The anaerobic contact process is a type of anaerobic digester. Here a set of reactors are created in series, often with recycling. This recycled material is pumped up into the bottom of the first reactor, an upflow reactor. The upflow anaerobic process is a large reactor which allows the waste to flow up from the bottom and separates the waste into 3 zones. At the very top is the biogas

Biogas

Biogas typically refers to a gas produced by the biological breakdown of organic matter in the absence of oxygen. Organic waste such as dead plant and animal material, animal dung, and kitchen waste can be converted into a gaseous fuel called biogas...

 zone where the gas is collected. Bacteria digest waste in the lowest portion of the upflow reactor; the bioreactor

Bioreactor

A bioreactor may refer to any manufactured or engineered device or system that supports a biologically active environment. In one case, a bioreactor is a vessel in which a chemical process is carried out which involves organisms or biochemically active substances derived from such organisms. This...

zone. In between these two stages is the clarifier zone where the which exports the stabilised waste .

A diagram of an anaerobic contact process can be found here.