WO2001021537A1

WO2001021537A1 - Method for purification of water which has been used for vehicle wash

Info

Publication number: WO2001021537A1
Application number: PCT/SE2000/001815
Authority: WO
Inventors: Björn MÄLARSTIG; Gunnar STRÖM
Original assignee: Alfa Laval Ab
Priority date: 1999-09-20
Filing date: 2000-09-18
Publication date: 2001-03-29
Also published as: SE9903363D0; SE514966C2; AU7695600A; SE9903363L

Abstract

Water which has been used for vehicle wash is collected in a first tank (1) for purification of the same. The wash water contains contaminants as particles and oil. From the tank (1) a flow of wash water is withdrawn which is mixed with a flocculent consisting of a high molecular cationic polymer and a coagulant consisting of a metal salt, which is environmentally friendly. The most part of the solid particles are separated in a filter (6) and the purified wash water is directed further to a second tank (7). From this a partial stream is withdrawn continuously/alternatively during a certain time, which is brought to pass a centrifugal separator (9). The purified wash is returned to the mentioned second tank (7) and is used again for vehicle wash.

Description

Method for purification of water which has been used for vehicle wash

The present invention relates to a method for purification of water which has been used for vehicle wash, which wash water contains contaminants as particles and oil, at which the wash water is collected and the contaminants are separated from the wash water, after which the water may be used again.

Vehicle wash demands large amounts of water from each wash. It is not suitable to direct the whole amount of used water to drainage neither for the consumers who pay for the wash or for the municipal treatment of drainage water. Depending on the time of the year or on the type of vehicles the presence of oil and the amount of particles in the wash water is varying.

The method which are used today for purification of used wash water are usually based on a separation by way of gravity, where the wash water is collected in one or several tanks in which a first separation of sludge takes place in a so called sludge well. From this first coarse purification the water is directed further to a second tank where free oil gradually is collected at the surface. The oil may be removed over an overflow or with some kind of band skimmer. The particles are collected on the bottom of the tank and the sludge is taken care of at fixed intervals. This purification demands large tanks for collection of the wash water, while the particles sink and the oil rises.

Consideration for the environment implies that wash water no longer may be discharged into municipal drainage systems without a satisfactory separation of oil and heavy metals. Changed standards for discharge and the development which has taken place of the purification agents which are used in the vehicle wash has brought about that purification only using gravity separation does not work satisfactory, since oil as well as fatty particles may be emulsified in the wash water. One has therefore been forced to resort to further purification steps as chemical precipitation, microflotation, membrane filtration or biological purification.

In DE 43 15 408 A1 there is described a plant for purification of wash water from vehicle washing stations comprising a collection tank for wash water from which the water is directed to a tank in a flocculation step. Suitable chemicals are added to the water in order to obtain a precipitation of oil containing contaminants. From the bottom in the tank the water is directed further to a filter station with a specially designed filter chamber, where the precipitated or flocculated particles are separated from the water. The purified water may be used for vehicle wash again.

In DE 196 35 005 there is described an arrangement for purification of wash water from a vehicle wash in which the collected wash water is brought to pass a separator where particles of dirt over a certain size are separated. The separator may consist of a static separator or of a centrifugal separator. Flocculating agents are added to the purified water and it is directed to a container in which floccules of dirt are collected at the bottom of the container, while light contaminants are collected on the surface and are removed by way of an overflow. After a predetermined sedimentation time the purity of the water is examined using turbidity measuring and when a satisfactory value is reached the water is directed further to a collection container possibly after a further purification in one or several filters. DE 196 27 718 describes an arrangement for purification of water from vehicle wash stations which comprises a sludge well, a separator for light contaminants and a storage vessel, from which the water is directed to an injector where dangerous substances are oxidised by means of ozone, after which the water is returned to the sludge well. From the storage vessel the water is pumped to a micro filter. The purified water may be used again for car wash.

As is described e.g. in DE 296 11 811 U1 it is known earlier to use mem- brane filtration for a finishing purification. The wash water is collected in a container which functions as sludge well from which the water is pumped to a basin provided with an oil skimmer. Heavy particles sink to the bottom of the basin while oil, which has collected on the surface of the water at regular intervals is transported to a separate oil tank by way of the band skimmer. In order to secure that also emulsified oil and ions of heavy metals are separated the wash water is brought to pass a membrane module for reverse osmosis. The water which has passed the membrane may be directed to drainage or used again for a car wash.

