WO1998054097A1

WO1998054097A1 - Process and agent for water purification

Info

Publication number: WO1998054097A1
Application number: PCT/SE1998/001027
Authority: WO
Inventors: Tord Georg Eriksson
Original assignee: Tord Georg Eriksson
Priority date: 1997-05-30
Filing date: 1998-05-29
Publication date: 1998-12-03
Also published as: SE9702056L; AU7794398A; SE9702056D0; EP0988254A1; SE512118C2

Abstract

Process for the purification of alkaline waste water containing anionic compounds that may react with calcium and form water insoluble precipitates. A precipitating agent is added to the waste water. The agent contains a soluble calcium salt and a cationic polymer with molecule weight in the range of 1 to 12 millions in proportion by weight from 10:1 to 100:1 in an amount corresponding to 0.001 to 3 g calcium ion and the corresponding amount polymer per litre waste water. The formed precipitate is removed by using known techniques for the separation of solid and liquid substances.

Description

PROCESS AND AGENT FOR

WATER PURIFICATION

From the cleaning of equipment for milk handling, tanks, restaurant dishes etc. waste waters with a very high content of organic material are produced. Even when only natural substances, which are easy to decompose biologically, are involved, such waste waters become a heavy burden on the waste water purification process

The problem is aggravated, when waste waters containing substances that may have negative influence on the purification process, are involved. This happens to a high degree at the washing of vehicles and especially at cleaning of lorries and tank cars, which have been used for differing freights and are soiled by road dirt and corrosion products from the chassis etc.

International application PCT/SE95/00477 proposes a method for reactivation and recycling of cleaning solutions containing cationic surface active agents by the addition of an easily soluble ortho-phosphate and purification by sedimentation, filtering and reuse of the wash water and the wash chemicals for new washes.

The method functions well at most automobile wash facilities, but may be disturbed by high concentrations of substances with surface activity of the anionic type. Such high concentrations occur sometimes during winter in wash facilities for common automobiles at a very high load and may be caused by the fact that many automobiles have been washed with anionic cleaning agents earlier or that an anionic de-fatting agent has been sprayed on the automobile before the wash. From cleaning of tank cars, where the tank is washed both on the outside and the inside, problems arise by the fact that many substances carted by tank cars are anionic surface active. For cleaning in the food industry, especially in the dairy and meat sectors, strongly alkaline cleaning agents are frequently used. Such agents cause partial saponification of fatty acids. Thus, the waste waters from such cleaning operations contain high concentrations of substances with surface activity of the anionic type.

European patent publication 0075241 concerns a process for the regeneration of water containing de-fatting and cleaning solutions. According to this process one starts by adding 1 to 5 g/l water soluble alkali metal phosphate and/or -silicate and proceeds by adding water soluble calcium and/or magnesium salts in such proportions that the concentration becomes .3 to 2 g/l calcium ions respectively .2 to 1.2 g/l magnesium ions, i.e. approximately equivalent proportions of phosphate ions and calcium ions. The publication tells that one frequently still has a comparatively high content of mineral oil in the solution after the separation. In passing, addition of a flocculant on poly-electrolyte basis is mentioned. Obviously this refers to a routine separate addition as nothing is said about concentration, proportions between the different additives and ionic characteristic for the polymer. EP 0075243 and US patent No 4,409, 119 refer to a regeneration method for phosphate or silicate containing cleaning solutions contaminated by oils of different kinds. The solutions must contain at least 1 g/l alkali phosphate and/or -silicate. The purification is done by adding solutions containing calcium and/or magnesium salts, especially nitrates, sulphates or chlorides, in such proportion that it is equivalent to a concentration of .3 - 2 (.6 - 1.2) g/l calcium ions respectively .2 - 1.2 (.3 - .7) g/l magnesium ions. Here too, an addition of a poly-electrolyte of the polyacrylamide type is mentioned with just as little data about essential details as in earlier mentioned publication. In most of the examples the remaining content of oils is far above the nowadays accepted limits for discharge to a municipal sewer. The inventors, which are the same in both European patents, do not deal with the practical complications of adding precipitation agent respectively flocculant. The methods appear to be laboratory products, which may be used for batch purification but are unsuitable for automatic operation of the type needed for routine use at vehicle washing, restaurants, dairies and similar applications. The methods appear not to have got any commercial use.

