When preliminary treatments have as objective the removal of coarse substances and a part of the solids in suspension, secondary treatments have as objective to eliminate the rest of the suspended solids and also a maximum of the dissolved salts. When a biological process is used, the removal of organic material (or carbon-load) is the principal objective. Simultaneous removal of a small part of the nitrogen and phosphorus-loads occur.
When a higher level of C, N et P-removal must been obtained, a tertiary treatment must complete the scheme.