SE512118C2 - Procedure for water purification - Google Patents

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

In many cases, a relatively high mineral oil content still remains in the solution even after the separation. In passing, the addition of a polyelectrolyte-increasing agent is mentioned. Obviously this is a routine separate additive as nothing is said about either concentration, ratio between the different additives or ionic character of the polymer EP 0075243 applies to a regeneration process for phosphate- or silicate-containing cleaning solutions contaminated with oils of various kinds. The solutions must contain at least 1 g / l of alkali phosphate and / or silicate. The purification takes place by adding solutions containing calcium and / or magnesium salts, in particular nitrate, sulphate or chloride, in such an amount that they correspond to a concentration of 0.3 - 2 (0.6 - 1.2) g / l of calcium ions and , 2 - 1.2 (0.3 - 0.7) gll magnesium ions. The addition of a polyelectrolyte of the polyacrylamide type with as little information on essential details as in the previously mentioned publication is also mentioned here. In this example, the residual content of oils is far above what is currently considered permissible for discharges to sewers.

The inventors, who are the same in both European patents, do not deal with the practical problems with the addition of precipitants and extenders. The methods are suitable for laboratory products, which may be suitable for batch cleaning but are not suitable for automation of the kind needed for routine use in vehicle washes, restaurants, dairies and the like. It does not appear that the methods have received any commercial application.

The present invention relates to an automated process for the separation of organic dirt including mineral oils, milk residues and more from cleaning solutions and drains of various kinds to levels low enough to allow, both reuse, with a substantial part of the active substances preserved, as well as discharges. to treatment plants with biological treatment of the type used for sewage from households and the like, without disturbing treatment and / or sludge management.

The process is suitable for virtually all alkaline wastewater. It is particularly suitable for solutions containing fatty acids and / or cleaning solutions containing surfactants of various kinds. The process uses a mixture of a readily soluble calcium salt, preferably calcium chloride and / or calcium nitrate, and a cationic polymer having a molecular weight in the range of 1 to 12 million.

A suitable mixture for the purpose is 90 to 99% by weight of technical grade calcium chloride containing 75 to 85% by weight of calcium chloride and 1 to 10% by weight of cationic polymer. The additive should be in the form of an aqueous solution which may contain from 1 to 20% by weight of the mixture. 0.1 to 10% by volume of the aqueous solution, calculated on the amount of wastewater, is suitably used.

Other easily soluble salts with valuable cations can be used to advantage. Calcium nitrate works very well but can sometimes be feared to give an unwanted addition of bound nitrogen to the wastewater.

Due to its nitrate content, calcium nitrate has the good property of preventing the emergence of anaerobic conditions. Calcium nitrate alone or a mixture of calcium chloride and calcium nitrate is therefore a good alternative if the onset of anaerobic conditions can be feared. Such a mixture is optimal when the business may be affected by interruptions with residual water in the system. A well-balanced addition of calcium nitrate will in most cases not involve any addition to the nitrogen load in the treatment plant, as the nitrate is consumed through biological activity with the formation of nitrogen gas. This biological activity without the presence of nitrate gives rise to hydrogen sulfide formation.

Hydrogen sulphide, even in low concentrations, has a very negative impact on the nitration processes used by modern treatment plants. It is therefore very likely that a moderate addition of calcium nitrate when there is a risk of anaerobic conditions, ie when sludge remains for a longer period of time, can result in reduced nitrogen load in the outlet from the municipal treatment plant.

Magnesium salts can be used but usually have a higher solubility than the corresponding calcium salts. Aluminum and iron salts can be used, but there is a risk that, at the pH that is optimal for heavy metal elimination, hydroxide precipitates are obtained which are almost impossible to filter and give very large sludge volumes due to high water content.