EP 876 999 describes a plant for purification of water from car washes in that the water in a number of tanks is purified using various chemicals. In a first tank there is added a flocculating agent, which results in a decantation in that floccules of sludge are collected at the bottom of the tank. The water which is drained off from the upper part of the tank is directed further to one or two tanks which work as an intermediate storage before the water is directed further to a first conditioning tank where a mixture of sand and a cationic polymer is added to the water. Then the water is directed further to a second conditioning tank with active carbon in which the water also is oxygenated. In the last conditioning tank there is again added active carbon, sand and cationic polymer. The water is sterilised at the outlet from the third conditioning tank by treatment with a sodium hypochlorite solution.

The flocculating agents which are used for purification of wash water consists usually of trivalent iron or aluminium salts or of poly-aluminium chloride together with some cationic polymer. Said metal salts demand, however, a pH-correction during the purification stage.

As is evident from the description above large tanks are usually used for the separation of oil, which implies high costs for the plant as well as large demands on the available space. According to the invention it is now proposed a method to carry through the treatment of the wash water which results in an efficient purification with considerably less demands on the available space. The proposed method may with advantage also be used in existing plants. According to the proposed invention special precipitation chemicals are used which efficiently bind colloidal particles in the wash water and makes it possible to separate these.

The method according to the invention is mainly characterised in that the wash water is collected in a first tank from which a flow of wash water is withdrawn which is mixed with a flocculent comprising of a high molecular cationic polymer with a high positive charge and a coagulant consisting of a metal salt that does not lower the pH which is environmentally friendly, such as barium, calcium or magnesium sulphate. The wash water is brought to pass a filter with a pore size of 15-60 μm, where the major part of the solid particles is separated. The flow of purified wash water is directed to a second tank from which continuously/alternatively during a certain time a partial stream is withdrawn which is brought to pass a centrifugal separator, in which free oil and small particles are separated and taken care of. The purified wash water from the centrifugal separator is returned to said second tank and used again for vehicle wash.

The positive charge of the cationic polymer is due to the presence of quaternary ammonia groups. The substitution degree is 0,5 - 1 ,0.

The pH of the wash water may with advantage be rised to 8,5 - 9,0 by addition of sodiumbicarbonate or of a mixture of triethanolamine and sodiumbicarbonate. This adjustment of the pH is beneficial in order to precipitate zincsalts. The adjustment of the pH is carried through in connection with the addition of the precipitating chemicals.

By using the metal salts mentioned above small colloidal particles are attracted to the metal ions and the repellent forces in the particles are eliminated. The addition of these metal salts does not bring about any lowering of the pH. The co-operation between these blocks of positive metal ions and small particles and the cationic polymer makes an efficient purification of especially the small particles obtainable. Larger particles which have not had managed to sink to the bottom of the first tank are separated mechanically by the filter.

According to a preferred embodiment of the invention the high molecular cationic polymer consists of a polyacrylate or carbohydrate polymer. The molecular weight of the polymer suitably exceedsl 0.000.000. The flocculent and the coagulant are both present in the form of a water solution.

The amount of precipitating chemicals which are added to the wash water depends among all on the kind of vehicles which are washed, the degree of dirtiness and the time of the year. The concentration may vary according to the dosing equipment but is usually 1-2 % polymer and 3-8 % metal salt.

In order to avoid growth of micro-organisms during the storage time up to the use of the precipitating chemicals the water solution of flocculent and coagulant suitably comprises biocides.

The method according to the invention is with advantage carried through such that each vehicle wash is ended by rinsing the vehicle with tap water, which implies a mixing of the purified water with tap water. A smaller amount of water follows the vehicle out from the wash plant.

In order to keep the amount of water constant a certain amount of purified water is discharged from the wash system to municipal drainage, which water is taken from the return pipe leading away from the centrifugal separator. The water which is discharged has during the treatment in the separator been cleaned both from oil and particles.

The filter area is suitably cleaned by back flushing with purified wash water.

The mixture of water and particles which have been separated in the filter are directed to a second filter under influence of the gravity where de- watering takes place, after which the separated water is returned to the first tank. Through this second de-watering the volume which must be taken care of due to considerations of the environment is diminished. The method according to the invention may if so is desired be used also if the wash system is completely closed, i.e. all the water which has been collected in the first tank is used again. Suitably, a membrane filter for reverse osmosis is installed in the pipe in which the purified water is returned for use again for vehicle wash.