US patent No 4,588,508 concerns a process intended for purification of process water from refineries, water containing oil residues, ballast water etc. containing oil residues and finely divided solid material, which can be organic or inorganic. A mixture of two different cationic polymers with defined molecule weights is used for the purification. Advantages of addition water soluble salts of multivalent cations are also mentioned. The sole salts, for which the use is exemplified are aluminium and zinc salts. The listed waters are almost without exception neutral, i.e. have pH between 6 and 8 and contain, as a rule, no compounds that can react with calcium salts for the formation of water insoluble compounds. The proportion between polymer and salt is from 4:5 to 1 :2. Here too, the laboratory method seems not to have been adapted for practical use.

According to JP patent No 56-76293, application No 54-54844, a calcium or magnesium salt, hydroxide or oxide is added to a polymer of acrylic acid or acrylic amide to simplify their handling. The polymer is anionic and nonionic. This invention concerns an automatized process for the separation of organic soil inclusive mineral oils, milk residues etc. from cleaning solutions and waste waters of different kinds to levels that are low enough to permit as well reuse, with essential part of the active substances preserved, as discharge to a purification plant with biological purification of the type used for sewage from households and similar places, without disturbances of the purification and/or sludge handling. The process is suitable for most alkaline waste waters. It is especially suitable for solutions containing fatty acids and/or cleaning solutions that contain surface active agents and phosphates of different kinds. At the process a mixture of a soluble calcium salt, preferably calcium chloride and/or calcium nitrate, and a cationic polymer with molecule weight in the range 1 to 12 millions is used. A suitable mixture for the purpose is 90 to 99 weight % calcium chloride of technical quality containing 75 to 85 weight % calcium chloride and 1 to 10 weight % cationic polymer. The addition should be done as a water solution that may contain 1 to 20 weight % of the mixture. 0,1 to 10 volume % of this water solution calculated on the waste water volume is a suitable quantity. Other soluble salts with multi-valent cations may be used. Calcium nitrate works very well but may sometimes be expected to give an unwanted contribution of fixed nitrogen to the waste water.

Calcium nitrate has because of its nitrate content the good quality that the appearance of anaerobic states is prevented. Therefor, calcium nitrate solely or a mixture of calcium chloride and calcium nitrate is a good alternative if anaerobic states may be expected. Such a mixture is optimal when the activity may include stops with water remaining in the system. A well-adjusted addition of calcium nitrate will usually not mean any contribution to the nitrogen loading at the purification plant as the nitrate is consumed by biological activity with generation of nitrogen gas. Without nitrate this biological activity causes the generation of hydrogen sulphide.

Even at low concentration hydrogen sulphide has a very negative influence on the nitrification processes modern purification plants are using. Therefor it is very plausible that a modest addition of calcium nitrate, when anaerobic states may be expected, i.e. when sludge remains for a long time, may result in reduced nitrogen loading at the outlet from the municipal purification plant.

Magnesium salts may be used but are less preferred. Magnesium ion salts with large anions are more soluble than the calcium ion salts. Magnesium has lower equivalent weight, which causes precipitates containing magnesium ions to sink more slowly. The cationic polymer should have high molecule weight. The range 1 to 12 millions is acceptable but a smaller range, 3 to 10 millions, is preferred. Suitable cationic polymers for this application are usually inter-polymers of neutral monomers and cationic monomers. The percentage of cationic groups may vary between just more than 0 and 100 calculated as numbers of different monomers. For this application moderately cationic active polymers are preferred, i.e. with percentages of cationic monomers between 20 and 80%. Very suitable polymers for the purpose are found in the group inter-polymers of acrylic amide and cationic active acrylic acid derivatives. Before the addition of the precipitating agent it is made sure that the solution is alkaline. pH in the range 9 to 11 , especially 9.5 to 10.5, is preferred, as the separation, especially for heavy metal ions and fat, is complete in this range. Suitable pH may be achieved by lye and/or trisodium ortho phosphate. The combination of lye and ortho phosphate is preferred even when the purification is not part in a recycling system for the wash water. Small quantities of ortho phosphate promote the efficiency of the purification process. Just ortho phosphate may be used, but is more expensive than the combination, without bringing corresponding advantages. Beside, alkalisation with just ortho phosphate may give an unwanted contribution to the phosphor loading at the purification plant and be cause for surcharges for waste water treatment in municipal plants.

