Purification of water

The purification of water is a whole of techniques which consist in purifying water either to recycle the Waste water in the Natural environment, or to transform natural water into Drinking water.

Waste water

Biological dies

The biological processes are generally used for the secondary treatment of urban and industrial waste water. In their basic configuration, they are primarily employed for the elimination of the carbonaceous compounds present in soluble form such as Sucre S, Graisse S, Protéine S, etc, for which the solutions by physicochemical way are often not very effective, expensive or difficult to implement. Those are harmful for the environment since their degradation implies the oxygen uptake dissolved in water and necessary to the survival of the aquatic animals. The goal of the biological treatments is to eliminate soluble organic pollution by means of micro-organisms, Bactérie S mainly. The micro-organisms Hétérotrophe S, which use the organic matter like source of carbon and energy, have a double action:

the organic matter is partly eliminated in gas form during the mineralization of carbon with production of CO in the aerobic processes and Biogaz (CO + CH) in the anaerobic processes,
and partly transformed into solid particles made up of micro-organisms resulting from the bacterial multiplication. These particles can be easily separate liquid phase by physicochemical means such as the decantation for example.

If necessary, the transformation of the ions ammonium (NH+) into nitrates (NO) or nitrification can be realized simultaneously.

These processes can also make it possible to eliminate nitrogen and phosphorus by biological way with the help of the implementation from additional stages in the die of treatment: installation of a basin of anoxia, a basin of anaerobe,….

The various processes used can be classified according to the conditions of ventilation and implementation of the micro-organisms. Thus, one distinguishes:

aerobic processes with free cultures or activated sludge,
aerobic processes with fixed cultures,
anaerobic processes with free cultures,
anaerobic processes with fixed cultures.

The load in organic pollutants is measured commonly by DBO5 (Request Biochemical Oxygenates some over 5 days) or DCO (Chemical Demand for Oxygen).

Aerobic treatment

The aerobic biological dies call upon the Micro-organisme S naturally present in the natural environment to degrade the Pollution. They miment properties of purification of the grounds (planted filters of reeds, sand filters) or of the rivers (lagunage, activated sludge). The contribution of oxygen can be natural (wind or system of cascade) in the small installations of Lagunage, or artificial (turbine or diffusion of microbubbles) in the purification plants of the " type; muds activées". The Bactérie S can be free (activated Boue, Lagunage) or fixed (bacterial Lit, planted filters, sand filters, Bio-filter) or biodisques.

Anaerobic treatment

See the article on the Methanisation.

Elimination of nitrogen

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: the 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 the CO of the air). They thus grow much more slowly than the heterotrophic ones. A communal purification plant must initially eliminate the organic compounds before being able to nitrify.

A third stage consists in denitrifying nitrates resulting from nitrification. For that, it is necessary to pump part of water charged with nitrates with the biological end of treatment and to mix it with the water of entry, at the head of treatment. The Dénitrification occurs in an engine anoxic, in the presence of organic compounds and of nitrates. The nitrates are reduced out of elementary nitrogen (N2) which escapes in the air in the form of bubbles. The nitrates are pollutants which are at the origin of the invasion of algae in certain seas, in particular the North Sea.

Physicochemical dies

The physicochemical dies use average physiques (decantation, floatation, filters and membranes) and/or chemicals, in particular of the coagulants (ferric Chloride, aluminum Sulfate…) and of the Flocculating S. One uses them for certain industrial effluents (toxic) or when one must manage fast variations of flows to be treated (case of the purification plants of tourist communes, or when with a unit network one wants to face the rainwater arrival). In the actual position of technologies, the membranes of Microfiltration and Nanofiltration are still reserved for the potabilisation of water.

In the complex installations having to treat several parameters, one can meet the two dies simultaneously.

Classically an urban purification plant with activated sludge includes/understands the following stages:

pretreatment: cleaning, desanding, primary education de-oiling
treatment: simple decantation with recovery of muds and creaming of the floating ones.
secondary treatment: ventilation and mixing, secondary decantation (known as also clarification). Starting from this last element, clarified water is rejected (except 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.
possibly tertiary treatment of Coagulation-flocculation or disinfection by the Chlorine or the Ozone (to eliminate the germs Pathogenic S).

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

The dies of water treatment can comprise a stage final, known as " tertiaire" treatment; , including one or more following processes:

disinfection by chlorine or ozone (to eliminate the pathogenic germs).
metals in solution in water can be neutralized: while varying the pH of water in certain beaches, one obtains a decantation of these pollutants.

But each stage produces in its turn of the by-products which also should be eliminated: coarse waste, sands and especially the muds consisted of dead bacteria.

Therefore, in parallel of the processing circuit of water, the factories of depollution also comprise a data processing sequence of muds. The goal of the treatment of muds is to stabilize these muds (to make them inert) by a means which perhaps physicochemical with for example of the lime, or biological while letting remain muds in " digesteurs" (large storage heated and brewed in Anaerobic). Treatment then including/understanding works of decantation (one speaks then about thickening), of storage and dehydration (press, filter press, centrifugal machine), even of drying, the valorization in Biogaz, or even of Incinération. Metals in solution in water can be neutralized: while varying the pH of water in certain beaches, one obtains a decantation of these pollutants. The digestion of muds produces Méthane (CH₄), commonly called gas for domestic use, which when it is produced in rather great quantity is used as energy (electric production, boiler, etc), and of the Hydrogen sulfide (H₂S), which can cause asphyxiations in confined surroundings.

