FR2513984A1 - Purificn. of highly polluted industrial effluent esp. from dye-works - by heat evapn. followed by physico:chemical treatment - Google Patents

Abstract

A process and an installation for the purificn. of polluted industrial effluent with high COD and contg. fine solids in suspension. Effluent is evaporated by heating and the condensate is subjected to further phyisico-chemical treatment as required, partic. coagulation/flocculation, decantation and filtration. Pref. the process includes using the vapour produced by evapn. of the effluent to preheat effluent on its way to the evaporator. Water obtd. by purifying the effluent is pref. recycled directly for use in the industrial process which gave rise to the polluted effluent. This applies partic. to effluent from the dyeing industry. The installation pref. comprises a multi-stage evaporator which receives the effluent, each stage being equipped with a separator which eliminates sludge from heating vapour. Effluent for treatment does not require the usual pre-screening and decanting. Degree of purificn. is such, e.g. 80-95% redn. of COD, that the purified water can be re-used in the process producing the effluent. All colour is removed from dye works effluent.

Description

Process and installation for the purification of industrial wastewater, in particular wastewater from dyeing works
The invention relates to a process and an installation for the purification of industrial wastewater, and it applies more particularly to dyeing wastewater. Actually, the processes commonly used to purify industrial wastewater make use of first phase of homogenization and summary purification by mechanical treatment (screening, decantation, etc .: :), a second phase of reduction of the pollutant load by simple processes, either biological or physicochemical (coagulation, flocculation then filtration), and possibly a third finishing phase to remove residual dissolved solids and using techniques. fine separation agents such as: separation by permeable or dialysis membranes, adsorption on a solid surface with a high specific surface area, ion exchange on resin, separation on. grafted celluloses.

for waters. at. high chemical oxygen demand (W0) and fine suspended solids, as are for example the dyeing water whose suspended solids have a dimension of 1 to 2 tenths of a millimeter, the coagulation-flocculation treatment is great importance.

 However, whatever the conditions under which this physico-chemical treatment is carried out, in particular regardless of the fineness of separation obtained beforehand by decantation, the reduction of LCO after this co-flocculation-treatment is limit to 40-50 0.

The COD of the treated water is thus in general much too important so that the water can be recycled in the industrial process of the plant whose discharges are treated so that it is rejected in the course of water, which causes significant water supply costs as well as the payment of relatively high taxes for the direct discharge of wastewater into protected watercourses.

In addition, and this applies in particular to sewage waste water, the usual treatments do not discolor the waste water, which irremediably pock any water recycling in the industrial process.

 Moreover, another problem inherent in the usual purification treatments lies in the elimination of sludge obtained at the end of treatment. However, for waters with a high COD and which therefore require for their depollution significant amounts of coagulating agent or flocculating agent, the volume of sludge obtained is considerable.

 It is an object of the invention to provide a waste water purification process which makes it possible to obtain a sufficiently clean water for recycling in the industrial process.

It is, in particular, an object of the invention to provide a process for the purification of waste water from dyeing plants which makes it possible to discolor the discharged water.

It is yet another object of the invention to provide a process which makes it possible to obtain, in highly concentrated form, the pollutant extracted from the water.

It is, finally, another object of the invention to provide a method and a wastewater treatment plant that are very simple and economical.

These objects are achieved, according to the invention, by virtue of the fact that before physicochemical processes of the coagulation-flocculation and filtration type are carried out, the polluted water is subjected directly to evaporation treatment by heating. , without having recourse to the usual pre-treatments which make use of a mechanical separation of the type of infiltration, decantation, etc.

One of the essential features of the invention is thus to subject the polluted water evaporation by heating.

However, the techniques of evaporation are only acceptable in an industrial installation if all the products obtained have a utility. In particular, in order for the operation to be profitable, it is necessary to recycle the distilled water obtained, to use the steam produced, to easily reject the sludge formed. The process of the invention fills all. these. conditions.

 the Applicant has in fact noticed that while in all the treatment treatments implemented hitherto the coagulation-flocculating operations do not make it possible to reach a yield, expressed as a reduction in COD, higher than 40-50 #, the preliminary evaporation significantly entails a considerable improvement in the effectiveness of these physicochemical treatments, the yield obtained being of the order of 80 to 95.

Evaporation-a on the physicochemical operations carried out have a clear synergistic effect, on the one hand, on the purity of the water obtained at the end of the treatment which is sufficiently cleaned up to be recycled directly in the industrial process-and, secondly, in the case of the dyeing industry, in terms of the quality of the water obtained and recycled, which has a content of encrusting materials (calcium, silica, etc.). lower than the content of drinking water, so that the quality of the dye achieved using recycled water is improved. This synergistic effect between the evaporation and coagulation-flocculation stages is also manifested in an improvement of the exchange during the vaporization, thanks to the very low silica and calcium contents of the floc water. recycled uleum.

