SE500177C2 - Process for simultaneous reduction of residues of heavy metals and oil in oil-laden wastewater - Google Patents

Abstract

The final concentrations of heavy metals and oil in oil contaminated wastewater intended for biological purification are reduced by adding an easily soluble sulfide and an easily soluble orthophosphate. The separation of oil and heavy metal sludge may be done in conventional, gravity based equipment for sedimentation as oil separators or lamellae separators.

Description

tapes with waste water of a type which, apart from the heavy metal content, is suitable for biological treatment together with household sewage.

The addition of alkali metal sulphide should be combined with the simultaneous addition of an easily soluble orthophosphate according to Swedish patent application No. 9101290-6 which applies to the addition of orthophosphate to improve emulsion breaking and reduction of residual oil content in wastewater from automatic car washes. There are indications that the precipitation of heavy metals also has a positive effect on the reduction of the residual oil content.

The precipitation of heavy metals and the precipitation of precipitate in sodium sulphide-containing waste water have surprisingly proved to be very effective, despite the prevailing conditions differing in many ways from those which are considered optimal for precipitation reactions. Example: the precipitation takes place at low temperature and in the presence of surfactants and complexing agents. These factors usually have a negative effect on the precipitation of metals. Even very small amounts, around stoichiometric or even sub-stoichiometric, give a significant reduction of the heavy metal bales in the water. Dosage should be taken into account that sulfides are easily oxidised. The conditions in the wash favor this oxidation. Evaporation of hydrogen sulphide if released can For this reason, the pH 8.5 should also lead to reduced efficiency. ensure that recirculating wash water is alkaline. to 11.5 and especially 9.5 to 11 is recommended.

Common levels of heavy metals in wastewater from automatic car washes can be up to 50 micromoles per liter or even higher. The sodium sulphide content should therefore not be less than 4 mg / l.

Within certain limits, the efficiency of the precipitation increases with increasing sulphide content. At high levels, however, problems with the smell of hydrogen sulfide can arise. Levels above 150 mg per liter are rarely justified for precipitation reasons and are avoided due to the risk of odor problems unless the levels of heavy metals are extremely high.

Suitable levels are usually in the range of 10 to 100 mg / l and especially in the range of 10 to 50 mg / l. The present invention thus relates to a process for the simultaneous reduction of heavy metal content and residual oil content in wastewater from workshops, car washes and similar plants by adding 4 to 150 mg per liter of alkali metal. or ammonium sulfide and 0.05 to 50 g per liter of readily soluble orthophosphate.

In principle, the precipitation could take place by the formation of sulphides "in situ" by introducing hydrogen sulphide directly into the oil separator, but in view of the control problems this would entail, this is generally not practicable.

Therefore, easily soluble sulfides are used. It is within the scope of the invention to also use acid salts, i.e. salts, where only one hydrogen ion in the hydrogen sulphide is neutralized, but this generally gives rise to odor problems. The preferred sulfide is disodium sulfide.

The precipitation can be separated by filtration, but this is not preferred, as filtration is a complicated process, which is often unsuitable for waste water of the type in question. In preferred processes, therefore, separation by sedimentation is used. This can usually be done in existing gravity-based oil and sludge separators.

Addition of sulphides and orthophosphates can take place separately, but it is preferable to add a mixture of the salts, as the orthophosphate has a stabilizing effect on the sulphides and reduces any odor problems. The proportions between the two types of salts can be varied, but it is preferred that orthophosphate be present in excess. The appropriate level of sulfide addition is between 5 and 20 weight X. That is, a weight ratio of orthophosphate to sulfide of between 4: 1 to 20: 1.

Example 1.

In a car wash for passenger cars, about 400 liters of water were consumed per washed car. Of this, about 300 liters were recycled. About 100 liters consisted of clean water. Common type of cleaner was used. Oil and sludge separator volume LTI T; 13 \; | was about 25 ma 'During a control period, samples of the wastewater were taken from the separator and analyzed for residual oil and heavy metals. The number of cars washed during this period varied between 25 and 50 per day. The total water samples contained 740 mg of oil per liter.

The heavy metal contents are shown in Table 1. After this trial period, a device was inserted according to Swedish patent applications 8904027-3 and 8904029-9. The container contained 3.5 kg of a mixture of 90% by weight of sodium orthophosphate and 10% by weight of sodium sulfide. The consumption of the contents of the container was controlled by the injection of detergent and adjusted so that one container was sufficient for cleaning about 150 cars. The average content of sodium sulphide in the wastewater was estimated to be 23 mg / l. This is a certain overestimation, as water from other parts of the plant, including a "do-it-yourself" wash, was also led to the oil separator. These water quantities have been neglected in the calculations because it has been impossible to achieve a reasonably reliable estimate. Samples were taken for two periods. During one, the weather was relatively favorable and the frequency washes relatively low, about 25 cars per day.

This period is denoted by I in the table below. During the second period (II) the weather was worse and the washing frequency increased to about 50 cars per day. In the samples from the first period, the residual oil content was 11 mg / l, the second period 18 mg / l, ie in both cases well below the guideline value of 100 mg / l.

Table 1.

Control I II T-met ug / l ug / l ug / l Lead 280 50 60 Zinc 2100 100 325 Cadmium 67 Chromium 63 Copper 810 50 140 Nickel 110

Claims (2)

Guide value (Stockholm Vatten AB) lead 50 ug / l, zinc 500 ug / l, cadmium 0.05 ug / l, chromium 50 ug / l, copper 500 ug / l, nickel 50 ug / l. Regarding cadmium, the analysis method used has a detection limit of 10 ug / l. It is therefore not possible to say whether the guideline value is below in this case. For other heavy metals, the measured values are close to or below the guideline values. The differences between the bathing periods can possibly be explained by the fact that the dilution of the water through additions from other sources became greater during period II, e.g. a. because an increase in the frequency of automatic washes is likely to be matched by a similar increase in the number of do-it-yourself washes. Another reason may be that the sedimentation time became shorter during the second period. In both cases, however, the reduction of the heavy metal contents is significant, above 89 and above 83 X. The wastewater becomes acceptable for most treatment plants after treatment. Patent claims Process for the simultaneous reduction of the residues of heavy metals and oil in oil-laden waste water, characterized in that heavy metals precipitate as sulphides by adding 4 to 150 mg per liter of waste water, easily soluble sulphide and the residual oil emulsions are broken by adding 0.05 to 50 g per liter of wastewater readily soluble orthophosphate. Process according to Claim 1, characterized in that the separation of oil and sulphide sludge from the aqueous phase takes place by sedimentation in an oil separator which can be supplemented with a lamella separator.