Vandgenvinding ved fotografiske processer hos CEWE COLOR DK 

Summary and conclusions

Photographic processes traditionally use large quantities of water for rinsing of films and photographs, which have been treated in developer, stop bath and fixing bath. CEWE COLOR DK, which is one of the largest photo laboratories in Denmark, has been working intensively during the last couple of years on reducing water consumption by implementation of water-saving rinsing processes.

In 1993 it was not possible to improve it further. If the water gets to concentrated it is not possible to remove silver efficiently from the wastewater neither by ion-exchanger nor by chemical precipitation because of strong silver thio-sulphate complexes. Therefore new methods has to be implemented in order to make room for further increase of the production load.

The most obvious solution is purification and reuse of the wastewater by means of reverse osmosis (RO). This is a membrane filtration process, where dissolved salts and compounds are removed from the water. This method is used in other types of industry, and it has been tested in some foreign photo laboratories as well, however, with poor results. The practical problem is quick fouling of the membranes, causing an unacceptably low capacity. Furthermore, cleaning of the membranes can be extremely difficult. For these reasons the method is still not commonly used.

CEWE COLOR and MILJŲ-KEMI have obtained financial support from the Danish Environmental Protection Agency’s cleaner technology programme for development and testing of reverse osmosis for treatment of photographic wastewater. The main idea of the project is to prevent precipitation of calcium carbonate in the membranes by chemical conditioning of the water. Furthermore, mechanical filtration should be established for removal of small particles, which might cause fouling of the RO-membrane.

It soon became obvious that the task was much more difficult to solve than anticipated, as it was impossible to remove the small particles from the wastewater efficiently by means of mechanical filtration. Filter bags, cartridge filters and sand filters have been tested, but a satisfactory filtration was not achieved and fouling of the RO-plant could not be prevented.

Flocculation of the wastewater by an organic polymer was also attempted. The polymer generated larger flocs in the wastewater, but still the filtration was not satisfactory.

Subsequently, it has been examined if it is possible to achieve a satisfactory pre-treatment of the water by micro-filtration (MF) before RO. Micro-filtration cannot be applied directly as fouling occurs too quickly. If on the other hand aluminium-containing flocculating agents are added to the wastewater, it is possible to perform an effective filtration with acceptably slow fouling of the micro-filter. But it is still not possible to achieve acceptable operational results on the RO-plant. Fouling still occurs a little too quickly.

In 1994-95 Fuji Hunt launched a new chemical for precipitation of silver in photographic wastewater. It is called SRP and is far more efficient than similar products in the market. Therefore, SRP was included in our investigations, hoping this was the solution to our problems. If we were able not only to remove the silver from the wastewater but also to achieve an acceptable operation of the RO-plant we would solve two problems at the same time.

SRP turned out to be an extremely effective precipitation agent for silver. Provided that the dosage is correct, SRP added to the wastewater makes it possible to reduce the concentration of silver residues to less than 0.1 mg/l, with a dosage of approx. 8 g of SRP per 1 g of silver. Large flocs are created, which are easily removed by ultra-filtration. On the other hand it is not possible to achieve satisfactory removal by means of the micro-filter, which was used previously in the project. Fouling of the MF is too quickly, and the RO-plant does not run satisfactorily with the filtered water.

The final pilot tests with UF at CEWE COLOR must be described as a great success. Here it was possible to reduce the silver content from 25 mg/l to approx. 0.2 mg/l by ultra-filtration. Subsequently, the filtered wastewater was treated in the RO-plant with a satisfactorily high capacity and with no tendency of fouling of the membrane.

The tests had a duration of 150 hours, and the average flux in the UF-plant was around 100 l/m² per hour for PES-membranes and 150 l/m² per hour for RC-membranes. No fouling problems were observed during the testing period, and chemical treatment of the membranes was not necessary. In the testing period all water from the UF-plant was treated in the RO-plant afterwards. During this process the RO-flux was quiet and constant around 40 l/m² per hour, and there was no tendency of fouling.

