A system can be designed to digest a wide variety of organic wastes, from kitchen scraps to sewage, to livestock manure, to industrial wastes. The ideal feedstock is a 6-8% slurry with a Carbon to Nitrogen ratio of about 30:1. Incoming waste material should be macerated, and as close to the operating temperature (95 degF) of the digester as possible. The small scale system described below will handle the toilet wastes produced by a family of 6, each flushing a 1-1/2 Pint/flush toilet 5 times per day.
Gas can be stored in low pressure gas bags (i.e. truck tire inner tubes, etc.), rigid tank(s) with floating cover and water seal, compressed and stored in pressure tank, and/or burned as it is produced, (minimizing storage requirements). For safety reasons, it is recommended that the gas be burned as soon as possible, avoiding the requirement to store and handle larger quantities of flammable gas.
The gas produced typically consists of about 30% CO2 and about 60-65% methane, depending on the content of the wastes. Small amounts of hydrogen, hydrogen sulfide, and nitrogen gas will also be produced, as well as water vapor.
One use of this gas is to heat domestic hot water in a gas-fired tank-type water heater installed between the cold water inlet and the conventional (backup) water heater. A small, weighted inflatable gas bag will be used to collect up to an hour's worth of gas (i.e. about 5 cu ft at design conditions), and to provide required pressure for proper burner operation.
Gas burner should be as small as possible, with intermittent ignition with continuous retry and maximum lock-out time. Sufficient hot water storage capacity should be provided to make use of all available energy without having to store flammable gas. This may require addition of one or more insulated storage tanks piped together, and a small circulator pump and controls.
Effluent from the digester will be returned to the conventional backup sewer system. A hydraulic loading rate (total liquid throughput) of about 60 gallons/ day is assumed. Estimated solids loading will be about 35 lbs/day. Assuming that volatile solids will be reduced by about 60-70% in the digester, additional volatile solids entering the sewer system will be about 35 - (35 x .65) ~ 12-15 lbs/day.
The volume of sludge solids accumulating in the digester will depend on the digestibility of the influent material and the extent to which digester contents are mixed (i.e. kept in suspension and discharged with effluent), or allowed to settle. Tanks are designed to facilitate sludge removal (e.g. quick disconnect fittings provided for connection to vacuum pump, etc.). To reduce solids loading on backup sewer system and recover sludge solids as a valuable soil amendment, a settling tank can be installed in the line between the effluent overflow and the sewer system