Micro purification station: operation pros cons and costs

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

System limits

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

  • rainwater, drainage or emptying of large basins
  • effluent loaded with petroleum products (gasoline, diesel, oil, etc.)
  • organic or welded solvents and their derivatives (paint, glues, drain cleaners, etc.)
  • large amounts of chlorine products
  • garbage disposals or other
  • products that are difficult to biodegrade (wipes, sanitary napkins, tampons, plastics, etc.)

Small wastewater treatment plants require a regular supply, but control, organic matter. They are therefore not very suitable for premises with occasional use, such as residencescondaires

The influx important and regular 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. Sewage 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 ANC treating a gross load
  • to standards European and French.

The preliminary study

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

Each file must include:

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

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

Principle of operation of a small wastewater treatment plant

The small wastewater treatment plant comes in the form of a sealed tank made of concrete or synthetic material. There are 3 main treatment techniques: free cultures, SBR and fixed cultures.

Free growing microstations

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 settle to the bottom where they are liquefied by colonies of anaerobic bacteria. At the same time, aerobic bacteria break down the fats floating on the surface. The liquid intermediate layer is directed towards 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 (clarificateur) and any subsequent ones complete the purification process. The mineral residues from the reaction and clarification stages are returned to the bottom of the first tank, where they mix with the sludge awaiting pumping.

Less sophisticated et less expensivethese materials are reliable but more sensitive to variations de charge.

Les microstations SBR (Sequencing Batch Reactor)

My SBR sequence reactor is a simple device for increasing the yield of free growing crops, of which it takes over the mode of operation. The whole process takes place in a single tank, both decanter, aerated reactor and clarifier. In this configuration, the bacterial support consists of floating elements made of plastic material offering a large surface for attaching bacteria and favoring their concentration. The effect is mathematical: more bacteria = more yield. The other originality is thealternation programmed air compressor on and off cycles. During the resting phase, the mineral sediments in suspension are deposited more quickly in the bottoms and the clearer water rises to the surface. This sequencing therefore decreases the clarification time or, at equal yield, the volume of facilities. The single tank dispenses with sludge transfer mechanisms.

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

Fixed crop microstations

In fixed culture, the tanks have two or three compartments. As in free culture, the first tank provides the primary treatment. The second concentrates the system-specific, large-area biological carriers. These are real bacteria nests, crossed by the forced circulation of the mixture of pretreated effluents and oxygenation bubbles. The aerobic microorganisms remain attached to the supports by a kind of natural glue made up of molecules that they excrete. Like the balls of the SBR system, these supports of different natures (honeycombs, minerals, rock wool or vegetable fibres, etc.), concentrate large quantities of bacteria in a small volume, without risk of clogging. A variant comprises discs with semi-emerged rotary movements. This technique aims to simplify maintenance operations.

Advantages of micro sewage treatment plants:

  • allows construction in areas not served by mains drainage,
  • low land occupation (minimum between 5 and 10 m²),
  • no release of foul odors,
  • very good depollution performance (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,
  • binding regular maintenance for the user,
  • annual maintenance by a specialized company, strongly recommended,
  • mandatory emptying, by an approved company, when the sludge reaches 30% of the volume of the tank.

Costs of installing wastewater treatment plants

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

Operating constraints and maintenance costs are decisive criteria for choosing a treatment plant. In fact, in these areas, there are great disparities depending on the techniques used.

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

An annual maintenance contract is offered, depending on the stakeholders between 150 et 400 € TTC.

The price for emptying sludge varies from 180 at 300 € TTC.

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