WO1996038385A1 - Method for purifying water containing pollutants and additive for implementing same - Google Patents

Abstract

Method for purifying water containing pollutants, including subjecting the water to be purified to at least one treatment step, particularly a clarification and/or biological activation step, followed by a separation step into a purified water component and an optionally activated sludge component. A mineral additive added to the water to be purified has the following properties: it is non adsorbent and chemically inert relative to the pollutants present in the water to be purified, it is insoluble or weakly soluble, i.e., with less than 5 % solubility, in water under normal treatment pH conditions, it has a specific surface area of more than 0.4 m2/g (as measured by BET), a microporosity of less than 0.1 cm3/g, a residual cloudiness of less than 300 NTU and a rate of fall of less than 35 m/h. The invention is useful for purifying industrial or municipal waste water and for producing drinking water.

Description

Process for purifying water containing a polluting load, and additive for implementing the process

The invention relates to a process for purifying water containing a polluting load, as well as an additive for carrying out the process.

The studies and experiments which made it possible to obtain the results of the invention were carried out thanks to the collaboration of the team of Mr. Jean CANTET of the University of Research of Biological Processes at the INSA of Toulouse and thanks to the collaboration of the team of Mr. Alain GRASMICK of the Department of Water Sciences and Technologies of the Institute of Engineering Sciences of Montpel¬ lier.

The invention applies generally to the purification and purification of water using treatment units operating chemically, physico-chemical, or even biologically.

The term “water” currently means either used water of industrial or municipal origin, or water to be made potable.

The term "purification" is intended to mean both a purification of waste water and a purification with a view to obtaining drinking water.

The expression "pollutant load" intends to designate at present all the compounds of all chemical, organic or inorganic natures making either a waste water unfit for fish life (therefore its discharge into the river), or a drinking water unfit for consumption . This polluting load is expressed by its flow at the inlet of the purification unit, that is to say the mass compared to the time unit, whether it describes carbon pollution (COD or BOD), pollution particulate matter (MES), nitrogen pollution (N-NH4, N-NO3, total N) or phosphorus pollution (P-PO4, total P).

COD indicates the chemical oxygen demand, expressed in mg / liter of oxygen equivalents.

BOD is the biological oxygen demand expressed in mg / liter of oxygen equivalents.

MES indicates the Suspended Matter expressed in mg / liter.

N-NH4 designates the ammoniacal nitrogen content expressed in mg / liter.

N-NO3 designates the nitric nitrogen content expressed in mg / liter.

N total denotes the total nitrogen content expressed in mg / liter.

P-PO4 designates the phosphorus content of orthophosphates expressed in mg / liter.

P total denotes the total phosphorus content expressed in mg / liter.

During their purification, the waters are usually subjected to a prior operation, called "pre-treatment", by screening and / or grit removal and / or grinding. The purpose of this operation is to eliminate solid particles of all sizes and of all kinds, which are carriers of pollution and create deposits or mechanical breakdowns in subsequent treatment facilities.

When it is a question of drinking water, the latter is first screened and sanded and can then undergo different complementary treatments by simple or accelerated decantation in the presence or not of chemical additives, by filtration on gravity bed or on support, or by flotation using air bubbles.

Then, the water to be made potable is subjected to specific treatments for removing pollutants by passage through resins, activated carbon or by ultra-filtration, nano-filtration, chlorination, ozonation, nitrification and denitrification, etc.

After screening and sand removal, the wastewater can undergo additional treatments by simple or accelerated decantation with or without the addition of chemical additives, by filtration, by flotation using gas bubbles (generally air), before being sent to biological tanks. As a variant, this wastewater can be sent directly to these biological basins after the sand removal.

In biological basins, also called "activation basins", the water to be purified is subjected to biological degradation in the presence of a biomass composed of purifying microorganisms, that is to say bacteria.

For the treatment of wastewater, biological degradation is carried out by bacteria of various natures in order to eliminate carbon and / or nitrogen and / or phosphorus pollution, this degradation taking place in one or more biological basins undergoing phases aerobic, anoxic and anaerobic. Microorganisms or bacteria generally agglomerate to form flocs which are separated from treated water by difference in density with the treated water or by size exclusion in a "secondary clarifier". The term "secondary clarifier" is understood to mean an apparatus in which separation takes place by flotation, by decantation, by hydrocycloning, by centrifugation or by filtration. The purified water can then be directly discharged into the river, while the thickened flocs called "sludge", collected in the secondary clarifier are partially recycled to the basins biological in order to maintain a high number of purifying micro¬ organisms and maintain biological activation, the other part being a waste to be treated.

In all types of treatment, the water to be purified is subjected to at least one particular operation, in particular by decantation and / or biological activation, with subsequent separation of purified water and optionally activated sludge.

