Low-Rate versus High-Rate Treatment of Animal Waste

Low-Rate Animal Waste Digestion

Many full — scale animal waste digester systems are low-rate anaerobic digesters, such as covered anaerobic lagoons, plug-flow digesters, and CSTRs. A typical VS loading rate (VSLR) for an unheated covered anaerobic lagoon is 0.24 g/L/day with a hydraulic retention time (HRT) of 65 days (Cheng et al. 1999). By feeding dairy waste at a VS concentration of 5g/L in a 4.5-L low-rate bioreactor, we achieved a maximum VSLR in a heated (34°C) and completely stirred anaerobic digester (semi-batch fed system) of 3.5 g/L/day and a minimum HRT and sludge retention time of 15 days (Hoffmann et al. 2008). A typical VSLR and HRT for a heated (35°C) CSTR-fed swine waste at a concentration of 20gVS/L are 1.4g/L/day and 15 days, respectively. Design parameters for a completely stirred system are dependent on the VS concentration in the influent, the operating temperatures, and composition of the biomass in the waste. Cow and swine wastes have a considerably different composition and require different operating conditions to achieve a stable and sufficient treatment perfor­mance. In addition, each farm is different, for example, in how the choice of bedding material for dairy cows will affect the required operating conditions (NRCS 1996).