The tests demonstrate that it is possible to remove silver from photographic wastewater by precipitation of silver by SRP followed by separation in an ultra-filter. The filtered water can be further purified by reverse osmosis and reused for rinsing. This way it is possible to recover and reuse 50-65% of the wastewater from photographic processes.

In 1997 CEWE COLOR DK was ready to implement a full-scale plant, but something had happened on the market for photo chemicals in the meantime. Therefore the German owners, Nordcolor, was not quite sure that it was still the optimum solution.

Now it is possible to have higher concentrations of chemicals in the rinsing tanks than earlier, because a new biocide "Superstab" had been introduced on the market. Dosing of Superstab to the rinsing tanks will prevent growth of micro-organisms, which is a big problem in semi-concentrated solutions. Using Superstab it should be possible to use counter-current rinsing systems with a much higher degree of concentration. This will reduce the water consumption to the same level that could have been reached by RO with water reuse. The Superstab solution will depend on a new SRP-precipitation technique for silver, because it will not be possible to purify concentrated wastewater using old treatment methods.

After a long assessment period Nordcolor decided in 1998 that CEWE COLOR should implement a wastewater treatment system based on SRP-precipitation of silver. RO should not be a part of this solution, because in the future water savings could be obtained as well by other new methods available. At that time Nordcolor believed that CEWE COLOR should implement Superstab which was now available. The key to water savings was drag-out minimisation and extended use of counter-current rinsing with a very high concentration of chemicals. In practice CEWE COLOR solved the problem without using Superstab. Instead the half concentrated rinsing tanks were emptied and cleaned regularly (once a week) to remove and prevent biological growth.

In April 1999 a complete plant was delivered from the German company Hauck, which is a regular supplier of wastewater equipment for the Nordcolor group in Germany. The treatment plant was put into operation at CEWE COLOR in the end of April.

The plant is based on SRP-precipitation followed by 3 mechanical filter systems, before the water is finally treated in a UF-plant before discharge to sewer. Cartridge filters are used for a kind of pre-concentration of sludge. All silver sludge is finally collected in the filter bags, which are emptied when they are full. Silver sludge is dried in the air before it is sent to external recovery. The function principle of this plant differs slightly from that of the pilot plant. In the pilot plant all sludge was removed by the UF-plant, but in the full-scale plant sludge is removed and collected in the filter bags.

In the period 04.05-17.08.1999 we have followed the start-up and test-period very closely. In this period 168 m³ of wastewater have been treated and 5.9 kg of silver have been removed. The silver concentration in the untreated water has been almost steady.

Over-dosing of SRP has been necessary to obtain an efficient precipitation of silver. Apparently the consumption of SRP increases with high concentration of iron in the water. Iron originates from bleach fixing chemicals. Today CEWE COLOR do not use very much of these bleaching chemicals, and therefore the consumption of SRP has decreased.

The treatment efficiency has changed quite a lot during the test period. For 75% of the time the silver concentration in the discharged water has been below 0.5 mg/l - often considerably lower. In 1993 the total amount of discharged silver in wastewater was 10 kg per year. Today it is reduced to 0.8 kg per year corresponding a reduction of 92%.

Based on the experience gained during the test period CEWE COLOR has made some adjustments of the daily operation, and the performance has now improved and become more steady. The maximum flow for the plant is 1.2 m³/h, but at 0.6-0.8 m³/h we have a better performance and a longer and more steady operation of the filters.

The consumption of water for developing of films and photographs has been reduced considerably. Before starting the project in 1993 the water consumption was 0.87 litres per metre film. Today the consumption is only 0.35 l/m - a reduction of 60%. The consumption of water for photographs was 3.6 litres per m² of paper. Today it is 1.8 l/m² - a reduction of 50%.

Altogether we can conclude, that the new treatment plant based on SRP-precipitation provides a much better treatment of photographic wastewater than ever seen before. Combined with other environmental improvements and water savings CEWE COLOR has obtained a total result matching the objectives for this cleaner technology project - and a little more.