Whatever the treated water, the ultimate sludges from the different settling operations are drawn off, thickened, then conditioned to reduce their volume and mass for agricultural use by spreading, landfill or incineration.

The operators of water treatment plants are faced with various problems, the most important of which are:

- incomplete purification due to improper sizing of the equipment with regard to the pollution to be treated;

- particular pollution which cannot be treated by known biological processes;

- in the case of a biological process, poor flocculation of microorganisms making separation difficult, random, or even impossible in the secondary clarifier; and

- in the case of drinking water, poor settling of the colloidal particles in suspension.

To solve these problems, the operators of treatment plants must increase the size of their treatment units, profoundly modify existing installations, or use coagulation and / or chemical flocculation and solid products with high adsorption or absorption of pollutants.

These solutions have the drawback, on the one hand, of introducing additional pollution, often harmful to micro¬ organisms in the case of chemical additives and, on the other hand, of causing permanent circulation of products strongly adsorbed or absorbed by pollutants.

We know, from HAMODA and FAHAIM, Environmental Pollution (Series A) 36 (1984) 283-294, a wastewater treatment process which includes the addition of activated carbon or kaolin as adsorbent of carbon pollution, in a biological basin.

However, this is an experimental process, unsuitable for the industrial scale, because the mineral used leaves a cloudy purified water, not compatible with discharge into the river.

In addition, in the particular case described, part of the pollution is transferred from water to sludge, not improving the purifying quality of the biological system.

The authors also noted a slight increase in the rate of sedimentation, of 20%, generally observed during the addition of dense minerals which compress, by simple piston effect, the flocs of microorganisms present.

Also known, according to PCT Application WO 94 20425, is a process for the purification of waste water of the activated sludge type making it possible to increase the purification yields.

This is a process using biological basins or activation basins, in which the wastewater is brought into contact with a biomass and with a hydrophobic powder of talc, pyrophyllite or mica, to ballast the micro flocs -organisms. This process is only applicable in the case of flocs presenting hydrophobic sites and the authors have not been able to extend the Patent Application to the physicochemical separation of solid or colloidal particles which have essentially hydrophilic characteristics.

The addition of clay in biological water purification basins is known from CHUDOBA and PANNIER, Environmental Technology, 1994, Vol. 15, pages 863-870, to sometimes improve the treatment of wastewater.

The authors note a reduction in the volume of sludge of 30% which, as in the case of the publication of HAMODA and FAHAIM, is due to a piston effect all the more important as the clay particles used are of size and weight important.

The particles of 100 to 200 micrometers used sediment very quickly at the bottom of the basins, requiring heavy and specific recirculation installations to avoid the formation of a compact and dense deposit. This problem is particularly damaging in installations brewed and aerated by gas diffusion at the bottom of the tank.

It is also known, from the publication of Japanese Patent JP-A-4 253 793 of August 24, 1992 in the name of SHIKOKU KASEI KOGYO, the use of gel and polyelectrolyte to agglomerate flocs of microorganisms. The authors recommend ballasting this synthetic gel preformed with clays. In this article, the authors do not prejudge the specific effect of mineral additives.

Also known, from EP-A-0 383 674, is a process for the biological treatment of waste water, which uses granular material at very high use rates and mass concentrations. This process therefore requires large quantities of additives, which poses recycling problems. Finally, the so-called "ACTIVELO" process is known, which is developed by the company OTV which, in primary basins, combines the addition of microsands with coagulants and chemical flocculants to weigh down colloidal particles.

This process is similar to the Japanese process described above but is intended more particularly for the primary settling of solid and colloidal fillers. It combines pairs of coagulating and flocculating chemical agents with ballast charges with coarse particles. This process is limited to the treatment of very dilute water.

The last two processes described require large quantities of chemical additives.

The object of the invention is in particular to overcome the drawbacks of known purification methods.

It is also an object of the invention to provide such a purification process which makes it possible to improve the purification performance of wastewater treatment units, operating by chemical route, by physico-chemical route, by biologi¬ that, without inducing additional pollution or recirculating inside biological basins, adsorbed pollution.

To this end, the invention provides a process for purifying water containing a polluting load, in which the water to be purified is subjected to at least one treatment operation, in particular by decantation and / or biological activation, with subsequent separation. purified water and possibly activated sludge.

In accordance with the invention, at least one mineral additive in the form of powder, granules or suspension is added to the waste water to be treated, and having the following properties: non-adsorption (as defined below) and chemical inertness with regard to the polluting load present in the water to be treated;

- insolubility or low solubility, less than 5%, in water, under the usual pH conditions (pH5 to pH9) encountered during treatment;

- specific surface greater than 0.4 m ² / g (BET method);

- microporosity less than 0.1 cm ³ / g;

- residual turbidity below 300 NTU, as measured in a pure water solution at 3 g / l after 30 minutes of decantation; and

- falling speed less than 35 m / h.