The steam produced during the evaporation by heating of the polluted water, after separation of the sludge, is used to preheat the polluted water conducted to the evaporator. This has a beneficial effect on the operation of the evaporating operation, minimizing the energy requirements for heating the water.

Finally, the volume of the sludge recovered at the end of the treatment is low because evaporation has, in fact, a reduction yield of COD of the order of 75 to 90%, the subsequent treatment with a flocculating agent. on a water already, well cleaned up, provides little sludge.

 The plant according to the invention comprises an evaporator, a separator connected to the steam outlet of the evaporator to separate the sludge from the. polluted water vapor formed, a coagulation-flocculation device connected directly to the evaporated condensed water outlet from the separator, optionally a flocculated water settling device and a flocculated water filtration device.

This installation is very simple. It removes the devices from screening, preliminary settling, etc.

 commonly used in common water treatment processes and still occupy large areas. It also eliminates the fine separation devices of residual materials contained in the treated water usually required in current processes.

the invention is now described in more detail by reference; to the accompanying drawings, in which Figure 1 is a diagram of an evaporation plant in which is performed the first phase of the method according to the invention; and Figure 2 shows schematically the sequence of water treatment operations performed after evaporation.

According to the invention, the water to be purified is subjected directly to evaporation by heating before further subjected to a physicochemical treatment of the coagulation-loculation type, the evaporation plant 10, shown in FIG. several stages, in this case seven the first stage 111, or upper stage, is supplied with a heat transfer fluid from a boiler 12 through a pipe 13 and returning through a pipe 14. The water-polluted from a main line 15 is supplied to the evaporator 111 via line 151 via an economizer b1 in which it is preheated. the dirty vapor produced in the evaporator flows out through a pipe which leads it to a separator S1, for example a centrifugal separator, at the outlet of which, on the one hand, sludge is obtained which pipe 17 and, on the other hand, clean steam.

This clean steam is used as heat transfer fluid in the evaporator 112 of the second stage. Saddle is fed through a pipe 181 and evaporated water which condenses out of the evaporator via a pipe 192 and then passes into an economizer b2 in which it is used to preheat the polluted water supplied from the pipe 15 into the The preheated polluted water exiting the economizer via the line 152 is introduced into the evaporator 112 where it is evaporated as a result of the heat exchange with the clean vapor and exits through a line 162. which brings it into a centrifugal separator S2 which provides, on the one hand, sludge collecting in the pipe 17 and, on the other hand, clean steam supplied by a pipe 182 in the evaporator 113 of the third stage.

Polluted water, clean steam and evaporated condensed water follow similar paths in stages 3 to 7.

The vapor leaving the last evaporator 117 via the pipe 167 is sent into the centrifugal separator S7 and the clean vapor obtained is then led through a pipe 21 where it condenses in the economizer M1 of the first evaporation stage, to preheat polluted water entering this floor.

It can thus be seen that the energy requirements of the evaporation plant are solely those of heating the heat transfer fluid of the first evaporation stage, the other stages operating with clean steam from the immediately preceding stage as heat transfer fluid. .

evaporated condensed-outgoing from each evaporator by the pipes 201 ... 207 collect in a pipe 20 connected to a pipe 22 for supply of water being purified in a physico-chemical treatment facility (Figure 2 )
At this pipe 22 are connected, in specific locations, a device 23 for dosing and adding distilled water to a pH adjusting product to bring the water to the pH-appropriate for performing a subsequent coagulation operation under satisfactory conditions, then a device 24 for dosing and adding to water the coagulant (s) used. The water then enters a coagulation apparatus 25: the coagulant product destabilizes the emulsions of pollutants contained in the evaporated water and which are in very finely dispersed form. These techniques being known, they will not be described in more detail.

 In line 26 of water outlet after coagulation treatment, is connected a device 7-dosing and addition to water of one or more flocculating products.

The water is then brought to a flocculation device 28 in which will be formed the precipitations or flocculations induced by the flocculating product
The flocculated water exits via a pipe 29, is optionally brought into a settling device 30.
exits through a pipe 31 which leads to a filtration device 32 comprising, in this case two filtration elements on sand 521 and 322. The filtered water is then ~ then recovered in a pipe 33 and it is sLLt recycled
in the current industrial process, be discharged into a protected stream.

 As a pH adjusting agent, sodium hydroxide, potassium hydroxide or the like, or formic acid, acetic acid or the like is used depending on the desired pH and the condensed evaporated water to be treated.

As coagulating product, according to the polluting substances contained in condensed evaporated water, the usual products such as alumina sulphate, ferric chloride, lime, calcium chloride, ferrous sulphate, magnesium chloride are used.
and the like.