An alternative to sodium hydroxide and/or sodium ortho phosphate for pH-control is sodium carbonate. Other possibilities are addition of calcium and/or magnesium carbonate and/or calcium hydroxide to the mixture of calcium salt and polymer.

When comparing with the mentioned US 4,588,508 one must observe that multi- valent cations react very differently at pH-changes. In the pH range less than 4 aluminium to the main part exist as Al²⁺, in the pH range 4 to 8,5 the main part exists as AI₂O3 or AI₂O₃.3H₂O, in the range more than 8,5 AIO₂ ^~ dominates. Therefor the power of aluminium salts to precipitate anions decreases with rising pH and is very small at pH more than 9. For zinc the circumstances are more favourable but the tendency is the same. Over pH 8 one gets a large reduction of the Zn²⁺' which is the active specie at precipitation. Thus, it is dubious if the compositions of the American patent can be used for alkaline solutions.

Calcium and magnesium exist mainly as Ca²⁺ respectively Mg²⁺ right to pH11.5. At lower pH their salts are comparatively soluble. Thus, they are less suitable for use at the process according to the US patent. Therefor, the difference between this invention and the process according to the US patent is not just the large difference in the proportion between salt and polymer but also the direction and usefulness for the different methods.

According to EP 0075243, US patent No 4,409,119 and JP 56-76293, application 54-54844, an acrylic amide polymer is used with calcium and magnesium salts. According to the latter publication an acrylic acid polymer is used, too. As those polymer species are nonionic respectively anionic one in both cases misses the synergistic effect that the mixture of calcium salts and cationic polymer gives at the addition to alkaline solutions containing large inorganic and organic ions, which can react with calcium and form water insoluble compounds. Addition should not be done continuously but surge-wisely, i.e. that 5 to 50% of the amount of wash solution respectively waste water is mixed with precipitating agent, then the formed mixture is stratified over the already added quantity waste water in the separator. Surge-wise addition reduces the need for precipitating agent and in applicable cases, i.e. at surplus of ortho phosphate and recycling of the wash solution, the consumption of ortho phosphate that the purification involves. Also, the method of addition differentiates this invention from the processes described in the mentioned publications. In those processes the precipitating agent is mixed, while agitating, with the whole quantity water that is to be purified. This is the usual procedure. Another important difference between common technique and this invention is the method to add the precipitating agent. A simultaneous addition of a calcium salt and cationic polymer as a water solution instead of separate additives brings unexpected advantages with respect to dosing and control of the addition. Simultaneous addition gives a synergistic effect, which makes that as well the calcium salt as the cationic polymer is used much better than if one, as is usual, adds the components separately.

This invention concerns a process for purification of waste water containing calcium precipitating substances, as inorganic anions with two or more valences but without complex forming power, salts of fatty acids, anionic surface active agents, proteins etc., by the adding of a water solution, which contains a mixture of calcium chloride and cationic polymer with molecule weight in the range 1 to 12 millions in proportion from 100:1 to 10:1.

This invention also concerns production and use, for the regeneration of wash solutions and purification of waste water, of a water solution containing 1 to 20 weight % of a mixture of 70 to 80 weight % soluble calcium salt, which may consist of calcium chloride of technical quality containing 75 to 85 weight % calcium chloride, and 1 to 10 weight % cationic polymer with molecule weight in the rangel to 12 millions, remainder water and contaminants in the calcium salts.

Production of polymer solutions for water purification is not without problems. To get maximum activity the solution must have some time (from some minutes to 3 - 7 hours) for maturing. Thus, it cannot be prepared and used immediately. The solution has limited shelf-life and should be used within one to two weeks to have optimal activity.

Even at large plants, where the need can be forecasted, the demand of maturing time and the limited shelf life cause problems, but they can be solved by usual known technical measures. For automatic automobile wash plants and similar applications, where the need cannot be forecasted, those known methods do not suffice. Therefor the inventor has developed a new, well functioning method for solving the problem. A container is filled with a mixture as above with 90 to 99 weight % calcium chloride of technical quality, i.e. containing 75 to 85% calcium chloride, and 1 to 10% cationic polymer. If the content of cationic polymer is less than 5%, no other measures are needed before use.

At higher contents of cationic polymer, 5 to 10 weight %, adding about 20 weight % water may be suitable. The result becomes a paste, which has very long shelf life. The paste can also be achieved by addition of a suitable, low molecule alcohol but water is preferred from cost and environment reasons.