The adsorption capacity of the pollutant load by the mineral additive is calculated, on the one hand, by measuring the difference in D.C.O. soluble between the raw water to be treated and this same water after 5 h of contact with 1 g of additive. This difference in D.C.O. soluble must be less than 60 mg / g of additive, to qualify this product as non-adsorbent.

On the other hand, the nitrogen adsorption capacity N-NH ₄ is determined in the same way. This adsorption capacity must be less than 10 mg per gram of additive.

The BET method designates a method by measurement of gas adsorption isotherm according to BRUNAUER, EMMET and TELLER, the gas used being argon.

The microporosity is determined by measuring the adsorption-desorption gas isotherm according to BET, the gas used being nitrogen.

The speed of fall is measured, when decantation takes place in a laminar regime, using a device called "Sedigraph" from the company MICROMERETICS, and, when decantation takes place in a non-laminar regime, using a one meter water column. The fall speed characterizes the sedimentation of 95% of the additive. The measurement with the "Sedigraph" device is carried out in a 4 cm high sedimentation cell. The calculation is carried out by monitoring the concentration as a function of time by reading the optical density.

NTU is the abbreviation for Nephelometric Turbidity Unit (see ASTM 1880-81).

Thus, the invention is based on the addition of at least one specific mineral additive with predetermined physicochemical characteristics, which makes it possible to modify the flocculation of particles and / or colloids in suspension, as well as the bio¬ flocculation of microorganisms , to improve the purification performance of water treatment units.

According to another characteristic of the invention, the mineral additive is a natural or synthetic product chosen from at least one of the chemical families formed by silicates, aluminates, carbonates, sulfates and insoluble or weakly soluble glasses with usual pH conditions described above.

The mineral additive can be synthesized by different routes to obtain particular physico-chemical characteristics, such as specific surface or rate of fall.

The mineral additive can be obtained naturally or else be ground before being chosen (switching to the selector) to obtain specific physicochemical properties such as porosity, turbidity or rate of fall.

The mineral additive is preferably chosen from one of the following chemical families, given without limitation and characterized by all the criteria described above: - aluminosilicates,

- magnesium aluminate,

- hydrated calcium carbonate,

- hydrated calcium and magnesium double carbonate,

- amorphous or crystallized calcium carbonate,

- hydrated magnesium silicate,

- hydrated aluminum silicate,

- hydrated magnesium and aluminum double silicate,

- hydrated calcium sulphate.

Advantageously, the mineral additive is chosen from at least one of the following groups of products, given without limitation:

feldspar, wollastonite, cristobalite, quartz, nepheline, micas, muscovite, chlorites, talc, sepiolite, serpentine, calcite, dolomite, giobertites, aragonite, gypsum, kieserite, kaolinites, pyrophyllite, olivine, andalusite, basalt or even alluvium. A particular example according to the invention consists in using all or part of the solid residues from treatment plants, when these residues contain significant quantities of one or more additives mentioned above.

Significant synergy is obtained when the mineral additive used in the process of the invention comes from mixtures of families of the minerals described above, whether this mixture is prepared in advance or that it is produced on site by successive and alternating additions of different types of mineral. The invention applies in particular to a process in which the water to be purified is water to be made potable and is subjected to a primary separation operation in a primary clarifier, with decantation.

The mineral additive according to the invention is then added before or during the primary separation operation, so that the daily mass (also called "daily mass") of the mineral additive added is equal to 1 to 500 times the daily mass of the pollutant load contained in the water. Thus, a drinking water station receiving 100 kg of D.C.O. per day will use 100 to 50,000 kg of additives per day.

The invention also applies to a process in which the water to be purified is waste water and is subjected to a primary separation operation in a primary clarifier, with decantation, and without treatment in biological basins.

According to the invention, the mineral additive is added before or during the primary separation operation, so that the daily mass of the added mineral additive is equal to 0.1 to 10 times the daily mass of the pollutant load contained in water.

In the two aforementioned processes, a mineral additive is used which advantageously has a residual turbidity in water generally less than 200 NTU and a falling speed of between 0.03 and 35 m / h.

The method thus makes it possible to improve the flocculation of the colloidal particles and of the particles in suspension present in the primary clarifier. This results in particular in the following advantages:

- improvement of the settling speeds in the primary clarifier, whether simple or accelerated (case of primary clarifiers); - increase in the reduction (reduction) of pollution in the primary clarifier, whether simple or accelerated; and

reduction or even elimination of the amount of chemical additive, namely iron or aluminum salts and cationic polymers based on acrylamide, acrylamide or epichlorohydrin.

The mineral additive is then introduced, either into the primary clarifier or upstream of it, to improve the performance of the yield, sedimentation, sludge height and sludge thickening, the introduction of the additive being made by direct route in the form of powder, granules or suspension.

