The methane biogas is collected from the first two stages of the AD vessel and utilized for fuel in the combined heat and power (CHP) gensets. These gensets are commercially available, reciprocating engines configured to burn biogas (under certain circumstances microturbines can be a viable generation alternative).
The electricity produced by these gensets can be used to offset commercial or on-farm power consumption; excess power can be sold directly to the utility as a source of green power.
The waste heat, in the form of hot water, is collected from both the engine jacket liquid cooling system and from the engine exhaust (air) system. Approximately 30 to 60% of this waste heat is utilized in the AD system. The remaining waste heat can be used by the farm as a replacement for hot water production (reducing the need for natural gas or propane purchases) and for in-floor heating of the farm and holding areas, as required.
The digester effluent is pumped from the effluent pit at the end of the AD vessel to a manure solids separator. The mechanical manure separator separates the influent digested waste stream into solid and liquid fractions. The solids are dewatered to approximately a 35% solid material. The separated solids, having the same odor and pathogen reduction characteristics as the liquid stream, may be utilized by the farm for bedding replacement (an expense reduction). Use of the separated solids for bedding typically comprises about 40 to 60% of the generated separated solids from a typical farm. The residual 40 to 60% of non-utilized separated solids may be sold (system-generated income) to other farms for bedding purposes or sold to after-markets, such as nurseries and composters, for soil amendment material.
The liquid from the manure separator, now with the majority of the large solids removed, gravity flows into the farm’s storage lagoon. A large advantage of the effluent from the AD treatment process is that the viscosity of the effluent is such, as opposed to the raw manure influent, that the liquid effluent can be pumped through an irrigation nozzle for field spreading. Because the digester consumes sticky starches and performs the natural process of mineralization within the AD vessel instead of in the soil, the plant-ready effluent can also be applied to a growing crop.