Waste waters from automatic car cleaning, work shops etc. ofte contain high concentrations of heavy metals. Heavy metals are undesirable in plants for sewage purification as they make the use of the sludge as soil conditioner more difficult and may make it necessary to make special arrangements for sludge deposition. High concentrations of heavy metals may influence the biological purifying process negatively by poisoning of the micro organisms that are reponsible for the degradation in the sewage plants.

The plants concerned are usually too small to be equipped with any kind of water purification beside the oil and sludge separators that are obligatory. These give, if no special measures are taken, no or just insigni icant elimination of heavy metals. If the sewage plants exercised strict criteria t the waste water they are accepting closing of automatic car cleaning plants would often be the only realistic alternative. Earlier known purification methods as micro flotation, membran filtration and biological purification involve too high investing costs to be supported by a car wash plant with a normal turn over.

The discussion about heavy metals in waste water has been lively, but constructive ways of handling the subject have bee missing. The inventor has tested a number of di ferent additive and has found that already very small amounts of alkali metal sulfide or ammonium sulfide give a nearly total precipitation of the heavy metals lead, zinc, cadmium, chro , copper and nickel, which are considered as causing most of the problems connected with car wash.

In. order to ensure that the precipitation will give lower concentrations of heavy metals in the waste water it must be done under such conditions that sludge containing precipitated heavy metals is effectively separated from the outgoing waste water. This demands buffering and additives, which are

effektively breaking the oil emulsions that stabilize the sludge suspensions in waste water from inter alia work shops and car wash plants.

Precipitation of heavy metals with the aid of hydrogen sulfide or sulfide salts has been used as analyse method for heavy metals and has been proposed as purification method for acid water solutions containing one of the elements arsenic, antimony, mercury and tin, SE 8001720-5, laid open publication no 420 487, and to eliminate mercury from acid industrial waste water, NO 814193, laid open publication no 154 010.

The methods according to the publications mentioned are complicated and give no hints to a fairly simple solution of the heavy metal problems connected with waste water of the kinds that, apart from the heavy metal content, are suited for biological purification together with domestic waste water. Of car wash plants it is demanded that at least part of the outflow from the oil separator should be reuseable and that investments and running expenses should be within reasonable limits, which makes the processes mentioned still more impossible.

Waste water from car wash plants and work shops contain oils and fats and the addition of alkali metal sulfide is therefore combined with addition of a highly soluble ortophoβphate for emulsion breaking according to Swedish patent application nr 9101290-6. According to this application addition of a highly soluble ortophosphat is used to enhance emulsion breaking and reduction of final oil content in waste water inter alia from automatic car wash plants. The ortophosphates give the buffering and emulsion breaking effect mentioned above. The precipitation of heavy metals influences the reduction of final oil content positively. The two additves thus have a synergistic effect.

The precipitation of heavy metals and the separation of the precipitate in sodium sulfide containing waste water has surprisingly shown itself to be very efficient in spite of the

fact that the prevailing conditions in many ways differ from those that are considered as optimal to precipitation reactions. For instance: the precipitation is done at low temperature and in the presence of tensides and complex formers. Those factors are usually considered to influence precipitation of metals negatively. Measurements show that the reduction of both oil and heavy metal content at the use of the proposed method is equal to the one that is the result of earlier known significantly costlier methods.

Even very small amounts of sulfide, about stoicheiometric or even understoicheiometric give a significant reduction of the heavy metal contents of the water. Vaporization of hydrogen sulfide if it is liberated may cause reduced efficiency. For this reason the recirculating wash water should be alkaline. pH 8.5 to 11.5 and especially 9.5 to 11 is recommended.

Common concentrations of heavy metals in waste water from automatic car wash plants may be from 10 up to 50 micro mole per liter or still higher. A significant reduction of the heavy metal content is obtained at sodium sulfide concentrations of 1 mg/1. If the concentrations in the waste water are high the efficiency of the precipitation increases within certain limits with increasing concentrations of sulfide. At high concentrations of sulfide problems with odour of hydrogen sulfide may arise. Concentrations over 150 mg per liter are seldom motivated by reason of precipitation and are avoided on account of risk of odour problems if the concentrations of heavy metals are not extremely high. Appropiate concentrations are often within the range 2 to 100 mg/1 and especially witin the range 4 to 50 mg/1.