The invention also applies to a process in which the water to be purified is water to be made potable and is subjected to a primary separation operation by flotation. In accordance with the invention, the mineral additive is added before the primary separation operation, so that the daily mass of the added mineral additive is equal to 1 to 100 times the daily mass of the pollutant load contained in the water.

Preferably, the mineral additive has a residual turbidity in water generally less than 250 NTU and a falling speed less than 5 m / h.

The invention also applies to a process in which the water to be purified is waste water and is subjected to a primary separation operation in a primary clarifier, with decantation and before treatment in biological basins. According to the invention, the mineral additive is added before or during the primary separation operation, so that the daily mass of the added mineral additive is equal to 0.1 to 10 times the daily mass of the pollutant load contained in water. Preferably, the mineral additive has a residual turbidity in water generally less than 300 NTU and a falling speed of between 0.01 and 35 m / h.

Also, the invention applies to a process in which the water to be purified is waste water and is subjected to an operation of primary separation by flotation before treatment in biological basins. According to the invention, the mineral additive is added before the primary separation operation, so that the daily mass of the added mineral additive is equal to 0.1 to 10 times the daily mass of the pollutant load contained in the water.

In this case, it is preferred to use a mineral additive having a residual turbidity in water generally less than 300 NTU and a falling speed less than 5 m / h.

In the case of a process in which the water to be purified is waste water and is subjected to a primary separation operation by flotation before treatment in biological basins, it is possible to use an excess of mineral additive relative to the pollutant load contained in the incoming water. Indeed, the process leaves part of the mineral additive in suspension in the water, entering the biological basins where it will react with the flocks of microorganisms ensuring optimization of the use of addi ¬ mineral mineral in the two stages of water treatment, namely the primary separation and the bio-flocculation of microorganism flocs.

The invention also applies to a purification process in which the water to be purified is subjected to a biological activation operation in at least one biological basin in the presence of a biomass composed of purifying microorganisms and a secondary separation operation in a secondary clarifier with subsequent separation of purified water and activated sludge. According to the invention, the mineral additive is added before or during the secondary separation operation, with a chosen quantity of mineral additive as a function of the biomass concentration at any time, the quantity of added mineral additive representing from 10 to 300% by weight of the biomass in the aeration tank.

In such a process, the mineral additive advantageously has a residual turbidity in water generally less than 300 NTU and a falling speed less than 35 m / h.

The mineral additive can be added in the different biological basins or between the primary clarifier and these, preferably in the aeration tank.

As in the previous variant, the mineral additive is introduced directly in the form of powder, granules or suspensions.

The mineral additive can also be introduced into the secondary clarifier in the event of very poor settling or to reduce the height of the sludge bed.

The mineral additive can, as a variant, be introduced into excess sludge before recirculation and / or thickening and dewatering.

To improve the yields of biological basins, the amount of mineral additive added is determined according to the biomass (mass of microorganisms present), the nature and size of the equipment, as well as the type of operation. low, medium or high load units.

As already indicated, the quantity of mineral additive to be added to the biological tank represents from 10 to 300% by weight of the concentration of the biomass in the aeration tank. At the start, the quantity of mineral additive necessary to obtain the desired performance is added. Then, an additional mineral additive is added, either continuously by slaving or discontinuously, depending on the amount of active biomass present at all times, or alternatively as a replacement for the amount of mineral additive in excess biomass, extracted as sludge from the secondary clarifier.

Thus, in the particular case of activated sludge purification processes, the mineral additive of the invention makes it possible to improve the bio-flocculation of the flocs of microorganisms.

The mineral additive of the invention, which is fine, inert, non-adsorbent and non-polluting, makes it possible to modify the size, the volume and the weight of the flocs of microorganisms in order to adapt them to the problems encountered by the operators of stations. of purification, and this without creating additional constraints.

The mineral additive must be, inter alia, characterized by its rate of fall and its residual turbidity in water to be compatible with the size of the microorganisms and the rate of fall of the flocs of microorganisms, in order to optimize the bio-flocculation and avoid any solid deposit in the water purification units.

Also, the mineral additive is determined by its specific chemical composition, structure and surface to bind preferentially to microorganisms and flocs of microorganisms in order to optimize bio-flocculation and avoid absorption. or the adsorption of impurities present in the water to be treated.

This is how the mineral additive is characterized, among other things, by its rate of fall, its chemical composition and its structure, in order to bind to microorganisms and form large diffuse flocs capable of being floated. In certain cases, it may be advantageous to add to the mineral additive a cationized additive with a short chain, that is to say the carbon chain of which is less than 25 atoms. The concentration of this additive must remain below 5% by weight of the mineral additive.

