Biogas produced by the digester is used to heat the digester to the desired temperature. Axium Foods has a full wastewater treatment plant that includes an anaerobic digester. This closed system is called an anaerobic digester, biodigester or a bioreactor. The slurry from the digester is pumped into fields. Bio-decomposition in the anaerobic digester involves a four-staged cooperative action performed by different microorganisms. Day also owns the largest green energy Anaerobic Digester in the UK. The problem was solved by the introduction of an anaerobic digester to process the whey. A similar but variant technology to UASB is the expanded granular sludge bed (EGSB) digester. Some waste heat from the engine is then used to heat the digester. The waste heat is, in general, enough to heat the digester to the required temperatures. The digester is an airtight circular pit made of concrete with a pipe connection. Neither approach has mixing inside the digester. This substantially reduces the volume that is required to be treated in the digester. One type of these system is the Sintex Digester.