Large quantities of silver are used, in particular in the photographic processing industry. This silver is lost because after processing of the films the silver-containing processing solutions are discharged into the sewage water and into the environment. Not only does this constitute a considerable waste of an expensive and scarce material, since silver exists but in limited amounts in nature, but in addition thereto the waste also causes inexcusable environmental pollution. Heavy metals cause problems of various kinds. In this respect, one problem which hitherto has been given but little consideration deserves to be mentioned, namely the fact that even minute amounts of silver in the sewage water have a detrimental effect on bacterial life, with the result that bacteriological filters in sewage purification plants become ineffective very rapidly. Of the silver produced around the world the photography in¬ dustry uses appr. 28% and at the moment only 8$ thereof 1s recovered. Other metals are also used in the photography industry, such as gold, aluminium and iron. With the aid of the methods hitherto used, such as the electrolysis process, 60% of the silver may be recovered, but none of the other metal ions may be recovered by prior-art methods.

Many attempts therefore have been made to find methods of recovering silver from waste water. These attempts include methods for deposition of the metals or evaporation to extract the silver from the evaporation remainders e.g. by burning, and so on. However, these methods are very complex and expensive, particularly as they require ver large amounts of energy.

The purpose of the subject invention is to provide a method of purification of the waste water from preferably photographic labora¬ tories and for recovery of silver therefrom as well as of other metals, the method being both efficient and inexpensive compared with prior-art methods.

This purpose is achieved in accordance with the invention by collecting the waste water from all film processing stages in a collection container, when a predetermined amount of liquid has been collected in said container, transferring said liquid to a compression chamber and passing it through a number of a filters operating on the

principle of reversed osmosis, channelling liquid from which the metal ions have been removed into a sewage while the concentrate present in the compression chamber is disharged at regular intervals into a collection tank to be transferred for further processing to recover metals present in the concentrate.

,)The invention also concerns an apparatus for performing the method of purifying waste water containing heavy metals, said apparatus comprising a collection container from which a liquid pipe leads to a compression chamber which is fitted with filters operating on the principle of reversed osmosis, said compression chamber having an outlet through which is dicharged water from which heavymetal ions have been removed, and a line connecting said chamber with a collection tank for collection therein of the concentrate of heavy metal ions.

The invention will be described in closer detail in the fol- lowing with reference to the acompanying drawing which is a schematical representation of a plant designed to perform the method in accordance with the invention.

Waste water is conducted from a laboratory 1 through a pipe 2 to a collection container 3 in which all waste water is collected, such as fixing solutions, developing solutions and rinsing baths. When a predetermined amount of waste water has been collected in the con¬ tainer 3, for instance 25 - 30 litres, it is supplied to a compression chamber 4 which in the embodiment illustrated herein is equipped with three filters 5, 6 and 7 operating on the principle of reversed os- mosis. After having passed through the filters 5, 6 and 7 the de- metallized water is supplied through a line 8 to a sewage pipe. The concentrate which remains inside the compression chamber 4 is supplied via a line 9 to a collection tank 10 in which the concentrate is collected. After a predetermined number of processing operations the tank 10 is sufficiently filled to be emptied or replaced by an empty tank and thereafter be transported to a plant in which its contents are further processed. In the case of laboratories of normal capacity the collection tank 10 probably will need to be removed for further processing of its contents at appr. monthly intervals.

SUBSTITUTE SHEET

By using the reversed osmosis principle to clean the waste _ water in the manner suggested herein it has become possible to purify waste water which is polluted with heavy metal ions in an efficient and inexpensive manner. An additional advantage is that the waste water may be discharged directly into the natural recipient without further processing, possibly after having passed through municipal sewage purification plants. Silver which is an expensive rare metal and in addition exists only in scarce amounts may be recovered and re-used in a plant which may be constructed at reasonable costs and yet be so efficient that the waste water discharged therefrom contain such minute amounts of metal ions that their presence cannot be proved.

The invention is not limited to the embodiment described and shown herein but a number of modifications are possible within the the scope of the appended claims. For instance, it is quite possible to vary the number of filters used in dependence on the desired or required degree of purification and to a certain extent on the composition of the salts present in the waste water. It should be obvious that the waste water may be subjected to other purification processes for removal of other salts than those containing ions of heavy metals. The compression chamber may be provided with windows to allow monitoring of the condition of the filters.