The invention is compatible with the use of other additives already known for their specific actions on toxic pollution or on soluble pollution that is not degradable by the usual biological purification processes. The additional additive can in particular be an activated or activated carbon, a zeolite, lignite, bentonite, and the like.

The improvement in the separation between the soluble phase and the colloids or particles in suspension, obtained according to the invention, makes it possible to improve the specific action of these complementary additives. This results in greater efficiency of these additives for adsorbing toxic cations, dyes, etc. on the soluble fraction to be purified.

The method of the invention makes it possible to improve the performance of water treatment installations. It leads to a standardization of the physico-chemical properties of the flocs, allowing a reliable separation of the purified water from the sludge and avoiding the displacement of the problem of pollution of the water to the sludge, whose management is increasingly difficult .

By improving the bio-flocculation of micro-organism flocs, the following performances are obtained:

- reduction of the volume of sludge in the secondary clarifier;

- reduction in the size of the secondary clarifier for a given mass of microorganisms; - Increase in the volume of decantable water for a secondary clarifier of given dimensions;

- reduction of losses of non-decantable filamentous microorganisms, with a compact or open structure, under the usual conditions;

- reduction of losses of non-flocculated or deflocculated microorganisms and therefore not settling under the usual conditions;

- reduction of foaming phenomena due to the presence of microorganisms on the surface of secondary clarifiers;

- increase in biological purification yields, carbon and / or nitrogen and / or phosphorus pollution;

- reduction in the size of the biological treatment basins for a given amount of carbon, nitrogen and / or phosphorus pollution;

- increase in the quantity of carbon and / or nitrogen and / or phosphorus pollution treated in a basin of given dimensions.

Furthermore, the presence of a predetermined quantity of mineral additive in the sludge from the primary clarifier and / or the secondary clarifier makes it possible to achieve the following performances:

- improvement of the thickening yields of excess sludge by gravity;

- reduction of the separation times between the biomass and the interstitial liquid, by sedimentation, or thickening, or flotation, in order to avoid any re-solution or transfer of pollution from the biomass to the interstitial liquid, this liquid can be found either in the outlet effluent, either at the head of the station or in the aeration tank;

- improving the yields of excess sludge dewatering equipment;

- improved handling of excess, thickened or dehydrated sludge, by reducing the sticky effect; and

- improvement of the behavior of excess, thickened and / or dewatered sludge during compaction or granulation operations.

The method according to the invention makes it possible to make so-called "high and medium load" water treatment systems reliable by reducing the mass load with, if necessary, external carbon and nitrogen (N-NH4) input.

According to the invention, it also appears possible to carry out operations for separating flocs of microorganisms by flotation and / or sedimentation in a suitable secondary decanter, as well as in installations for processing the extracted sludge, such as flotation separators. , assisted or not, and thickening silos.

The mineral additive of the invention also allows the following performances:

- improvement of the performance of chemical additives called "coagulants" or "flocculants" (iron salts, aluminum salts, cationic or anionic acrylic polymers, epichlorohydrin polymers, etc.) in secondary decanters by reducing the volume of sludge produced and increased settling rates;

- reduction, or even elimination, of chemical additives known as "coagulants" or "flocculants" during secondary decanting operations in the presence of lamellar processes; - Improvement of the separation yields between the treated water and the flocculated microorganisms when using dynamic devices such as lamellar settling tanks, hydrocyclones, centrifuges, etc .; and

- improvement of the separation yields of the filtration processes by gravity beds or on supports.

The process of the invention appears particularly advantageous because it makes it possible, by a judicious choice of the mineral additive, to improve the performance of primary clarifiers without chemical additive, mainly in the case of lamellar clarifiers where it can make it possible to double the speed of reduction (reduction) of pollution.

The method of the invention also makes it possible, always by the judicious choice of the mineral additive, to control the size, the volume or the weight of the flocs of microorganisms in a predictable manner with unique performances for the treatment of 'water. The method makes it possible, by complementary or successive additions, to flocculate the microorganisms first with an appropriate mineral additive, then to increase their density by adding a second suitable mineral additive.

It is also important to point out that the process of the invention is particularly specific, given that it overcomes the volumes of water treated so as to take into consideration only the masses of microorganisms present in the biological basins or pollution flows at the inlet of the primary clarifier.

The performances obtained by the implementation of the process of the invention are completely unexpected.

In fact, the process of the invention brings into contact fine mineral additives whose sedimentation is weak, or even zero, with flocs of microorganisms (or colloidal suspensions) whose sedimentation is difficult. even impossible, to produce clear and limpid purified water (or to increase the speed of abatement of pollutants). In other words, the process makes it possible to sediment a weakly decantable product by adding a weakly decantable additive.

The performances are also unexpected because the addition of the mineral additive makes it possible to produce flocs whose buoyancy performances by assisted flotation or mechanical flotation are superior to those of the unweighted flocs.

