The sewage works

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Tohannic sewage works

Built and commissioned at the end of 1996, the Tohannic works, which has a rated load of 60,000 equivalent inhabitants, is capable of treating 9,500 m3 per day using biological processing of sludge activated by extended aeration.

The six stages of treatment

1. Pre-treatment

The main role of this first stage is to protect the equipment and the treatment facilities themselves:

  • The largest waste is trapped by two rakes.
  • The grit settles naturally to the bottom of the grit chamber and is then washed and drained.
  • Greases, whose floatability is optimised by air injection, are raked off the surface before undergoing the process in stage 5.

2. Biological treatment

Effluent then enters the two basins for phosphate removal and aeration.

In these basins, pollution is transformed by naturally occurring bacteria. According to the type of pollutants, some micro-organisms need oxygen, others do not.

The basins are therefore split into two parts, one of which is aerated with very small air bubbles at a rate of 5,000 m3 per hour.

The continuous development of these bacteria allows sludge to be removed on a daily basis. It is then treated in stage 6.

3. Clarification of treated water

The two clarifiers are designed to separate the sludge produced in the previous stage from the water purified by natural settling.

The surface water collected is returned to the natural environment after first being analysed by the station's laboratory.

The sludge, on the other hand, goes back to stage 2 to absorb the pollution that is continuously arriving.

4. Odour treatment

Sewage and its by-products produce odours.

To protect the environment and improve operating conditions, the air is sucked in when there is a risk of odours, and washed with acid, bleach and then caustic soda.

5. Grit and grease treatment

The grit is drained into a screw conveyor.It is taken to a waste disposal site.

The greases, which are stored in a treatment basin, are used to feed the bacteria. Their growth is encouraged by aeration and added fertilizer.They return to stage 2.

6. Biological sludge treatment

The sludge taken from the "water treatment process" is useful as it has a propensity for reducing nitrogen and phosphorous levels significantly.It is concentrated in two stages:

  • Floatation, which multiplies the initial concentration by eight
  • Centrifuging, after which it nevertheless contains 80% water (or has 20% dry solid content).

The dehydrated sludge is taken to a liming workshop and is then stored in covered silos. All the raw material that is transported is weighed on a weighbridge.

In parallel with the conventional process, Vannes town council has implemented innovative new technology known as Biolysis®E with the aim of reducing the production of sewage sludge by 70%.

The process can be summarised as follows:an important quantity of sludge is continually removed from the aeration basins, this flow is taken in to the special reactor where it stays for more than 40 hours at a temperature of 65°C before being returned to the basin it was taken from. At the end of 2006, this project was still in its initial stages.

Tohannic is capable of processing human waste collected in Vannes urban area.

Prat sewage works

Located in Prat industrial park, this works opened in 1981, brought up to the requirements of EU standards in 1995 and has a  rated capacity of 35,000 equivalent inhabitants, which enables it to process a daily biological oxygen demand (DBO5) of 2,100 kg

This station is equipped to process 5,500 m3 per day with the biological processing of sludge activated by extended aeration 

The five stages of the treatment

1. Pumping and straining

The raw sewage arrives at the station along two gravity mains. It is then pumped, and undergoes automatic straining before entering the preliminary treatment system to remove the largest waste (rags, sponges, bottles, etc.)

2. Degritting and degreasing

The grit and greases must be trapped before they enter the aeration basin so as to prevent deposits, clogging of pipes and pumps, and in order to eliminate non-biodegradable greases.

This treatment is carried out in a sloped longitudinal basin where the grit deposited at the bottom is pumped to a hydrocyclone, then into a grit-washing machine.

The greases, whose floatability is increased by air scouring with air micro-bubbles, are raked off and stored in a tank before being taken away to a disposal site.

3. Aeration basin

In this 6,000 m3 basin, the effluent to be treated is brought into contact with the purifying micro-organisms that are present in the sludge.This is the main stage of the treatment.

4. Clarifier

To ensure the separation of the purified water and the sludge, the mixture is fed into the central part of a circular basin 28 metres in diameter which has a volume of 1,530 m3.

The sludge settles on the basin bottom while the purified water is drained off by means of an overflow.In order to ensure the sludge returns quickly to the aeration basin, it is sucked out of the bottom of the clarifier by a suction bridge.

The excess sludge is discharged into a static thickener before mechanical dehydration on a centrifuge which allows the sludge to attain a dry solid content of 15%.The dehydrated sludge is limed and sent to a composting centre.

5. Odour treatment

Sewage and its by-products give off odours.

To protect the environment and improve operating conditions, the air is sucked in when there is a risk of odours, and washed with bleach and then caustic soda.

Before being discharged in the natural environment, a recording instrument measures the amount of purified water.



Place Maurice Marchais

Tél. : 02 97 01 60 00



Monday to Friday from 8h to 12h15 and from 13h15 to 18h.Saturday morning from 9h to 12h.



7 rue Joseph Le Brix

Tél. : 02 97 01 60 00


Ouverture :


Monday to Friday from 8h15 to 12h15 and from 13h15 to 17h.


Permanence of the state civil service, Saturday 9am to 12pm.Are given priority: the declarations of birth and death as well as passport applications (by appointment).


Saturday morning, home phone from 9h to 12h.