At use the container is placed upside down. The paste is sprayed with water at its underside according to a program, which gives suitable quantity solution for each portion waste water. With suitable design of the nozzle for the water spray a fixed amount of water gives a comparatively fixed amount precipitating agent. Cutting water saving types of nozzles are preferred. Such nozzles have a flat comparatively narrow jet pattern.

The outlet from the dosing equipment contains undigested polymer solution. This can be seen as turbidity caused by floating and water-suspended polymer particles. The stream is collected in an intermediate container. In this container the mixture is agitated by air injection to hasten the maturing. The container is provided with a discharge pipe, which begins at the lower region of the container, bends upwards along the container up to desired liquid level, bends again and ends in a pipe, which adds desired quantity precipitating agent to a suitable mixing point with the cleaning solution respectively the waste water. If dosing at each addition of waste water is wanted, the discharge pipe is provided with an opening that lets in air at its highest point. Undigested polymer particles are lighter than water and gather in the upper part of the container even at moderate agitation. In this way one gets matured calcium salt/polymer solution, dosed by overflow. The liquid level is adjusted to give suitable maturing time.

If one instead, as preferred, wants dosing in surges the flow from the dosing equipment is collected until a suitable level in the maturing container has been obtained and is then emptied at once by siphon effect in the pipe to suitable mixing point with wash solution respectively waste water. Here, the diameter of the discharge pipe must not be large enough to let air in the back way. Besides the air opening at the top must be closed. The surge frequency for dosing may, in this case, be determined just by the choice of container and height adjustment of the outlets highest point. The intermediate container should be provided with an arrangement for conductivity measuring, which gives a signal that can be used to tell when it is time to change the paste container. The measuring point should at surge-wise dosing be placed under the outlet not to be disturbed by the siphon emptying.

Emptying by siphon effect, respectively overflow, may be replaced by dosing via pump.

If one instead chooses to prepare the solution batch-wisely in a container one should start from a dry mixture of polymer and calcium salt. This mixture is poured in the water in the container. The container should be provided with an agitator, and/or arrangements for injecting finely divided air in the lower part of the container. The dry mixing allows that the powder can be tipped in the container. If one instead wants to dissolve a dry, not mixed polymer, in a satisfactory way in reasonable time the powder has to be sprinkled slowly over the water surface. The separation of the formed precipitate is, with advantage, done in gravity-based oil separators of similar type as those that are usually used at automatic automobile wash plants and similar installations. At large amounts of sludge the emptying frequency of conventional separators of the mentioned type will be very high. Using some type of direct separation instead may then be suitable. Often continuous band filters of suitable types may be appropriate. This especially when the precipitate may have commercial value and therefor should not unnecessarily be subjected to biological-chemical influence that the dwell time in a conventional separator unavoidably causes.

Other suitable methods may be centrifuging and use of so-called wet cyclones. Another very suitable method is flotation, i.e. air injection in the basin's lower part and separation of formed froth. This is especially true when the waste water is free from heavy particles such as sand and similar. This may, of course, be a method for taking charge of lighter and heavier components in the waste water separately, but requires then afterwards sedimentation or other suitable method for to separate the heavier pollutants from the waste water. Flotation makes almost immediate separation possible before useful substances in the waste water have been destroyed by biological or chemical influence.

The precipitating agent should be mixed with the waste water before the separator. As mentioned addition should be done surge-wisely, i.e. that addition is done during just a part (1 to 50%) of the time waste water is delivered to the separator. The addition should be done when waste water is delivered to the separator so that the precipitating agent mixes fast with the part of waste water that is in the mixing zone. If the addition is done in a zone with a very turbulent flow, for instance in the channel in an automatic automobile wash plant, no separate arrangements are needed if the channel and the following pipe to the separator are long enough. In other cases using some suitable type of mixer may be advantageous. Usually a so- called "static mixer", i.e. a mixer without moving parts, will give sufficiently good mixing.

Continuous addition and also addition directly to the separator itself gives dilution and does not lead to equally good purifying results. The explanation to this may be that calcium salt and polymer with components in waste water form a three- dimensional net with large capacity for catching soil particles even if these are of nearly colloidal size. At larger dilution the meshes of the net become larger and the efficiency diminishes.