It is also surprising that the fine, non-adsorbent mineral additive of the invention has, in pure water, residual turbidities incompatible with the standards required for discharges at the outlet of water treatment stations. However, the mineral additive makes it possible to produce purified water whose turbidity is lower than that measured in the secondary decanter in the presence of flocs of microorganisms alone, therefore in the absence of the mineral additive.

The process of the invention may include, in certain cases, an operation of partial or total recycling of the mineral additive.

The recycling of the mineral additive is carried out using hydrocyclones, centrifuges, or other separation devices commonly used for the separation of solids and liquids in industry.

In another aspect, the invention also relates to the mineral additive used for carrying out the process.

We now refer to the accompanying drawings, in which:

- Figure 1 shows the diagram of a water purification installation comprising a primary separation, with biological basins, a secondary separation and a sludge thickener which allows the implementation of the process of the invention; and

- Figure 2 shows the diagram of a water purification installation by lamellar settling which allows the implementation of the method of the invention.

With reference to the installation of FIG. 1, the water to be purified E passes successively through a screening device 1 and through a desanding device 2 before reaching a decanter or primary clarifier 3 in which decanted water forms 4 located between a layer of supernatant grease 5 and a layer of mud 6 at the bottom of the decanter. The decanted water 4 is drawn off and sent, via a line 7, successively to a first biological tank 8 (optional), called "anoxic tank", a second biological tank 9 (optional), called "anaerobic tank" and a third biological basin 10 operating in aerobic mode. The basin 10 is aerated by means of a stirring device 11. In these different biological basins, the water to be purified is placed in the presence of a biomass composed of appropriate micro-organisms playing a purifying role. The water is then sent to a secondary decanter 12 which makes it possible to separate the water 13 from activated sludge 14, which is, at least in part, recycled in the biological basins through a line 15.

Furthermore, the sludge is collected at the base of the primary settling tank 3 by a line 16 and at the base of the secondary settling tank 12 by a line 17 to be sent to a treatment line making it possible to possibly reduce the putrescible power of the sludge via a digester 18 (optional). From there, the sludge can be sent through a line 19 to a device 20 used for dewatering the sludge (possibly after a thickener). The role of this device is to dewater the sludge mechanically, for example by centrifugation or by pressing. At the outlet of the device 20, the sludge dices tees are sent either to the landfill or to the incinerator.

In accordance with the invention, the mineral additive A can be added to the primary clarifier 3 or upstream of it, in one of the biological tanks 8, 9, 10, preferably in the aeration tank 10, or in the secondary clarifier 12. The additive can also be introduced into the excess sludge before thickening and dewatering before line 19 and the device 20.

Reference is now made to the installation of FIG. 2 which allows the implementation of a process with lamel¬ milk settling tank and column in contact with sludge, the process also applying to lamellar settling tanks with direct counter-current, co-current and cross-current.

The water to be purified E is brought into contact with an additive A previously suspended in a stirred tank 21, before being injected, via a pump 22, at an entry point 23 located at the base of a flocculation column 25 (also called sludge column). The water to be purified crosses, from bottom to top, the column of sludge 25 to penetrate into a decanter 26 and come into contact with the lamellae 27 thereof.

The purified water leaves the decanter 26 through an outlet S arranged at its upper part.

The accumulated sludge is drawn off from pour points 24 provided in column 25 and the settling tank 26.

The invention will now be described with the aid of the following nonlimiting examples.

Example 1

This example shows the increase in efficiency of a lamellar settling tank with sludge contact column, by introduction of a mineral additive. It is implemented in a laboratory pilot having the following characteristics:

Flocculation column:

Height: 1.1 m Width: 0.14 m Depth: 0.12 m Section: 0.0168 m ² Volume: 0.1848 m ³ .

Lamellar settling tank;

Number of slats: 4

Tilt angle: 60 °

Slat width: 1.25 m

Width: 0.12 m

Depth: 0.12 m Section: 0.0144 m ² .

The example refers to wastewater of municipal origin characterized by its Chemical Oxygen Demand (D.C.O.), its Suspended Matter (M.E.S.) and its Turbidity expressed in F.T.U. (Formazine Turbidity Unit). This last characteristic is measured by absorption of light through the sample to be tested with reference to formazine standards.

The effectiveness of the purification device is essentially measured by the reduction of turbidity continuously on incoming water and decanted water.

The mineral additive used was prepared in the laboratory and has the following characteristics:

40% hydrated magnesium silicate, 46% hydrated aluminum and magnesium double silicate, 2% calcium and magnesium double carbonate and 12% quartz. Water solubility at pH 7 is less than 0.1%, the specific surface is 2.5 m ² / g, the porosity is less than 0.1 mg / cm ³ , the residual turbidity in water is 160 NTU; and the fall speed is less than 35 m / h. This additive has a residual turbidity in water of 60 FTU, the index used for this example.

