Small wastewater treatment plant: operation, advantages, disadvantages and costs


The micro-treatment plant is a private autonomous device for the collection and treatment of domestic wastewater. It carries out, by a biological process, all the sequences of purification of black water and gray water. It is the non-collective sanitation installation requiring the smallest land area.

System limitations

The non-collective sanitation by VSAT accept all the wastewater produced in the house. However, as with all these biological purification devices, the balances are fragile. The large inputs of water, biocidal products and solid waste are sources of serious dysfunction. It is therefore necessary to exclude or limit the contributions:

  • rainwater, drainage or emptying large basins
  • effluents loaded with petroleum products (gasoline, diesel, oils, etc.)
  • organic or welded solvents and their derivatives (paint, glues, unblockers, etc.)
  • large amounts of chlorinated products
  • garbage disposals or other
  • hardly biodegradable products (wipes, sanitary napkins, tampons, plastics, etc.)

Small wastewater treatment plants require a regular supply, but control, in organic matter. They are therefore poorly suited to premises with occasional use, such as residences arecondaires.

The influx important and regular of edible fats and oils is contraindicated. In anticipation of intensive use of the kitchen (large kitchens, restaurants, caterers), it is essential to provide a grease trap upstream of the station. Black water must be collected after the grease trap.

Regulations relating to small wastewater treatment plants

Equipment not exceeding 1.2 kg per day of gross load must be approved by the competent ministry and must bear a CE marking, in accordance with:

  • under the terms of the ministerial decree of March 7, 2012, modifying that of September 7, 2009 relating to ANCs treating a gross load
  • to the standards European and French.

The preliminary study

These installations are subject to preliminary authorisation. A file must be attached to building permit applications for both new and rehabilitation work. The Public Non-Collective Sanitation Service (SPANC) of the municipality is able to help you with the presentation of the project and will be responsible for checking the conformity of the installations before commissioning.

Each file must include:

  • Study of the installation, determining the types of equipment and accessories used.
  • A geological and hydraulic study, confirming the conformity of the nature of the soil to the regulatory constraints in force.

Note : For any other use of the premises than those intended for individual housing, the file must be produced by a specialized design office.

Operating principle of a small wastewater treatment plant

The micro-treatment plant is in the form of a sealed tank made of concrete or synthetic material. There are 3 main treatment techniques: free cultures, SBRs and fixed cultures.

VSATs in free cultures

In free culture or by activated sludge, the stations are partitioned into 3 or 4 compartments. In the first (primary settling tank) the raw effluents are divided into layers: the heavier solids are deposited at the bottom where they are liquefied by colonies of anaerobic bacteria. At the same time, aerobic bacteria break down the fat floating on the surface. The liquid intermediate layer is directed to a second compartment (aerated biological reactor) passed through fine air bubbles produced by an air compressor. This hyper oxygenation maintains a bacterial fauna active on the organic elements in suspension. The third compartment (clarifier) and any subsequent ones complete the purification process. The mineral residues from the reaction and clarification steps are returned to the bottom of the first tank, where they mix with the sludge awaiting pumping.

Less sophisticated and less expensive, these materials are reliable but more sensitive to load variations.

SBR (Sequencing Batch Reactor) VSATs

The SBR or sequential reactor is a simple device for increasing the yield of free cultures, from which it resumes the operating mode. The whole process takes place in a single tank, both settling tank, aerated reactor and clarifier. In this configuration, the bacterial support consists of floating plastic elements offering a large surface area for attaching to bacteria and promoting their concentration. The effect is mathematical: more bacteria = more yield. The other originality is thealternation programmed air compressor operating and shutdown cycles. During the resting phase, suspended mineral sediments settle faster in the bottom and clearer water rises to the surface. This sequencing therefore decreases the clarification time or for the same yield, the volume of installations. The single tank eliminates the need for sludge transfer mechanisms.

SBR VSATs can achieve decontamination rates close to 99%. Their field of application extends to certain semi-industrial activities or to small communities. Not approved, these large volume installations (up to 30 m3), must be the subject of a special authorization.

VSATs in fixed cultures

In fixed culture, the tanks have two or three compartments. As in free culture, the first tank ensures the primary treatment. The second concentrates the biological supports of large surface, specific to the system. These are veritable nests for bacteria, crossed by the forced circulation of the mixture of pretreated effluents and oxygenation bubbles. Aerobic microorganisms remain attached to the supports by a kind of natural glue made up of molecules which they excrete. Like the beads of the SBR device, these different types of media (honeycombs, minerals, rock wool or plant fibers, etc.), concentrate large quantities of bacteria in a small volume, without risk of clogging. A variant comprises discs with semi-emerged rotating movements. This technique aims to simplify maintenance operations.

Advantages of small wastewater treatment plants:

  • allows construction in areas not served by mains drainage,
  • low land occupation (minimum between 5 and 10 m²),
  • no offensive odors,
  • very good depollution efficiency (between 90 and 99%),
  • some models can be used in sensitive areas (consult the approval notice for each model and the SPANC).

Disadvantages of small wastewater treatment plants:

  • essential power supply,
  • continuous use, even if certain technologies tend to overcome this drawback,
  • regular maintenance that is binding on the user,
  • annual maintenance by a specialized company, strongly recommended,
  • compulsory emptying, by an approved company, when the sludge reaches 30% of the volume of the tank.

Installation costs of wastewater treatment plants

The speed of installation constitutes an asset to be credited to the small wastewater treatment plants. They are delivered functional, ready to plug in and require few earthworks due to their small footprint. However, the management of rejections, the nature of the land or local constraints, can lead to significant additional costs.

Operating constraints and maintenance costs are decisive criteria for the choice of the wastewater treatment plant. In fact, there are great disparities in these fields depending on the techniques used.

Depending on its treatment capacity, the average price of an installation, in permeable ground, is between 5,000 and € 7,000 including tax.

An annual maintenance contract is proposed, according to the stakeholders between 150 and 400 € incl..

The price of sludge emptying varies from 180 to 300 € TTvs.

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