Thus this invention concerns a process for the simultaneous reduction of heavy metal content and final oil content in waste water from work shops, car wash plants and similiar plants by the adding of 1 to 150 mg per liter highly soluble sulfide and 0.01 to 50 g per liter highly soluble ortophosphate.

In principle the precipitation could be achieved by formation of sulfides "in situ" by letting in hydrogen sulfide direktly into the oil separator. By reason of the control problems this would give rise to this method is generally not practically possible. In preferred ways of carrying out the invention thus easily soluble sulfides as alkali metal and ammonium sulfides are used. It is within the scoop of the invention to use acid salts i. e. salts where only one of the hydrogen ions in the hydrogen sulfide i neutralized, but this way will frequently cause odour problems. Preferred sulfide is disodium sulfide.

At the process one oil rich phase, one water phase and one heavier sludge will be formed. In conventional gravity based oil and sludge separators an acceptabel separation of the three phases will usually be achieved. If the oil separator is undersized separation efficiency can be amended further by complementing the installation with la ell separators.

Addition of sulfides and ortophosphates may be done separately, but it is preferred to add a mixture of the salts, as the ortophosphate stabilizes the sulfides and reduces eventual odour problems. The proportions between the two kinds of salts may be varied, but it is preferred to use ortophosphate in surplus. Appropiate level of the sulfide content lies between 1 and 20 weight 7.. I. e. a weight proportion of ortophosphate to sulfide at between 4:1 to 99:1.

Example 1.

In a car wash plant for passenger cars around 400 liters water were used per washed car. Around 300 liters were recirculated. Around 100 liters were clean water. Common type of cleaning agents was used. The oil and sludge separator had a volume of about 25 τn ³ ' During a control period samples of the waste water were taken from the separator and analysed with respect to final oil and heavy metal content. The number of washed cars varied during this period between 25 and 50 per day. The combined water samples contained 740 mg oil per liter. The contents of heavy metals are shown in table 1. After this test

period an arrangement according to the Swedish patent applications 8904027-3 and 8904029-9 was installed. The container held 3, 5 kg of a mixture of 90 weight 7. sodium ortophosphat and 10 weight 7. sodium sulfide. The using up of the container content was controlled by the injection of cleaning agents and adjusted in the way that one container was sufficient for the cleaning of about 150 cars. The average concentration of sodium sulfide in the waste water was calculated to be 23 mg/1. This is some overestimate, as water from other parts of the plant including one "do-it-yourself"- wash was directed to the oil separator. Those water quantities have been neglected at the calculation as it has been impossible to make a fairly good estimate. Samples were taken during two periods.. During the first period the weather was rather fine and the frequency of washes comparativly low, abou 25 cars a day. This period is marked with I in the table below. During the second period <II) the weather was less fine and th frequency of washes increased to about 50 cars a day. In the samples from the first period the final oil content was 11 mg/1, the second period 18 mg/1, i.e. in both cases much unde the target value 100 mg/1.

Table 1.

Target values (Stockholm Vatten AB) lead 50 μg/1, zinc 500 ug/1, cadmium 0,05 ug/1, chrom 50 μg/1, copper 500 ug/1, nicke 50 μg/1.

Regarding cadmium has the analytic method used a detection limit at 10 μg/1. Therefore it is not possible in this case to tell if the measured concentrations are smaller than the

target. For other heavy metals the measured values are near to or under the target value.

The differenses between the two periods may possibly be explained by the fact that the dilution of the water by contributions from other sources became bigger during period II, inter alia as an increase of the frequency of automatic washes probably is followed by a corresponding increase of the number of "do it yoursel "-washes. Another cause may be that the sedimentation time became shorter during the second period. In both cases however the reduction of heavy metal concentrations is considerable, above 89 respectively above 83 V.. The waste water becomes after treatment acceptable to most sewage plants.