 As the flocculating product, inorganic flocculants such as activated silica, bentonite, metal hydroxide with a polymeric structure, or natural flocculants such as water-soluble polymers, are also used depending on the pollutant content of the evaporated condensed water. cationic or neutral anionic feedstock, starch derivatives (corn starch, potato starch, etc.) po'lysacchn-- wrinkles, alginates, etc., or else synthetic flocculants such as polyacrylamides copolymerized with acrylic acid (anionic), amine (cationic) compounds, polyethylenimines, amine polyamides, polyamides, ethylene polyoxides, sulfonated compounds and the like.

We will describe below, by way of non-limiting way, an example of treatment according to the invention of a dyeing water.

The average composition of the polluted water was as follows- - COD 2500 at 3o00 mg / liter
DB05 about 459-600 mg / liter; the ratio DC01bB05 is here too high to allow degradation of the pollutants by biological treatment; - dry extract 0.4%; composition of the dry extract: mineral matter 50 g
organic matter 50%.

the mineral substances are: silica SiO2 18%%
calcium CaO li%
copper or 3%
iron Fe203 6 #
sulphite S03 9%
phosphate P205 3 -R #.

organic materials include surfactants of various polarities and substances from raw materials and dyestuffs.

As surfactants there are anionic products (synthetic sulfonates), non-ionic products (ethoxylated alcohols and fatty amines), cationic products (quaternized fatty amines).

the substances from the raw materials consist of lubricating oils, oligomers of synthetic fibers, waxes and cotton pectics.

the dyestuffs are azo #, poly azotic, anthraquinone, soluble or insoluble substances.

This water has been treated according to the invention.

The polluted water was first evaporated in an installation of the type described above with reference to FIG.

the operating conditions of the evaporation plant were as follows

<tb>% <SEP> (residual <SEP> vapor <SEP>
<tb><SEP> water <SEP> polluted <SEP>
<Tb>
Number of floors = 7
Polluted water temperature: 5000
Distilled water temperature: 6500;
Saturated residual vapor pressure: 7 bars
Saturated residual steam temperature: 17000
Temperature at # 1 and return of the heat transfer fluid of the first stage: 260 - 2404C
Temperature of the outlet steam of the first stage: 22500
Temperature of the exit steam of the following stages decrease from 7 to 800 per floor.

The evaporated condensed water obtained at the outlet of the evaporation plant is then subjected to a coagulation-flocculation-filtration treatment.

For such water, sodium bicarbonate and / or alumina sulfate are suitably used as the coagulating agent.

As the flocculating agent, polyamine type compounds are used.

the average characteristics of the treated water were as follows
Characteristics of water after evaporation
COD 600 mg / liter dry extract: 0.040 so composition in 0 identical to polluted water turbidity: UJ 65.

Average characteristics of flocculated water
COD: 40 mgllitre dry-ester not measured turbidity: UJ 1 to 2 better than drinking water).

hardness # 0f alkalinity XAa 120f chloride 53.6 mg / liter silica 2 mg / liter.

 Overall, this results in a reduction of the COD of 98%.

Water can be recycled directly into the industrial dyeing process. In addition, this water has a percentage of encrusting materials lower than that of drinking water, which will improve the quality of dyeing during the next cycle of the industrial process.

The coagulation-freeze treatment carried out on the evaporated condensed water, provides a reduction of the COD of 93%, considerable progress compared to the usual yields of coagulation-flocculation treatments and which generally reach only 40 to 50%,

Claims (7)

1. A process for the purification of industrial wastewater with a high chemical oxygen demand and with fine suspended solids, characterized in that the physico-chemical processes of the coagulation-flocculation and filtration type have to be carried out, polluted sludge is directly subjected to evaporation treatment by heating, without having recourse to the usual pretreatment of wastewaters using mechanical separations of the screening, decanting, etc. type. 2. The method according to claim 1, characterized in that the vapor produced during the evaporation of the polluted water, after separation of the sludge, is used to preheat the polluted water. 3. Process according to claim 1 or 2, characterized in that the water is depolluted recycled directly into the industrial process that led to the discharge of water to be purified. 4. Application of the process according to any one of claims 1 to 3 to the waste water of the dyeing plants. 5. Installation for carrying out the process according to claim 1, characterized in that it comprises an evaporator connected directly to the outlet pipe of the waste water, a separator connected to the steam outlet of the evaporator for separating the sludge. of the polluted water vapor formed, a coagulation-flocculation device connected directly to the condensed evaporated flanged outlet pipe, optionally a settling device, and a filtering device.  6. Installation according to claim 5J characterized in c # e it comprises an evaporator at least two stages, the vapor outlet of the last-#age being connected, through a separator, the entry of polluted water in the first stage of the evaporator to preheat the water to be purified. 7. Installation according to claim 5 or 6, characterized in that it comprises an evaporator with at least two stages, the vapor outlet of each stage apart from the last one being connected, passing through a separator, to the evaporator of the next stage to bring the vapor as a fluid Coolant, then to the entry of polluted water in this stage for purposes-preheating the water to be purified.