Anaerobic digesters: bio-methanation that turns food waste into biogas and digestate.

Composting is aerobic: it needs oxygen, gives off heat and carbon dioxide, and produces compost. Digestion is anaerobic, it happens in a sealed vessel without oxygen and produces biogas plus a digestate. Digestion recovers energy, which composting does not, but it needs a more consistent feed, a more capable operator and a plan for the digestate.

High-moisture, readily degradable food and kitchen waste with clean segregation. Fibrous garden waste, packaging, plastics and inerts do not digest and cause problems in the feed system. A premises whose wet waste is mostly garden trimmings is a composting case, not a digestion case.

Most commonly to displace LPG in a kitchen through a dedicated burner line, and in larger installations to run a generator or a boiler. The utilisation route has to be decided at design stage, because it determines gas storage, cleaning, piping and safety requirements. Gas with no use is gas that gets flared or vented, which loses the main advantage of the technology.

It leaves the plant continuously as slurry and sometimes as separated solids, and it needs a destination: land application on the premises' own landscape, dewatering and use as a soil conditioner, or an agreed offtake. Digestate handling is where under-designed plants fail, because the volume does not stop while a destination is being found.

The daily quantity of suitable feed, its composition and moisture, and the retention time the process needs. Because retention time is measured in days, a digester is physically larger than a mechanical composter of comparable daily throughput. Consistency matters as much as quantity, a plant sized for a steady canteen load behaves badly if fed erratically.

Biogas is flammable, so gas storage, piping, ventilation, leak detection, flame arrestors and electrical classification around the plant have to be designed properly and maintained. Whether a plant additionally requires State Pollution Control Board consent depends on its scale, its gas utilisation and the state's position, and should be confirmed with the Board before construction rather than after.

It needs a trained operator on the premises with a defined daily routine (feeding, monitoring gas production and pressure, checking the slurry line, and recording readings) supported by a maintenance contract for the mechanical and gas systems. Responsibility for compliance stays with the generator under the Solid Waste Management Rules 2016, whoever runs the plant day to day.