WO2006057617A1

WO2006057617A1 - Process for purifying water from a vehicle wash installation

Info

Publication number: WO2006057617A1
Application number: PCT/SE2005/001791
Authority: WO
Inventors: Lars Porshagen; Claes Rydlander
Original assignee: Conlib Ab; Icr Kemi Ab
Priority date: 2004-11-29
Filing date: 2005-11-29
Publication date: 2006-06-01
Also published as: EP1784362A1; EP1784362A4; SE0402889D0

Abstract

Process for purifying waste water from a vehicle wash installation in which, after a coarse separation, the waste water is treated batchwise in a relatively small process tank (16) by adding a lime suspension as precipitation chemical such that a pH of approx. 11, as well as a lime excess, are obtained in the water. A flocculation chemical is then added to the water in the process tank so that a pH of from approx. 7.5 to 8 is established. In a subsequent, substantially larger sedimentation tank (46) , relatively large floccules are formed, together with the lime excess, such that precipitated contaminants are collected and separated out and sediment on the bottom of the tank (46) . Sludge from the purification process is treated with polyelectrolyte and filtered in sack filters (62).

Description

Process for purifying water from a vehicle wash installation

BACKGROUND TO THE INVENTION

Technical field

The present invention relates to a process for the batchwise purification of waste water from a vehicle wash installation, in which purification chemical precipitation is applied in combination with using fresh water, with this achieving a degree of purification which is such that there is no negative effect on municipal purification plants.

Prior art

In connection with purifying waste water from car wash installations, the authority in Sweden having environmental responsibility has recently departed from its previous requirement for 80% recirculation of the quantity of washing water, which is maximized to 50 liters for each automating wash, because of the serious problems which this entails, particularly in the period from October to March in Sweden and other countries having an equivalent climate since the salt concentration in the washing water has risen to high levels, with this having a harmful effect both on the washing equipment and on vehicles which have been exposed to the water, with this applying, in particular, to high-pressure washing. Personnel who have handled the washing equipment have also suffered diseases and other unpleasantness since the recirculation has created a breeding ground for bacterial growth and, as a consequence, an obnoxious odor in the installations. Extremely high bacterial contents have been measured in several installations, especially in the period from April to September. Measurements have shown that the content of E coli bacteria is well above permitted values in several purification installations and that legionella has also been encountered.

Another disquieting factor is the ever-increasing contents of heavy metals in the washing water; this has resulted in efforts being made to reduce these contents before the water is released into waste water conduits leading to municipal purification plants.

There are nowadays a number of suppliers of different types of purification installations for vehicle washes. Only a few of these suppliers have achieved a reasonably acceptable process without, for all that, fully meeting the authorities' requirements. The purification methods can be divided into the following main categories: chemical precipitation biological treatment - oxidation

These methods differ from each other in a large number of unit operations such as centrifuging with a centrifuge or cyclone, flotation, sand filtration, membrane filtration, absorption, distillation, sterilization with UV, ozone, silver ions, etc.

Chemical precipitation

In known installations, the precipitation is normally carried out continuously with a continuous influx of waste water and additions of chemicals, with this providing certain advantages but also a number of disadvantages such as: difficulty in controlling the metering-in of the precipitation chemicals and thus the precipitation pH; the floccules are not formed in adequate size and are small and difficult to separate; untreated water passes through the system; recirculating water raises the salt content, with this interfering with the flocculation process; the sludge sediments and does not compact satisfactorily and takes up too great a space in the sedimentation tank, with this in turn interfering with the purification effect, especially if emptying only takes place infrequently; the sludge has to be "suction-exhausted" and as a rule contains 85-95% water, something which is expensive.

While the precipitation pH is normally regulated with sodium hydroxide (NaOH) solution, it is also in certain cases regulated with lime. Lime is difficult to handle and the handling of dry lime is dangerous to health. The methods which have thus far been used for preparing the lime have not resulted in the suspension of small lime particles which is required for a good precipitation result.

Biological treatment

Aerated tanks containing plastic supporting material are almost exclusively used in the first place for breaking down biological material. The installations suffer from the following disadvantages: little or no reduction in heavy metals; if there is any reduction, it takes place by means of an absorption effect in the supporting material rather than in the biomass, with this decreasing efficiency; bacterial aggregates which become detached from the supporting material require expensive post- filtration using sand filters, for example; the biological activity is sensitive to temperature and, during the winter, expensive heating is required for the breakdown to be satisfactory; the powerful aeration results in high contents of bacteria and molds in the environment. Oxidation

The oxidation takes place by means of powerful aeration, frequently with subsequent flotation or using ozone from an ozone generator or in a combination of air and ozone. These installations require an aftertreatment in the form of expensive filtration.

