Context, issues and problems

Principle of a wastewater treatment plant

A wastewater treatment plant consists of one or several systems designed to extract the different pollutants contained in the wastewater it processes, so that the discharge does not deteriorate the natural milieu into which it is discharged.

This wastewater can be municipal or industrial (or agricultural) or the plant can treat both municipal and industrial wastewater. The connection of an industrial facility must be authorised by the authority or body managing the plant.

Wastewater treatment plants designed to only treat municipal wastewater are outside the scope of the nomenclature of classified installations.

What treatments for what problems?

In municipal or industrial wastewater, the traditional pollutants to eliminate are suspended solids, dissolved organic materials, nitrogen and phosphorus, metals. However, knowledge of the chemical composition of the wastewater is necessary to set up a water treatment matching the applicable legislation.

Certain industrial activity branches generate very specific types of pollution, for example:

• paper / cardboard industry: suspended solids, BOD (biological oxygen demand) and COD (chemical oxygen demand), AOX;
• chemical industries: COD, AOX, hazardous substances;
• surface treatment industries: metals, COD;
• food processing industries: BOD, COD, suspended solids, nitrogen, phosphorus;
• livestock farms: nitrogen, phosphorus

Various technologies are used to more or less specifically eliminate the concentrations of substances contained in the wastewater:

• pretreatment: screening, grit separation, oil and grease removal
• primary or physical-chemical treatment: settling, flotation, filtration, precipitation, metals,
• biological or secondary treatment: activated sludge, aerated lagoon, bacterial bed, biofiltration, anaerobic treatment,
• tertiary treatment: coagulation – flocculation (aluminium salts), adsorption (activated carbon), oxidation (ozone treatment), electrochemical processes, irradiation

The combination of processes is calculated according to the nature of the wastewater collected in the network and the type of pollution to be treated.

Emission limit values

Regardless of the destination of the wastewater generated by a classified installation (natural environment or sewer system), this wastewater must comply with the regulatory framework provided by the legislation on classified installations. The operational requirements of a prefectural permit order with regard to the quality of wastewater (emission limit values in concentration by substance, flow rates etc.) are based on:

• the national regulation applicable to classified installations (Ministerial order of 02/02/1998 in general or specific Ministerial orders if the activity of the classified installation is outside the scope of the Ministerial order of 02/02/1998)
• the conclusions of the environmental impact assessment
• the guidelines of the SDAGE general water scheme