The cationic polymer should have a high molecular weight. The range of 1 to 12 million is sufficient, but a slightly narrower range, 3 to 10 million, is preferred. Cationic polymers suitable for this use are generally copolymers of neutral monomers and cationic monomers. The proportion of cation groups can vary between just over 0 and 100% by number. For this application, moderately cationic polymers are preferred, i.e. with a proportion of cationic monomer between 20 and 80%. Very suitable polymers for this purpose are found in the group of copolymers of acrylamide and cationically active acrylic acid derivatives. Before the addition of the precipitating agent, make sure that the solution is alkaline. The pH in the range 9 to 11, especially 9.5 to 10.5, is preferred, since the separation, especially in the case of heavy metal ions and fats, is most complete in this range. Appropriate pH can be adjusted with lye and / or trisodium orthophosphate. The combination of lye and orthophosphate is preferred even when the treatment is not included as part of a recycling system for the washing water. Small amounts of orthophosphate benefit the efficiency of the purification process. Only orthophosphate can be used, but is more expensive than the combination, without bringing corresponding benefits. In addition, alkalisation with orthophosphate alone can make an undesirable contribution to the phosphorus load in the treatment plant and be the basis for additional fees for wastewater treatment in municipal facilities.

Sodium carbonate can be used as an alternative to pH control with sodium hydroxide and / or sodium orthophosphate. Other possibilities are the addition of calcium and / or magnesium carbonate and / or calcium hydroxide to the calcium salt / polymer mixture.

Addition should not be made continuously but intermittently, ie 5 to 50% of the amount of washing solution or waste water is mixed with precipitating agent, after which the formed mixture is layered over the amount already added in the separator. Batch addition reduces the need for precipitants and, where appropriate, ie. in case of excess orthophosphate and recirculation of the washing solution, the consumption of orthophosphate that the purification entails.

A significant difference between the prior art and the present invention lies in the way in which precipitants are added. A simultaneous addition of calcium salt and cationic polymer in the form of an aqueous solution instead of separate additives entails surprising advantages with regard to dosing and control of the additive. Simultaneous addition has a synergistic effect which means that both calcium salt and cationic polymer are utilized considerably better than if, as is usual, the components are added separately.

The present invention relates to a process for the purification of waste water containing calcium-precipitating substances, such as divalent or inert inorganic anions without complexing ability, salts of fatty acids, anionic surfactants. proteins, etc., by adding an aqueous solution containing a mixture of calcium chloride and cationic polymer having a molecular weight in the range of 1 to 12 million in a ratio of from 100: 1 to 1021.

The present invention also relates to the preparation and use for the regeneration of washing solutions and the purification of waste water of an aqueous solution containing 1 to 20% by weight of a mixture of 70 to 80% by weight of readily soluble calcium salt, which can be extracted. 1 1 8 5 is made of technical grade calcium chloride containing 75 to 85% by weight of calcium chloride, and 1 to 10% by weight of cationic polymer having a molecular weight in the range of 1 to 12 million, residual water and impurities in the calcium salts.

Preparation of polymer solutions for water purification is not without problems. For maximum activity, the solution must have a certain time (from a few minutes to 3 - 7 hours) for ripening.

It can thus not be prepared and used immediately. The solution has a limited shelf life and should be used within a couple of weeks for its activity to be optimal.

In larger plants, where the need can be foreseen, the requirements for ripening time and use within a limited time can cause problems, but these can be solved with standard known technical measures. For automatic car washes and the like, where the needs cannot be foreseen, these known methods are not sufficient. The inventor has therefore developed a new, well-functioning method for solving the problem. In a container, a mixture as above is filled with 90 to 99% by weight of technical grade calcium chloride, i.e. containing 75 to 85% of calcium chloride, and 1 to 10% of cationic polymer. If the cationic polymer content is below 5%, no further action is required before use.

At higher levels of cationic polymer, 5 to 10% by weight, it may be appropriate to add about 20% by weight of water. The result is a paste that has very good shelf life. The paste can also be obtained by adding suitable, low molecular weight alcohols, but water is preferred for cost and environmental reasons.

For use, the container is placed in an upside-down position. The paste is sprayed from below with water according to a program that provides the appropriate amount of solution for each portion of wastewater. With a suitable design of nozzles for water spraying, a certain amount of water gives a relatively uniform amount of precipitant. Cutting water-efficient types of nozzles are preferred. These nozzles have a rather narrow beam image.

