Treatment of waste water

The Waste water are all water charged with various elements owing to the fact that they were used for washing or the Toilette S, which are likely to pollute the mediums in which they will be poured. This is why, to ensure respect of these various mediums, treatments are carried out on these effluents.

These treatments can be realized in a collective way in a Purification plant or in an individual way. The majority of the purification plants function according to the same basic processes, but of the more or less important differences can exist in the manner of setting up these processes. The treatment is generally divided into several stages.

Pretreatment

The pretreatment consists of three principal stages which make it possible to remove water the elements which would obstruct the following phases of treatment. All the purification plants are not inevitably equipped with the three, only cleaning is generalized, the others are desanding and de-oiling.

Cleaning

Cleaning makes it possible to withdraw water the large Déchet S such as the branches, the bottles out of plastics or the quills. With this intention, the waste water passes through one or more grids whose meshs are increasingly tight. The grids are in general equipped with automatic systems of cleaning to avoid their filling.

Desanding

Desanding makes it possible by Décantation to withdraw the Sable S mixed in water by Ruissellement or brought by the erosion of the Canalisation S. This material if it were not removed would further deposit, obstructing the operation of the station and, causing a faster wear of the machine elements as the Pompe S. extracted sands can be washed before being put in discharge, in order to limit the percentage of organic matters, its degradation causing of the odors and a mechanical instability of material.

De-oiling

The floatation of oils allows their removal of the effluent by simple foaming (i.e. which one removes foam to create by agitation). It is important to limit to the maximum the quantity of grease in the works to downstream avoid for example a clogging of the works, in particular of the drains, problems of rejections of lubricating particles, difficulties of decantation or disturbance of the gaseous exchange, oxygenation in particular.

Desanding and de-oiling are generally carried out in the same work. Sands elutriate at the bottom of this one while greases go up on the surface.

primary education Treatment

In purification of waste water the primary education treatment is a simple decantation which makes it possible to remove the major part of the suspended matter. These are the matters which are responsible for the disorder of waste water.

The operation is carried out in mud tanks whose size depends on the type of installation and the volume of water to be treated. Same manner, the residence time of the effluents in this basin depends on the quantity of matter to eliminate and the capacity from the installation to eliminate them.

The decantability of the matters in a basin is determined by the Indice of Mohlman. This index is given each day in the important purification plants in order to check the good performance of the system.

At the end of this treatment, the decantation made it possible to remove approximately 60% of the suspended matter, approximately 30% of the biological Demande out of oxygen (DBO) and 30% of the chemical Demande out of oxygen (DCO). This DBO share removed was induced by the suspended matter. The organic load remaining to be treated is reduced of as much.

The removed matters form at the bottom of the decanter a bed of muds called primary muds .

Secondary treatment

The secondary treatment is done most usually by biological way. A physicochemical way can replace it or be more often added to it to support the Floculation and Coagulation of muds or to allow the fixing of the Phosphate S.

Treatment by biological way

Treatment of the organic compounds

The simplest degree of the biological treatment consists in eliminating the organic compounds such as Sucre S, grease S, Protéine S, etc Those are harmful for the environment since their degradation implies the oxygen uptake dissolved in water necessary to the survival of the aquatic animals. The load in organic pollutants is measured commonly by the DBO5 (request biological (or biochemical) Oxygenates of it over 5 days) or DCO (Chemical Demand for Oxygen). The bacteria responsible for the degradation of the organic compounds are heterotrophic.

Nitrification

If the biological engines allow a time of sufficient contact between the effluents and the bacteria, it is possible to reach a second degree of treatment: nitrification. It is about the oxidation of nitrite ammonia, then out of nitrate by nitrifying bacteria. Ammonia is a poison for piscicultural fauna. The nitrifying bacteria are Autotrophes (they fix themselves carbon necessary to their growth in CO2 of the air). They thus grow much more slowly than the heterotrophic ones. A purification plant must initially eliminate the organic compounds before being able to nitrify.

Denitrification

A third optional stage consists with to denitrify (or dénitrater) the nitrates resulting from nitrification. This transformation can be done by pumping part of water charged with nitrates with the biological end of treatment and mix it with the water of entry, at the head of treatment. Denitrification occurs in an engine anoxic, in the presence of organic compounds and of nitrates. The nitrates are reduced in Diazote (N2) which escapes in the air. The nitrates are nutrients which are at the origin of the invasion of algae in certain seas, in particular the North Sea. Denitrification is generally done on the small stations of purification in the same basin as nitrification by syncopage (stop of ventilation, phase anoxia). This stage tends to spread to protect the natural environment.

Treatment by physicochemical way

It gathers the ventilation and the mixing of water but also a secondary decantation (known as also clarification).

Starting from this last element, clarified water is rejected (except possible tertiary treatment) and elutriated muds are returned in greater part towards the basin of ventilation, the surplus part being directed towards a circuit or a specific storage.

The secondary treatment can comprise phases of Anoxie (or a separate part in anoxia) which makes it possible to degrade the Nitrate S.

Tertiary treatment

Déphosphatation

The treatment of phosphorus can be required on the stations superiors of 10.000 living equivalent. It can be asked on more small stations of purification according to the sensitivity of the receiving medium.

Two types of treatment are possible:

  • physicochemical treatment
  • biological treatment.
A biological treatment supplemented by a physicochemical treatment is often used.

Bacteriological treatment

The tertiary treatment is not always carried out. This stage makes it possible to reduce the number of bacteria, therefore of germs Pathogènes present in treated water. It can be asked in zone of bathe or to protect a collecting from drinking water. This treatment can be carried out by ozonization or a treatment UVc.

Bacteriological treatment by UVc radiation

There exists a certain variety of systems on the market. The traditional principle of disinfection per radiation UV consists in subjecting water to be treated with a radiation source UV by making it forward through a channel containing a series of submerged lamps. For a few years, one has found also, especially for the small stations of treatment of waste water, a system based on engines monolampe, which offers advantages to the level of maintenance and costs of implementation.

Treatment by physicochemical way

Tertiary treatment included one or more of the following processes:

  • disinfection by the Chlorine or the Ozone (to eliminate the germs Pathogenic S).
  • neutralization of metals in solution in water: while varying the pH of water in certain beaches, one obtains a decantation of these pollutants.

Internal bonds

External bonds

  • UV for waste water: photographs of sterilizers UV for tertiary treatment.

  • activated carbon in the waste water treatment

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