The amount of mineral additive added was adjusted to 0.5 times and 0.7 times the incoming suspended matter (M.E.S.) respectively for a D.C.O. 600 mg / 1.

The efficiency of the lamellar settling tank without additive was determined according to the initial turbidity, because this device is quickly at the limit of efficiency when it is less than 350 F.T.U.

It can be seen from the tests carried out that, firstly, the additive makes it possible to maintain an efficiency of the order of 45% by treating 60% more water. Second, at high speed, greater than 15 m / h, the efficiency of the decanter is doubled.

It is also important to know that if the mineral additive increases the mass flow of sludge, the volume flow of sludge to be extracted and treated is reduced by 20 to 30%, which increases the period of use of these lamellar settling tanks with discontinuous operation.

The example is further illustrated by Tables I and II in the appendix.

Example 2

This example deals with the characterization of the bio-flocculation of microorganism flocs by the use of a mineral additive. This property is obtained in the laboratory after the mineral additive has been brought into contact with microorganisms for 3 minutes with rapid stirring (100 rpm), then 10 minutes with slower stirring (60 rpm) in cylindrical reactors, followed by non-agitated decantation for 30 minutes.

The observation is carried out under an optical microscope for measuring the diameter of the flocs of microorganisms and the decantation is determined by the mud index.

The mineral additive concentration is 100% by weight of the biomass with which it is contacted. The additives in this example have the following characteristics:

Additive A

Magnesium silicate hydrate 95%, double silicate of aluminum and magnesium hydrate 3%, double carbonate of calcium and magnesium 2%. Specific surface: 3.5 m ² / g, residual turbidity: 160 NTU and falling speed: less than 5 m / h.

Additive B

Double aluminum and magnesium silicate 95%, hydrated magnesium silicate 4%, double calcium and magnesium carbonate 1%. Specific surface: 10.5 m ² / g, residual turbidity: 200 NTU and falling speed: less than 3 m / h.

Additive C

94% hydrated calcium carbonate, 3% hydrated magnesium silicate, 3% hydrated magnesium aluminum double silicate. Specific surface: 1 m ² / g, residual turbidity: 300 NTU and falling speed: less than 30 m / h.

All these additives have porosities of less than 0.1 cm ³ / g and adsorption capacities of COD and N-NH ₄ respectively of less than 60 mg / g and 10 mg / g. Example 2 is further illustrated by Table III in the appendix.

By acting on the flocculation of microorganisms, the additives improve the sedimentation of these microorganisms (measured by the mud index), which was expected with regard to the information previously known. The mineral additive also modifies the size of these microorganism flocs to create very large, diffuse and stable flocs with flotation properties using air bubbles, far superior to microorganism flocs. not modified.

In fact, the sludge introduced into a flotation cell shows an increase of 50% and 30% respectively for the additives A and B. On the other hand, the additive C mainly favors sedimentation.

TABLE I

Raw water turbidity> 350 FTU

Without additive Additive

0.5 times the MES entered 0.7 times the MES entered

Turbidity Speed Efficiency Turb. Vit.col. Eff. Turb. Vit.col Eff. incoming column

FTU m / h% FTU m / h% FTU m / h%

365 10.5 47 425 9.8 57 430 13.7 57

425 11.9 47 370 10.7 50 430 14.3 52

370 15.5 37 350 15.7 38 370 16.7 45

350 15.5 23 400 19 40

TABLE II

Raw water turbidity <350 FTU

Without additive Additive

0.5 times MES 0.7 times MES

Turbidity Vit.col Efficiency Turb. Vit.col. Eff. Turb. Vit.col Eff.

FTU m / h% FTU m / h% FTU m / h%

270 10 47 320 12.2 40 350 16.7 40

300 10.5 50 310 13.1 39

320 11.9 37 350 15.7 38

320 12.5 27

320 15.5 19

300 18.4

⁵

TABLE III

Biomass of Additive A Additive B Additive C reference 100% of the 100% of the 100% of the biomass biomass biomass