The discharge from the dosing equipment does not yet contain mature polymer solution. This can be seen by turbidity caused by fl surface and floating polymer particles. The flow is collected in an intermediate container. In this, the mixture is stirred by air blowing to accelerate the ripening. The container is provided with an outlet which starts from the lower region of the container, curves upwards along the container up to the desired liquid level, bends again and opens into a line which supplies the desired amount of precipitant to a suitable mixing point with cleaning solution and waste water. If dosing is desired with each supply of wastewater, the outlet is provided at its highest point with an opening that lets in air. Undissolved polymer particles are lighter than water and collect in the upper part of the container even with moderate stirring. In this way, mature calcium salt / polymer solution is obtained, dosed through excess.. The liquid level is set to provide a suitable ripening time.

If, instead, preferential dosing is desired, the outlet is stratified from the dosing device until a suitable level in the maturation tank is reached and then emptied at once by siphon action in the line to a suitable mixing point with washing solution and waste water. In this case, the diameter of the outlet must not be so large that air can enter the back door. In addition, the aeration at the top must be closed. The impact frequency for dosing can be determined in this simple way only by selecting the container and adjusting the height of the highest point of the outlet. The intermediate container should be equipped with conductivity measuring equipment that provides a signal that can be used to indicate when it is time to change the pasta container. The measuring point should be below the outlet for shock dosing so as not to be disturbed by the siphon emptying.

Emptying with siphon effect, or over fl fate, can be replaced with dosing via pump.

If you instead choose to prepare the solution batchwise in containers, you should start from a dry mixture of polymer and calcium salt. This mixture is poured into water in the container. The container should be equipped with agitators and / or devices for blowing distributed air into the lower part of the container. The dry mixture allows the powder to be overturned in the container. If you instead want to dissolve dry, unmixed polymer, in a satisfactory manner within a reasonable time, the powder must be slowly sprinkled over the water surface.

The precipitate formed is advantageously separated in gravity-based oil separators of a similar type to those usually used in automatic car washes and the like. With large amounts of sludge, the emptying frequency of conventional separators of the type mentioned becomes very high. It may then be appropriate to use some type of direct separation instead. In many cases, suitable types of continuous belts may be suitable. This is especially true when the precipitate may have commercial value and therefore should not be unnecessarily exposed to the biochemical impact that the residence time in a conventional separator inevitably entails.

Other suitable methods may be centrifugation and the use of so-called wet cyclones.

Another very suitable method is fl otation, ie air blowing into the lower part of the pool and separation of formed foam. This is especially true when the wastewater is 51.2 iivá 7 free of heavier particles such as sand and the like. This can of course also be a method for separate disposal of lighter and heavier components in the wastewater, but then presupposes post-sedimentation or another suitable method for separating the heavier pollutants from the wastewater. Flotation enables more or less immediate separation before useful substances in the sewage are destroyed by biological or chemical influences.

The precipitant should be mixed with the waste water before the separator. Preferably the mixing should take place in pulses, i.e. the addition takes place during only a part (1 to 50%) of the time sewage is supplied to the separator. The addition should take place in such a way that the precipitant mixes quickly with the wastewater. If addition can take place in a zone with very turbulent flow, for example the gutter in an automatic car wash, no special devices are needed. In other cases, it may be advisable to use a suitable type of mixer. l fl ertalët case gives a so-called “static mixer”, ie mixers without moving parts, good enough mixing.

Continuous supply as well as supply directly to the separator itself provides dilution and does not lead to equally good purification results. The explanation for this may be that calcium salt and polymer together with components in the wastewater form a three-dimensional network with a large capacity for trapping dirt particles, even if these are of almost colloidal size. Supply in such a way that the dilution increases makes the meshes of the net larger and lowers the efficiency.

Both calcium ions and cationic polymer react with negative ions, including orthophosphate, in the wastewater. If calcium ions and cationic polymer are added in intimate mixture, their precipitating properties are utilized much better than if those which have hitherto been added separately.

Contrary to what one would expect, mixing calcium salt and cationic polymer increases the dispersibility of the polymer in water. A 20% solution of a mixture of 98% technical grade 98% calcium chloride and 2% polymer is admittedly quite viscous, but still light enough to surface for dosing with a suitable pump. Such a mixture contains 0.4% polymer and 19% calcium chloride. To achieve an equally low viscosity in an aqueous solution, without calcium chloride, its polymer content must not exceed 0.2%. The maximum content of cationic polymer aqueous solutions of calcium chloride and polymer suitable for dosing without a special pump is around 0.8%.