Disadvantages: - heavy metals are only separated off to a certain degree; ozone excess in the premises is harmful to health; there is a great risk of high contents of bacteria and molds and the method has to be combined with UV irradiation or some other form of sterilization; the apparatus is sensitive to moisture and several instrument breakdowns have been reported; the treatment frequently has to be supplemented with the expensive injection of oxygen; - high energy consumption.

US 6 042 730 discloses a method for the batchwise purification of water from a car wash installation, in which method preferably sodium hydroxide (NaOH) solution is added to the water in a batch treatment tank in order to raise the pH of the water up to 10, but preferably to between 8.2 and 8.6 and most preferably 8.4. A flocculation or coagulation agent, preferably polyaluminum chloride, is added until the pH in the mixture has fallen to approx. 6.2-7.0 or most preferably 6.8. This batch mixture is then allowed to sediment in the relatively large batch treatment tank for a predetermined time of at least 1 hour but preferably for up to 3 hours or more. Even if slaked lime is mentioned as a possible chemical for raising the pH of the wash water, a precipitation process in which a lime excess is used for substantially facilitating the separation and handling of the resulting sludge is not, however, proposed. At said pH of 8.4, there is no lime excess, with this making the sludge separation more difficult and necessitating expensive investment in filtration.

Summary of the invention

An object of the present invention is to propose an improved process which applies chemical precipitation with lime excess in combination with using fresh water, with this achieving a degree of purity which is such that there is no negative effect on municipal purification plants, i.e. that heavy metals can be removed such that they do not accumulate in the digested sludge.

The process according to the invention is distinguished for this object by: a first step in which heavier and lighter contaminants, such as sand and oil, are separated off coarsely from the waste water by means of gravitational separation in a first tank;

- a second step in which coarsely purified water is pumped intermittently from the first tank to a separate process tank in order to fill this latter batchwise with a defined volume of water; a third step in which a precipitation chemical in the form of a lime suspension is added to the water in the process tank in such a quantity, and is stirred for intermixing and dispersion in the water, that, on the one hand, a pH of approx. 11 is obtained in the water, in order to cause the metal ions which are present in it to precipitate out, and, on the other hand, an excess of free lime particles is obtained in the water; a fourth step in which a flocculation chemical is then added to the content in the process tank in such a quantity that the pH of the water becomes from approximately 7.5 to 8; a fifth step in which the water in the process tank, which water is mixed with precipitation and flocculation chemicals and with an excess of lime particles, is then transferred to a third tank which serves as a separate sedimentation tank and which is substantially larger than the process tank; - a sixth step for forming relatively large floccules together with the lime excess in the sedimentation tank, where precipitated contaminants are collected and the lime excess accelerates the sedimentation and separation of these contaminants from the water; and a seventh step in which the purified water in the sedimentation tank flows over into an overflow or outlet conduit.

The proposed batchwise treatment of the waste water in a process tank of relatively small size, with an accurately adjusted metered quantity of lime suspension firstly being added and intermixed such that there is a lime excess in each liter of water (at a pH of approx. 11) , can firstly cause a maximum quantity of heavy metals to precipitate out over 2-3 minutes before the flocculation chemical is added to and mixed in the tank. The dwell time for the resulting liquid mixture in the process tank is only approx. 5-7 minutes before it is transferred to a separate sedimentation tank which is substantially larger than the process tank and where relatively large floccules are formed together with the lime excess such that precipitated contaminants are collected and sediment and are separated out more rapidly from the water due to the lime excess. The lime excess also contributes to a more compact and more readily handled sludge being formed on the bottom of the sedimentation tank.

Preference is given to using an approx. 20% lime suspension as the precipitation chemical. In order to facilitate the preparation of this lime suspension, slaked lime which is in powder form and has a water content of approx. 1% is mixed, in a step prior to said third step in accordance with the invention, with water at room temperature in order to obtain a readily handled lime granulate which is in gravel or grain form, which does not give off dust and which contains approx. 5-15%, preferably approx. 10% water, after which this readily dispersed lime granulate is intermixed with water in a lime preparation tank to form a suspension having the desired lime concentration.