In the cases where problems with bacteria growth are noted in the circulating wash water means may be taken which lead to a reduction of bacteria.

The method according to the invention is described further with reference to the attached drawing, which shows a plant for vehicle wash.

The wash water is collected in a first tank 1. In the tank 1 there is a first separation in that sand and heavier particles are collected on the bottom of the tank. The sludge on the bottom is taken away at predetermined time intervals through the outlet 2.

A partial stream of wash water is pumped from the tank 1 and mixed with precipitating chemicals in a static mixer 3. The chemicals are added from a storage 4 by means of a dosing pump 5. The chemicals used according to the invention consist of a mixture of a flocculent and a coagulant together with biocides in order to prevent growth of micro-organisms in the storage vessel. From the mixer the wash water is directed to a filter 6 with a pore size of 15-60 μm.

In the embodiment shown on the drawing the filter consists of a drum filter with a straining cloth and the wash water is directed into the inner of the drum and passes consequently outwards through the filter cloth. The precipitated particle flocks are collected within the filter which at regular intervals is back flushed with purified wash water. The wash water which has passed the filter cloth is directed to a second tank, while the separated particles are directed further to a de-watering arrangement 8 of suitable kind.

A centrifugal separator 9 is connected to the tank 7. When the wash water is collected in the tank there is an enrichment of oil at the upper liquid surface and the discharge of water from the tank 7 to the separator 9 takes place by way of the surface sucking means which flows on the liquid such that the surface layer is brought to pass the centrifugal separator. In this remaining small particles and free oil are separated. Depending on the amount of oil in the wash water the oil phase may be taken away continuously from the separator or be taken care of together with the particles. The purified wash water is returned to the tank 7 by way of a pipe 10. In this pipe there is a three way valve 11 and at a suitable time purified water may be directed to municipal drainage through the pipe 12. The purified water is returned to the vehicle wash from the tank 7 through the pipe 13. Usually the main wash is carried through using return water while the final rinse of the vehicle takes place with tap water.

The water which has been separated in the de-watering arrangement 8 is returned to the tank 1 through the pipe 14.

When a vehicle arrives at the wash plant the pump 15 starts and sucks water to the car wash. When the level in the tank 7 sinks the pump 16 which transports wash water from the tank 1 as well as the dosing pump 5 starts. The centrifugal separator 9 works and sucks water from the tank 7. When the water has reached an upper level indicator the pump 16 stops, while the centrifugal separator continuous to work. After a certain time interval during the washing period the valve 11 opens and a predetermined amount of water is led to municipal drainage through the pipe 12. Since the vehicle wash is ended with a rinse with tap water the amount of water which leaves the created circuit corresponds to the amount of water which is added during the rinsing with consideration of the minor amount of water which follows the vehicle.

Claims

1. Method for purification of water which has been used for vehicle wash, which wash water contains contaminants as particles and oil, at which the wash water is collected and the contaminants are separated from the wash water, after which the water may be used again, characterised in that the wash water is collected into a first tank, from which a flow of wash water is withdrawn which is mixed with a flocculent consisting of a high molecular cationic polymer with a high positive charge and a coagulant consisting of an environmentally friendly metal salt which does not lower the pH as barium-, calcium- or magnesium sulphate, after which the most part of the solid particles are separated in a filter with a pore size of 15-60 μm, that the purified flow of wash water is directed further to a second tank, from which continuously/alternatively during a certain time a partial stream is withdrawn which is brought to pass a centrifugal separator, in which free oil and small particles are separated and taken care of, while the purified wash water is returned to the mentioned second tank and is used again for vehicle wash.

2. Method according to claim 1 , characterised in that the high molecular cationic polymer consists of a polyacrylate or a carbon hydrate polymer.

3. Method according to claim 1-2, characterised in that the mixture of flocculent and coagulant also contains biocides.

4. Method according to 1-3, characterised in that the mixing of purified wash water with tap water takes place in that each vehicle wash is finished with a rinsing of the vehicle with tap water.

5. Method according to 1-4, characterised in that a certain amount of purified water is directed to municipal drainage, which water is taken from the return pipe from the centrifugal separator.

6. Method according to claim 1-5, characterised in that the filter surface is cleaned by reverse flushing with purified wash water.

7. Method according to claim 1-6, characterized in that the mixture of water and particles separated in the filter under influence of the gravity is directed further to a second filter where a de-watering takes place, at which the separated water is returned to the said first tank.