There is no good method for continuously controlling the content of aqueous solutions of polymers. The mixture with calcium chloride solves this problem. A simple conductivity measurement gives information about the content of calcium chloride and thus indirectly about the content of polymer. This is especially important with automatic dosing, as this way you can get the alarm when the dosing container is empty.

Example 1 A dairy produces 350 m ° of wastewater daily containing a cleaning solution consisting of surfactant, alkali and complexing agent (NTA), together with milk, milk fat, milk proteins and other dirt. The drain is collected in two tanks of 50 m each, which are used and emptied alternately. 2000 liters of a 10% solution of calcium chloride and cationic polymer are used per day for water purification. The mixture is prepared by dissolving 100 kg of a dry powder mixture containing 98% of technical grade 98 chloride (75 to 85% CaCl 2) and 2% of cationic polymer in water with stirring by blowing. The mixture is allowed to mature 2 to 3 hours before use. At each emptying of the collection tanks, approximately 290 liters of the solution are added. The addition takes place via a static mixer in the gutter between the collection container and the following separator, which consists of two series-connected containers of 60 m ° and 50 m respectively ”. 290 liters of the said solution contain about 23 kg of calcium chloride. Calculated on 50,000 liters of wastewater, the content of calcium salt is about 0.45 g / l. that is, about 0.16 g of calcium ion per liter.

In experimental operation, BOD has been reduced from about 400 mg / l to 1 mg / l. The precipitation is largely free of environmentally harmful substances and can be used to advantage as fertilizer or even for small animal breeding.

Example 2 In a plant for cleaning tankers, a system for washing with recirculating washing water is used broadly according to the above-mentioned PCT application, ie with a recirculating washing solution containing cationic surfactant and added with sodium orthophosphate and NaOH as emulsion-breaking and heavy metal precipitating agents. In the recycling solution where the pH was kept between 10 and 10.5, there was an orthophosphate content of the order of 2.0 g / l.

Due to the highly variable content of the car's tanks, difficulties arise in achieving treatment down to the level of 100 mg / l that constitutes the limit for emissions to the municipal treatment plant. To improve the separation, 20 to 2000 g per washed car of the mixture mentioned in Example 1 consisting of 95% water and 5% of a mixture containing 98% of technical grade calcium chloride and 2% of cationic polymer are added. The additive was varied with regard to the degree of dirt and size of the cars. The highest additives were used for cars that transported oil. -1l 10 15 20 25 30 35 5121135 Washing takes place both externally and internally in the tanks. Water turnover is estimated at 1500 to 2000 liters per car. The addition takes place before the oil separator and intermittently, ie only for a short part of the time the oil separator is supplied with wastewater.

A study over a longer period of time with wastewater containing virtually all raw materials and products that may be relevant for transport in a tanker has shown that regardless of which substances are in the washing water, the new purification method provides water that is well suited for both reuse and washing water and discharges to sewage treatment with common methods for biological treatment.

As an example of the very varied tank content that occurs, the following list relating to a week can serve: - spirits - oil - isopentane ~ polymer emulsion - uncured polyester solution ~ adhesive solution ~ glycerin - gelling agent ~ glycol - NTA solution ~ naphtha ° textile fi nish - diesel oil ~ concrete additive - base oil ~ xylene.

Example 3. In an automatic car wash with a washing system according to PCT / SE95100477, about 10 g per washed car of a mixture consisting of about 98% of technical grade calcium chloride (road salt) and about 2% of a cationic polymer having a molecular weight in the range of 6 to 12 million. Per wash, 2.8 g of calcium ions and 0.16 g of cationic polymer were dosed.

Dissolution of the mixture took place according to the previously described procedure.

The addition was pulsed by collecting the calculated amount of precipitant for 5 car washes in llllll ll i .ri 512118 10 15 20 25 30 35 35 and at once emptied by siphon action when the fifth car was washed. The circulating washing solution contained about 2.0 g / l of sodium orthophosphate, i.e. a significant stoichiometric excess relative to the amount of calcium ions and polymer. Consequently, the entire amount of calcium ions and polymer precipitated and there were no measurable amounts left in the recycle solution. For a normal wash, a total of about 500 liters of wash water is sold, consisting partly of recycled water and partly of fresh water for the final rinse. On average, 0.006 g of calcium ions were consumed per liter of washing solution in each wash cycle.