As well calcium ions as cationic polymer react with negative ions, including ortho phosphate, in the waste water. If calcium ions and cationic polymers are added intimately mixed, their precipitation power is used much better than if they as usual are added separately.

Contrary to what one should expect the mixing of calcium salt and a cationic polymer promotes the dispersion of the polymer in water. A 20% solution of a mixture of 98% calcium chloride of technical quality and 2% polymer is comparatively viscous, but enough thinly flowing for dosing with a suitable pump. Such a mixture contains .4 % polymer and 19% calcium chloride. To obtain equally low viscosity in a water solution, without calcium chloride, its polymer content must not surpass .2 %. The maximum content cationic polymer in water solutions of calcium chloride and polymer suitable for dosing without a specially designed pump lays around .8 %. No good method for control of the content of polymers in water continuously exists. The mixing with calcium chloride solves this problem. A simple conductivity measuring device gives information about the content of calcium chloride and thus indirectly about the content of the polymer. This is especially important at automatic dosing as one in this way may get warning when the dosing container is empty.

Example 1

3

A dairy produces 350 m waste cleaning solution containing surface active agents, alkali, complex formers (NTA), milk, milk fats, milk proteins and other soils each

3 day. The waste water is collected in two tanks of 50 m each. The tanks are filled and emptied alternately. Each day 2000 litres of a 10% solution of calcium chloride and cationic polymer are used to purify the waste water. The purification mixture is produced by dissolving 100 kilos of a dry powder mixture containing 98% calcium chloride of technical quality (75 to 85% CaCI₂) and 2% cationic polymer in water while agitating by air injection. The mixture is let to mature 2 to 3 hours before use. At each emptying of the collection tanks about 290 litres of the solution are added. The addition is done via a static mixer in the channel between the collection tank

3 and the follow-separator, which consists of two containers in series of 60 m

3 respectively 50 m . 290 litres of the mentioned solution contain about 23 kilos calcium chloride. Calculated on 50000 litres waste water the concentration of calcium salt will be about 0,45 g/l. , i.e. about 0,16 g calcium ions per litre.

At trials BOD has been reduced from about 400 mg/l to 1 mg/l. The precipitate becomes essentially free from substances that may disturb the environment. It can be used as fertilizer or even as food for small animals.

Example 2 A facility for the cleaning of tank lorries uses a system for washing with recycled wash water essentially according to the mentioned PCT-application is used, i.e. by a recycling wash solution containing cationic surface active agent and mixed with sodium ortho phosphate and NaOH as emulsion breaking and heavy metal precipitating agent. The recycling solution had a pH between 10 and 10.5 and a ortho phosphate content of about 2,0 g/l.

The very varying content in the tanks of the lorries causes difficulties to achieve purification down to the level of 100 mg/l that is limit for outlet to municipal waste water purification plant. To improve the separation one adds 20 to 2000 g per washed lorry of the, in example 1 mentioned, mixture consisting of 95% water and 5% of a mixture containing 98% calcium chloride of technical quality and 2% cationic polymer. The addition was varied with respect to the soiling degree and size of the lorries. The highest additions were used for lorries that had transported oil.

Washing was done as well outside as inside the tanks. The water turnover was estimated to 1500 to 2000 litres per lorry. The addition was done before the oil separator surge-wisely, i.e. just during a small part of the time when waste water is delivered to the oil separator.

Testing during a long period with waste water containing essentially all raw materials and products that may be of interest for transport in a tank lorry has shown that despite what substances the wash water contains the new purification method gives water that is well suited for recycling as wash water and also for discharge as waste water for purification by usual methods for biological purification.

As example of the very varied tank content that was found the following list, referring to just one week, may serve: spirit ^• oil iso-pentane polymer emulsion unhardened polyester solution glue solution • glycerine gelling agent glycol

NTA-solution naphtha • textile finish diesel oil concrete additive base oil xylene.

Example 3.

In an automatic automobile wash facility with a wash system according to PCT/SE95/00477 about 10 g per washed automobile of a mixture consisting of about 98% calcium chloride of technical quality (road salt) and about 2% of a cationic polymer with molecule weight in the range 6 to 12 millions were added. Per wash 2.8 g calcium ions and .16 g cationic polymer were added. The dissolution of the mixture was done by earlier described process. The addition was done surge- wisely by letting the calculated amount precipitating agent for 5 washes collect in a digesting container, which was emptied at once by siphon effect when the fifth automobile was washed.

