In an aquatic environment, a natural process of decomposition of the organic matter by living microorganisms (bacteria, algae, protozoa) with light and dissolved oxygen is set up. It is the assimilative capacity.

A treatment plant is but an amplifier of the process, where time is replaced by a considerable concentration of microorganisms in the presence of oxygen in non-limiting quantity (through continuous oxygen injection).

Urban effluents treatment plants may vary in their treatment capability at a quantitative level (capability to purify a certain amount of water, uttered in population equivalent) as well as at a qualitative level (extent of the remediation or type of the remediation). Generally, a water pre-treatment is done. Then, three (possibly four) types of units have to be distinguished: primary, secondary, tertiary and quaternary treatment units.


Through screening, grit removal and deoiling, pre-treatment suppresses big particles and floating waste. It is always made.

Primary treatment

The primary treatment, purely mechanical or enhanced by the addition of chemicals (flocculant and coagulant), consists of the settling of solids suspended in water. This step is carried out in a big basin, so as to let the suspended particles settle at the bottom of the basin and evacuate them (primary sludge).

Water is then carried to other basins where the secondary treatment takes place.

The primary treatment enables to suppress a lot of the suspended matter.

Secondary treatment

The secondary treatment consists of having microorganisms degrade the organic matter.  If those microorganisms are free, it is an “activated sludge”; if they are fixed to a substrate, it is a “trickling filter” or “biodisc”.

Then, the water undergoes a clarification during which it rests in a separate clarifier and the sludge from the secondary treatment is collected (secondary sludge).

At the end of this secondary treatment, the BOD5 will decrease since the majority of the organic matter will have been suppressed, up to 95%.

Tertiary treatment

The whole Walloon Region being designated as “sensitive area”, the urban wastewater treatment directive obliges a tertiary treatment for treatment plants of more than 10,000 PE.

It consists of reducing the total nitrogen and phosphorus concentration in the discharged water under a certain threshold. To do this, according to the plants and the desired objective, water may go once or more than once through bacterial treatment ponds, alternatively under aerobic and anaerobic conditions.

The tertiary treatment therefore mainly enables the suppression of nitrogen and phosphorus.

Quaternary treatment

Though the purified water goes through a clarifier before being discharged in the external environment, it contains significant quantities of bacteria, protozoa and other microorganisms that played a part in the purification of the effluent. After a delay principally varying because of local conditions, the microorganisms are naturally suppressed from the stream as a result of ultraviolet rays, natural predators, etc.

In specific conditions, however, (in particular if the discharge takes place upstream from a bathing area), the purified water must undergo an additional disinfection treatment in order to reduce the amount of discharged microorganisms. Such a disinfection can be carried out with an ultraviolet system, etc.

The processed water may then be discharged in the receiving environment (stream, river...)


Treatment plants

See the following pages