When the clarification sludge is free from toxic products, one can recycle them in agriculture with the help of a conditioning suitable to facilitate their handling and their storage on site (treatment the lime). When they are polluted, it is necessary to put them in Décharge. An elegant solution for the local government agencies is of the Compost er with the green Résidus or to carry out a Méthanisation to produce Biogaz. In other countries, the dies of elimination can vary. In Switzerland for example, the setting in mud discharge is prohibited and agricultural valorization ended on October 1st, 2006 (with extension by two years in certain cases) this because of the risks for health and the grounds and under the terms of the precaution principle. The only authorized die is thermal elimination (factories of incineration of the household refuse, cement factories).

Lastly, a third circuit (optional) ensures the treatment of the polluted air. It can be him also biological or chemical.

Collective cleansing - cleansing not-collective

Lawful concept

Distinction established by the L.2224-8 article of the general code of the territorial collectivities, concerning the purification of domestic waste water. The collective cleansing is that which is dealt with completely by the community (the commune, or the public corporation of inter-commune co-operation - EPCI - to which it delegated this competence): collect, transport, treatment, rejection in the natural environment of treated water, and elimination of the by-products. The cleansing not-collective is that which does not profit from this assumption of responsibility. The common one has nevertheless the obligation to exert control of it (control of design, execution, good performance, good maintenance) and can, if she wishes it, deal with maintenance of it.

The L.2224-10 article forces the communes to define, on the whole of their territory, the zones which will concern the collective cleansing or not-collective.

Technical concept

The techniques of collective cleansing are described herebefore. There is no technique of cleansing not-collective strictly speaking, since it is about a lawful concept, and nontechnical. However, for the purification of waste water of an individual dwelling (some inhabitants), there exist specific techniques, which one will describe as individual cleansing, or autonomous. These techniques call upon biological dies exclusively. Four elements in an autonomous installation of cleansing:

Collection : it is a question of making leave waste water the building, to guide them towards the point where the pretreatment will take place. It is thus the whole of the drainage pipes since each water point of the building.
Pretreatment : the objective is to change the nature of waste water to make possible their purification by the die of treatment to the downstream towards which they will be then directed. Water is directed, at exit of building, towards a large closed container and most of the time buried, named " pit all eaux". In this pit, the floating ones (whose greases) will be retained, the heavy solid particles also (they fall at the bottom), and of the processes of fermentation liquefy the organic solid matters, and break the macromolecular chains. A certain abatement of pollution takes place already in the pit, by the retention of floating and of the solid matters. It can reach 30%.
Traitement : At exit of pit, the effluents are directed towards a filter (bacterial colonies on fixed support, to see higher) which ensures purification. Composed of sand, it must be ventilated permanently to allow the breathing of the purifying bacteria. It should not thus be too much buried, and surfaces it which recovers it should be neither compacted, nor waterproofed (tar or cement is proscribed). The distribution of the effluents in the filter is allowed by the water run-off in a series of filter pipes which recover the filter.
Evacuation

Production of drinking water

Purification in France and Europe

Downstream from Paris, in Yvelines, the purification plant of Achères treats theworn ones of 6 million Franciliens. This station would not respect, in 2007, a directive adopted in 1991, on the urban waste water treatment.

Amphitria, an of the department of Var purification plant. It is, to date, the only factory of the Mediterranean basin (in France?) with being in conformity with the European regulation.

The denomination purification plant is an objective and not a reality: in the analyzed effluents, we raised the presence of 26 pharmaceutical agents belonging to six therapeutic classes: antibiotic , Béta-blocking S, of disinfectants, the Antiépileptique S, the Anti-inflammatory drug S and the regulators of lipids”, explains Roberto Andreozzi, of the University of Naples “Federico II”, coordinator of the project. The researchers passed to the fine comb the behavior of six of these products chosen among most common: Carbamazépine, Acid clofibric, Diclofénac, Sulfamétoxazole, Ofloxacine and Propanolol HTTP: /ec.europa.eu/research/rtdinfo/40/article_481_fr.html

See too

Lagunage
Methanisation
Clarification sludge
W: Sewage treatment: The purification of water such as it is described on anglophone Wikipédia east seems it rather different from what is presented in this article, because of the use of the process of Méthanisation which digests pollution and asepticizes it. The elimination of the Nitrate S comes then. This process, very widespread in the Sixties, is not used practically any more in France for the urban effluents. In Europe, the priority is given to the elimination of nitrates, which is not inevitably the case in all the countries.

External bonds

the methanisation of the industrial effluents Site of information for natural purification by methanisation and the production of renewable energy: Biogas.
Water treatment by solar energy [[Ekopedia]] (This bond should rather be useful for filtration of drinking water that for purification) June 2nd, 2007 with 08:21 (CEST)
Filtrage of water by phytoepuration (or lagunage) Ekopedia
http://cartel.oieau.fr/
Service of Data administration and Référentiels on Water (PIKE PERCH): Main features of the purification plants in autonomous France
Purification plant with biological cleansing Information, French laws and an explanatory cartoon film on the process in aerobe and anaerobe.
Association EDEN of promotion of the methanisation and the valorization of biogas
Association AQUATERRE Any knowledge on purification by the plants
the Laboratory of Biotechnology of the Environment: Laboratory public of research of the INRA whose sets of themes of research relates to the water treatment. Internet site rich in contents and downloadable documents.

References

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