Additional features and advantages of the present invention will be evident from the subsequent detailed description and patent claims and with reference to the attached figure drawing.

Brief description of the drawing

The sole figure drawing shows a diagram of a vehicle purification installation in which the process according to the invention is applied.

Detailed description of preferred embodiments in accordance with the invention

In the drawing, 10 denotes in a general manner, a water purification installation in which the process according to the invention is applied to contaminated water from automatic car washes and/or wash halls in which the customer himself washes his car. The installation 10 includes, in a known manner, a first section in the form of a large coarse separation tank 12 which is the first to receive the contaminated water and in which heavy contaminants such as sand and the like, and a lighter phase such as oil or other petroleum products are separated off by means of gravitational separation. After this coarse separation, the water is pumped batchwise by means of a level- regulated pump 14 to a process tank 16 whose outlet 18 is for the present closed. Firstly, precipitation chemical and then flocculation chemical, in a quantity which is adapted, as will be described below, to the volume of the process tank 16 and the prevailing degree of contamination, are added to the water in this process tank 16 which can, for example, have a volume of approx. 3 m³.

In the process according to the invention, the precipitation chemical which is used is an approx. 20% lime suspension which is preferably prepared in a manner which will now be described. Preferably large sacks 22 containing approx. 700 kg of slaked, what is termed powder, lime having a water content of approx. 1% are delivered to a premises 20 which is preferably at a distance and which can be a separate factory. Each large sack 22 is lifted up and lowered into a stand 24 which has a supporting frame for the sack and an inlet hopper 26 leading to a motor-driven transport screw 28 of the paddle type. Water, which is in the main at room temperature, is metered in and added under pressure at the arrow 30, approximately in the middle of the screw 28, which mixes the lime and the water at the same time as it transports the mixture as a result of its screw blades being interrupted. The rotational speed of the screw transporter 28 is adjusted such that the water content increases to between approx. 5 and 15%, preferably approx. 10%. Large granules, which cannot be dispersed, are formed at water contents of more than approx. 15%. The water- containing lime, which Is now in the form of a lime granulate in gravel or grain form, is filled into relatively small portion vessels 32 which are suitable for being transported to water purification installations and whose volumes are adapted for a relatively small lime preparation tank 34 in the purification installation. In this connection, the volume of a portion vessel 32 can, for example, be approx. 14 liters. The lime preparation tank 34, whose volume can, for example, be approx. 200 1, is filled with water and with readily-dispersed lime from the portion vessels 32 in order to form an approx. 20% suspension during agitation with an agitator 36.

The process tank 16, which has in the meantime been filled with a batch of waste water at room temperature from the tank 12 using the pump 14 and a level- regulating device 38, which is shown diagrammatically, is provided with time-controlled agitators 40 and a measuring device, which is not shown, for measuring the pH of the contaminated water which has been aliquoted in. The lime suspension, which was prepared in the lime preparation tank 34 and whose concentration can be between approx. 15 and 30%, preferably approx. 20%, is added to the water in the process tank 16 by activating a pump 41, with the quantity of added lime suspension being controlled by the volume of water in the tank 16 and by the pH measuring device in such a way that, during agitation in the tank 16, the pH of the water firstly becomes approx. 11 and, in addition, a lime excess is obtained in the water in the tank 16. The lime suspension is thus added in a quantity which is such that the concentration of lime in the water exceeds 150 mg/1, preferably approx. 200-300 mg/1. The quantity of approx. 20% lime suspension required for achieving a pH of approximately 11 normally amounts to from approx. 1.0 to 1.5 1/m³ of water. In this way, a maximum quantity of heavy metals can firstly be caused to precipitate out over a period of approx. 2-3 minutes. A flocculation chemical, which is preferably based on polyaluminum chloride, is then added to the water in the tank 16 from a container 42 in a quantity which is such that the pH of the flocculated water in the tank 16 is adjusted to a lower level of approx. 7.5 to 8. At the same time, the agitators 40 bring about an effective agitation and dispersion of the precipitation and flocculation chemicals in the water. The chemical addition is consequently adapted to the prevailing degree of contamination of the water in a process tank 16 and the dwell time of the batch in the tank 16 amounts in all to only a few minutes, normally approx. 5-7 minutes.