The cycling, wash solution contained about 2.0 g/l sodium ortho phosphate, i.e. an important stoicheiometric surplus in proportion to the amount calcium ions and polymer. Thus the total amount of calcium ions and polymer was precipitated and no detectable amounts were left in the recycling solution. Totally, the turnover at a nor- mal wash is about 500 litres wash water consisting of recycled water and fresh water for the final rinsing. At average .006 g calcium ions per litre wash solution were consumed at each wash cycle.

Example 4.

The test of example 3 was repeated in another similar facility with the difference that the calcium chloride/polymer solution was prepared in two 100 litres' containers, which were used alternately. The solution was prepared by first filling the container with water and then emptying a bucket containing 5 kilos of a dry-mixed mixture of calcium chloride and cationic polymer in the container.

Of this solution 1 litre was dosed at each third wash. The recycling solution contained between .5 and 3 g/l ortho phosphate. Two different mixtures of calcium chloride and polymer were used. One contained 96 weight % calcium chloride and 4 weight % cationic polymer, the other 98 weight % respectively 2 weight % of the mentioned components.

The recycling water solution of example 2 to 4 with surplus of ortho phosphate is well prepared for further purification by a combination of ultra filtration and reverse osmosis.

Example 5.

In a dairy with similar production conditions as the one of example 1 the waste water was collected in a buffer tank. The waste water was transferred as batches from this buffer tank to a flotation container.

At trials with help chemicals for purification two different products were used as additives. One product (I) contained a mixture of 95 weight % trisodium ortho phosphate and 5 weight % sodium hydroxide, which had been dissolved in water to a solution with 7.5 weight % of the mixture. The other product (II) consisted of a 7.5 % water solution produced by adding 15 kilos of a dry mixture of 98 weight % calcium chloride (technical quality) and 2% cationic polymer to 200 litres water and agitating.

Three different tests were done:

At the first trial 2 volume % of product I was added to the waste water at the inlet to the flotation tank. Afterwards, when the tank was nearly full, 1 volume % of product

II was added. The percentages were calculated on the volume in the flotation tank.

At the second trial 1% of product I was added at the inlet and 1 ,5 % product II added with the air injection for the flotation.

At the third trial one first added 0,5 % product I and after agitation 1 % of product II to the buffer tank before the transfer to the flotation tank.

The first test resulted in a COD reduction from 2500 mg/l to 428 mg/l. The second test gave a COD reduction from 14972 mg/l to 1246 mg/l. At the third test the corresponding reduction was from 4430 mg/l to 275 mg/l. The third test gave precipitate in the buffer tank already and was therefor judges as less successful. Thus, the COD reductions at the three tests became 83, 92 and 94%. BOD₇ followed the same pattern.

Claims

Patent claims

1. Process for the purification of alkaline waste water containing anionic compounds, which can react with calcium and form water insoluble precipitates, characterized in that a precipitating agent is added to the waste water, this agent consisting of a water solution containing a soluble calcium salt and a cationic polymer with molecule weight in the range 1 to 12 millions in the proportion by weight of 10:1 to 100:1 , which is added in an amount that corresponds to .001 to 3 g calcium ions and the corresponding amount polymer per litre waste water, after which the formed precipitate is removed by using known techniques for the separation of solid and liquid substances.

2. The process of claim 1 characterized in that the precipitating agent is produced by dissolving a powder mixture of a calcium salt and cationic polymer in water.

3. The process of claim 1 and 2 characterized in that the soluble calcium salt consists of calcium chloride or calcium nitrate or a mixture of calcium chloride and calcium nitrate in proportions from 10:1 to 1 :1.

4. The process of claim 1 , 2 and 3 characterized in that the water solution contains 2 to 20 weight % of the calcium salt/polymer mixture.

5. The process of claim 1 , 2, 3 and 4 characterized in that the addition of the precipitating agent is done surge-wisely, i.e. that the precipitating agent is mixed with a limited volume of the wash solution respectively waste water (5 to 50%) and that this mixture is added to already added amount waste water without precipitating agent in the separator.

6. The process of claim 1 to 5 characterized in that the formed precipitate is removed by sedimentation, continuous filtration, centrifuging or by using wet cyclones.

7. The process of claim 1 to 5 characterized in that the formed precipitate is removed by flotation.