Flock size

100 ± 20 260 ± 40 240 ± 40 160 ± 20 from micro-organisms to micro-meters

mud index ml / g 110 90 80 60

Claims

claims 1.- Process for purifying water containing a polluting load defined by its flow, in which the water to be purified is subjected to at least one treatment operation, in particular by decantation and / or biological activation, with subsequent separation of '' purified water and possibly activated sludge, characterized in that at least one mineral additive is added to the water to be treated in the form of powder, granules or suspension, and having the following properties: - non-adsorption and chemical inertness with regard to the polluting load present in the water to be purified, - insolubility or low solubility, less than 5%, in water under the usual pH conditions (pH5 to pH9) encountered during treatment, - specific surface greater than 0.4 m ² / g (BET method), - microporosity less than 0.1 cm ³ / g, - residual turbidity in water less than 300 NTU, and - falling speed less than 35 m / h. 2.- Method according to claim 1, characterized in that the mineral additive is a natural or synthetic product chosen from at least one of the chemical families formed by silicates, aluminates, carbonates, sulfates and insoluble glasses or poorly soluble at the usual pH conditions encountered during treatment. 3.- Method according to one of claims 1 and 2, caracté¬ ized in that the mineral additive is chosen from at least one of the following chemical families: - aluminosilicates, - magnesium aluminate, - hydrated calcium carbonate, - hydrated double calcium and magnesium carbonate, - amorphous or crystallized calcium carbonate, - hydrated magnesium silicate, - hydrated aluminum silicate, - hydrous magnesium and aluminum double silicate, and - hydrated calcium sulphate. 4.- Method according to one of claims 1 to 3, wherein the water to be purified is water to be made potable and is subjected to a primary separation operation in a primary clarifier, with decantation, characterized in that one add the mineral additive before or during the primary separation operation and the daily mass of the added mineral additive is equal to 1 to 500 times the daily mass of the pollutant load contained in the water. 5.- Method according to one of claims 1 to 3, wherein the water to be purified is waste water and is subjected to a primary separation operation in a primary clarifier with decantation and without treatment in biological tanks ques, characterized in that the mineral additive is added before or during the primary separation operation and that the daily mass of the added mineral additive is equal to 0.1 to 10 times the daily mass of the pollutant load contained in water. 6.- Method according to one of claims 4 and 5, caracté¬ ized in that the mineral additive has a residual turbidity in water generally less than 200 NTU and a fall speed between 0.03 and 35 m / h. 7.- Method according to one of claims 1 to 3, wherein the water to be purified is water to be made potable and is subjected to a primary separation operation by flotation, characterized in that the mineral additive is added before the primary separation operation and the daily mass of the additive 32 added mineral is equal to 1 to 100 times the daily mass of the polluting load contained in water. 8.- Method according to claim 7, characterized in that the mineral additive has a residual turbidity in water generally less than 250 NTU and a falling speed less than 5 m / h. 9.- Method according to one of claims 1 to 3, wherein the water to be purified is waste water and is subjected to a primary separation operation in a primary clarifier, with decantation, and before treatment in basins biological, characterized in that the mineral additive is added before or during the primary separation operation and that the daily mass of the added mineral additive is equal to 0.1 to 10 times the daily mass of the pollutant load contained in water. 10.- Method according to claim 9, characterized in that the mineral additive has a residual turbidity in water generally less than 300 NTU and a falling speed between 0.01 and 35 m / h. 11.- Method according to one of claims 1 to 3, wherein the water to be purified is waste water and is subjected to a primary separation operation by flotation before treatment in biological basins, characterized in that one add the mineral additive before the primary separation operation and the daily mass of the added mineral additive is equal to 0.1 to 10 times the daily mass of the pollutant load contained in the water. 12.- Method according to claim 11, characterized in that the mineral additive has a residual turbidity in water generally less than 300 NTU and a falling speed less than 5 m / h. 13.- Method according to one of claims 1 to 3 and 7 to 12, wherein the water to be purified is subjected to an operation biological activation in at least one biological basin in the presence of a biomass composed of purifying microorganisms and a secondary separation operation in a secondary clarifier with subsequent separation of purified water and activated sludge, characterized in that the mineral additive is added before or during the secondary separation operation with a selected quantity of mineral additive according to the biomass concentration at all times, which represents from 10 to 300% in weight of biomass in the aeration tank. 14.- Method according to claim 13, characterized in that the mineral additive has a residual turbidity in water generally less than 300 NTU and a falling speed less than 35 m / h. 15.- Method according to one of claims 13 and 14, characterized in that the mineral additive is added according to the amount of active biomass present at any time or alternatively as a replacement for the amount of mineral additive in the biomass in excess, extracted in the form of active sludge from the secondary clarifier. 16.- Method according to one of claims 1 to 15, caracté¬ ized in that one adds to the mineral additive a short chain cationized additive. 17.- Method according to one of claims 1 to 16, characterized in that one adds to the mineral additive an additional additive known for its specific action on toxic pollution or non-degradable soluble pollution by biological pathway, whose specific action is increased by synergistic effect due to the mineral additive. 18.- Additive for implementing the method according to one of claims 1 to 18, characterized in that it is a mineral additive as defined in one of claims 1 to 3. 19.- Additive according to claim 18, characterized in that the mineral additive is mixed with a cationized short chain additive. 20.- Additive according to claim 18, characterized in that the mineral additive is mixed with an additional additive chosen according to the treatment operation.