The water, which is mixed with precipitation and flocculation chemicals in the tank 16, is then emptied, by way of the outlet valve 18, down into one end of a sedimentation tank 46 after the agitators 40 have been kept inactive for a minute or so. The volume of the sedimentation tank 46 is approx. 5-8 times larger than that of the process tank 16. Growing floccules are now formed in the tank 46 and sink, due to the lime excess, in a forced manner towards its bottom since there is no significant flow in this tank between the batches. The floccules form a type of filter which collects contaminant particles, and the suspended lime helps, in this connection, to add weight and to increase the rate of sinking and also compact the sludge at the bottom of the tank. The tank 46 can have compartments 48, or the like, such that any readily-floating sludge which might possibly be present, and which the floccules have not been able to collect, is collected on the surface. Sensors (not shown) which detect this lightweight sludge can indicate when this sludge should be removed, something which normally only has to be carried out on the odd occasion during a season. The tank 46 preferably has a sloping or V-shaped bottom where the sludge can collect in a manner which is as concentrated as possible. At the end of the sedimentation tank 46 opposite the end for the entry of the water there is a wide outlet 50 where the purified water flows down into a sampling well 52 and, as required, onward to an external, municipal purification plant by way of a waste water network as indicated by the arrow P.

The sludge in the tank 46 is pumped intermittently, in a gentle manner by means of a hose pump or membrane pump 54, to a relatively small tank 56 whose volume is approx. 200 liters and which has a level control which governs the pump 54. When the tank 56 has been filled with a batch of sludge, a metered quantity of thickener (polyelectrolyte) is added, during agitation, from a container 57 such that the flocculated sludge becomes concentrated into larger aggregates. After approx. 5-8 minutes, a bottom valve 58 in the tank 56 is opened in connection with which the sludge is able to trickle down into a sack filter unit 60. The sack filter .unit 60 contains a number of filter sacks 62 which have a volume of, for example 70 liters, which are arranged in parallel and which are suspended in a container 64, having an open outlet valve 66. The mesh width of the filter sacks 62 is such that compressed sludge is retained in the sack 62 while particle-free liquid can pass out of the container and be fed to the sampling well 52 for further transport to the external purification plant. When the sacks 62 are full, they can be changed and transported (the arrow F) for example in containers, to a storage site where the contents can be air-dried such that the volume decreases still further and the dry matter content increases. After that, the dried sludge can be transported to an installation for destruction. Two examples of practical tests which were carried out for reducing heavy metals and other contaminants in the washing water from car wash installations are given below.

Example 1

In connection with a larger investigation, water samples were taken at the outlet from about a dozen local purification installations during the winter half of the year. Weekly samples were taken in December, February and May. All the samples were analyzed for the substances which the Swedish authority (the Swedish Nature Conservancy Board's (NVs) "general advice 96:1") considers harmful to the environment, namely cadmium, zinc, the sum of chromium and nickel, and also mineral oil.

On each sampling occasion, the reduction in oil and heavy metals was substantially superior in the case of the described installation as compared with other purification installations.

Mean values for all the samples:

Parameter Discharge Reduction % NV s target mg/car mg/car

Lead 1.1 86

Cadmium 0.05 81 0.25

Copper 8 84

Chromium 3.5 62

Nickel 1.2 59

Zinc 28 88 50

Sum of lead/ 5.8 10 chromium/nickel

Mineral oil 900 83

Example 2

Similarly good results were obtained , in another investigation which lasted for 5 days and used automatic samplers. The samples were taken at the end of February and beginning of March.

Results:

Parameter Discharge Reduction % mg/car

Cadmium 0.02 87

Zinc 17 94

Sum of lead/ 4.0 87 chromium/nickel

Mineral oil 5CM) 94 It may be mentioned that, during the winter months, each car in central Sweden discharges approx. 10 grams of salt (NaCl) , which provides some conception of the salt concentration which arises if the water is recirculated without being de-salted. The cost of removing salt is at present too high when using the only methods which are available, i.e. distillation and reverse osmosis.

