production [88]. The superiority of these digesters remains to be determined.

Recent studies have focused on improvement of VS degradation and concomitant increase in biogas production. Co-digestion with food wastes or crop residues was found to dramatically increase (by 2-3 folds) biogas production [51, 59]. This is attributed to the increased input of readily degradable substrate from these wastes. Temperature-phased AD (TPAD) also substantially improves AD [78], and TPAD of dairy manure slurry can be completed within a short HRT. The increased conversion rates at elevated temperature (55°C) are responsible for the improvement observed in TPAD systems [91].