Example 4.

The experiment of Example 3 was repeated in another similar plant with the difference that the calcium chloride / polymer solution was prepared in two 100 liter containers which were used alternately. The solution was prepared by first filling the container with water, after which a bucket containing 5 kg of a dry-mixed mixture of calcium chloride and cationic polymer was emptied into the container.

1 liter of the solution was dosed at every third wash. The recycle solution contained between 0.5 and 3 g / l of orthophosphate. Two different mixtures of calcium chloride and polymer were used. One contained 96% by weight of calcium chloride and 4% by weight of cationic polymer. The other 98% by weight and 2% by weight, respectively, of the mentioned components.

In Examples 2 to 4 where there is an excess of orthophosphate, the recirculating aqueous solution is well prepared for further purification by means of a combination of ultra-filtration and reverse osmosis.

Example 5. In a dairy with similar production conditions as in Example 1, the wastewater was collected in a buffer tank. From this buffer tank, the wastewater was transferred batchwise to an ationsation tank. In experiments with the addition of auxiliary chemicals for purification, two different products were used. One product (I) contained a mixture of 95% by weight of trisodium orthophosphate and 5% by weight of sodium hydroxide dissolved in water to a solution of 7.5% by weight of the mixture. The second product (II) consisted of a 7.5% aqueous solution prepared by pouring 15 kg of a dry mixture of 98% by weight of calcium chloride (technical grade) and 2% of cationic polymer into 200 liters of water and stirring. Three different experiments were carried out: In the first, 2% by volume of product 1 was first added to the waste water at the inlet to the ttation tank. Then, when the tank was almost full, 1% by volume of product II was added.

The percentage was calculated on the volume in the stotation tank.

In the second experiment, 1% of product I was added at the inlet and 1.5% of product II together with the air injection for the totation.

In the third experiment, 0.5% of product I was first added and after mixing 1% of product II to the buffer tank before transfer to the fltation tank.

In the first experiment, COD was reduced from 2500 mg / l to 428 mg / l. Second, the COD content was reduced from 14972 mg / l to 1246 mg / l. In the 3rd experiment, the corresponding reduction was from 4430 mg / l to 275 mg / l. The third attempt gave precipitation already in the buffer tank and was therefore judged to be less successful.

The reductions in COD in the three trials were thus 83, 92 and 94%. BOD7 followed the same pattern.

Claims (1)

1. j. ._......... ^. I | alni ... Jul i. ~ __.-. 512118/2 Patent claim Process for the purification of alkaline waste water containing anionic compounds, which can react with calcium and form water-insoluble precipitates, characterized by adjusting to a suitable pH between 9 and 11, and adding a precipitating agent consisting of an aqueous solution, as prepared by dissolving a powdered mixture of a readily soluble calcium salt and a cationic polymer having a molecular weight in the range of 1 to 12 million in a weight ratio of 1011 to 10021, in such an amount as to correspond to 0.001 to 3 g of calcium ion and the corresponding amount of polymer per liters of wastewater, and o removal of the formed precipitate using known techniques for the separation of solids and fl surfactants. Process according to Claim 1, characterized in that the easily soluble calcium salt consists of calcium chloride or calcium nitrate or calcium hydroxide or a mixture of calcium chloride and / or calcium nitrate and / or calcium hydroxide. Process according to Claims 1 and 2, characterized in that the aqueous solution contains 2 to 20% by weight of the calcium salt / polymer mixture. Process according to Claims 1, 2 and 3, characterized in that the addition of precipitating agent is pulsed, i.e. the precipitating agent is mixed with a limited volume of the washing solution and the waste water (5 to 50%) and that this mixture is added to already supplied amount of wastewater without precipitating agent in separation clean. Process according to Claims 1 to 4, characterized in that the precipitate formed is separated by sedimentation, continuous filtration, centrifugation or the use of wet cyclones. Process according to Claims 1 to 4, characterized in that the precipitate formed is separated by flotation.