Summary

In contrast with known methods, the process according to the invention treats the contaminated water batchwise, such that a lime suspension is added to the water during agitation in a process tank having a relatively limited volume so that a pH of approx. 11 is obtained in the water and, in addition, a lime excess is achieved in order firstly to cause a maximum quantity of heavy metals to precipitate out from the water before the flocculation chemical is added and admixed in the tank so that the pH falls to approx. 7.5 to 8. In this connection, care is taken to ensure that all the water is treated similarly with optimal quantities of precipitation and flocculation chemicals (lime suspension and aluminum chloride) , with this being regulated by the pH values. In a larger sedimentation tank, the dispersed lime together with the flocculent purify the water by separating off, binding and "floccule-filtering" the contaminants in connection with which the floccules which are formed are weighed down by lime particles and contaminants with this making possible, due to the batchwise process, gravitational separation without interference from flowing water. - Simple equipment is employed, with it frequently being possible to use existing tanks, which means that the level of investment is moderate and capital costs are thereby low.

The process removes more than 80% of environmentally dangerous heavy metals.

For good purification results, the process requires a special preparation of slaked lime in which a lime granulate containing approx. 10% water is prepared in portion vessels.

Sludge from the purification process is treated with polyelectrolyte and filtered in sack filters and subsequently air-dried in order to facilitate storage and destruction.

Claims

Patent: claims

1. A process for purifying waste water from a vehicle wash installation, comprising: - a first step (12) in which heavier and lighter contaminants, such as sand and oil, are separated off coarsely from the waste water by means of gravitational separation in a first tank (12); a second step in which coarsely purified water is pumped intermittently from the first tank (12) to a separate process tank (16) in order to fill this latter batchwise with a defined volume of water; a third step in which a precipitation chemical in the form of a lime suspension is added to the water in the process tank (16) in such a quantity, and is stirred for intermixing and dispersion in the water, that, on the one hand, a pH of approx. 11 is obtained in the water, in order to cause the metal ions which are present in it to precipitate out, and, on the other hand, an excess of free lime particles is obtained in the water; a fourth step in which a flocculation chemical is then added to the content in the process tank (16) in such a quantity that the pH of the water becomes from approximately 7.5 to 8; a fifth step in which the water in the process tank (16) , which water is mixed with precipitation and flocculation chemicals and with an excess of lime particles, is then transferred to a third tank (46) which serves as a separate sedimentation tank and which is substantially larger than the process tank (16) ; a sixth step for forming relatively large floccules together with the lime excess in the sedimentation tank (46) , where precipitated contaminants are collected and the lime excess accelerates the sedimentation and separation of these contaminants from the water; and a seventh step in which the purified water in the sedimentation tank (46) flows over into an overflow or outlet conduit.

2. The process as claimed in claim 1, characterized in that the precipitation chemical comprises a lime suspension, preferably an approx. 15-30% lime suspension but, in particular, an approx. 20% lime suspension.

3. The process as claimed in claim 1 or 2, characterized in that the lime suspension is prepared by slaked lime in powder form and having a water content of approx. 1% being intermixed with water in order to obtain a lime granulate in gravel or grain form containing approx. 5-15%, preferably approx. 10% water, after which this readily-dispersed lime granulate is intermixed with water in a lime preparation tank (34) to form a suspension having the desired lime concentration.

4. The process as claimed in claim 3, characterized in that the lime granulate is prepared, and for transport purposes packed, in vessels (32) having a volume which is adjusted to the volume of the lime preparation tank (34) before the granulate is intermixed with water in the tank.

5. The process as claimed in claim 2, characterized in that lime suspension is added in a quantity which is such that the concentration of lime in the water exceeds 150 mg/1, but preferably amounts to approx. 200-300 mg/1.

6. The process as claimed in any one of claims 1-5, characterized in that sedimented, flocculated sludge which has collected on the bottom of the sedimentation tank (46) is fed intermittently and batchwise to a fourth tank (56) , in that a thickener is added to the sludge in the fourth tank (56) during concurrent agitation in order to form a compressed sludge, and in that the compressed sludge is then caused to trickle down into at least one filter sack (62) which is suspended in a container (64) and which has a mesh width which is such that water which is essentially particle-free can pass through the filter sack and be fed to the waste water or effluent conduit while compressed sludge is retained in the filter sack (62) .

7. The process as claimed in claim 6, characterized in that the thickener comprises an approx. 25% polyelectrolyte solution.

8. The process as claimed in claim 6 or 7, characterized in that the compressed sludge in the filter sack (62) is transported to another, preferably outdoor, site in order to be air-dried at that site down to a moisture content which is suitable for further destruction.

9. The process as claimed in any one of claims 1-8, characterized in that the flocculation chemical is based on aluminum chloride.