Biological treatment processes are a secondary treatment stage to remove dissolved organic matter from wastewater. In pulp and paper applications, the biological treatment process is preceded by one or more primary stages such as screening, clarification or dissolved air flotation. Biological treatment is followed by clarification or dissolved air flotation as other secondary treatment steps. If needed, secondary treatment may be followed by tertiary treatment.

In a biological treatment process micro-organisms are using the dissolved organic substance as food which is subsequently converted to gaseous or solid by-products. For living microorganisms need ammonia, phosphorus and trace elements as well. In contrast to municipal wastewaters, waters from pulp and paper mills do not contain enough of nitrogen and phosphorus. Thus these have to be added.

There are two basic types of biological treatment being; anaerobic (without oxygen) or aerobic (in the presense of oxygen). Both types are treatments are commonly used in pulp and paper waste water treatment applications.

Anaerobic treatment is preferred when dissolved organic concentrations in untreated waste water are high. Aerobic treatment may follow an aerobic stage or be used as the sole secondary treatment. Traditional aerobic treatment consists of activated sludge processes or oxidation lagoons. The size of these can be reduced and tolerance against fluctuations and toxics increased by adding a step with moving bed bioreactors (MBBR) prior to the active sludge treatment.

EWT has experience in different forms of biological treatment with hundreds of worldwide references in most industry sectors.