digestion is a two-step process. In the first step, bacteria and
sludge are mixed together in a heated tank called a digester.
This is where the biological activity takes place and biodegradable
organic matter is broken down producing gases and inert organic
matter. The sludge is then delivered to the second step. This digester
is not mixed and the sludge separates with gases at the top of the
digester, liquid in the middle and solids on the bottom. Gases that
have risen to the top are collected and burned. The liquid is returned
to the beginning of the plant to be treated again. Solids are sent
to the next step for dewatering.