This paper discusses the process of nanofiltration for recovering chemicals and recycling wash water used in photographic processing.

Historically, recycling wash water has not been necessary in most areas of the world. However, in many countries today, the cost of water is increasing and regulatory standards are becoming more stringent. It has, therefore, become increasingly important to address the issue of recycling the water used in these processes.

Two filtration techniques, which have been widely used to clean up wash water, are reverse osmosis and ultrafiltration. Reverse osmosis is capable of producing very clean water and high concentrate retentate. However, the process is very expensive, due to the relatively sophisticated technology it utilizes. Ultrafiltration, on the other hand, is relatively inexpensive but is not effective enough to meet stringent recycling standards. The process of nanofiltration is an effective compromise between reverse osmosis and nanofiltration. Nanofiltration utilizes membranes with pores that fall between those of reverse osmosis and ultrafiltration in size but which are still effective in removing ions from the used wash solution.

Nanofiltration is easier to implement and less expensive than reverse osmosis. The membranes are less costly, and the process utilizes a working pressure of 20 bars, compared to the 65-bar working pressure required for reverse osmosis. The illustration below shows the effective range of the different filtration processes.

Nanofiltration experiments

I. Nanofiltration equipment

Based on the experimental results obtained by TECHNO-MEMBRANES studies, commercial equipment was selected for treating the wash water. The ECP processing machines used produced wash water flows of approximately 650 l/h and 1400 l/h respectively. (They did not utilize rem-jet or soundtrack wash water.) The goal of the trials was the recovery of at least 80% of the wash water using nanofiltration. The diagram below illustrates the nanofiltration system used.

The nanofiltration membranes used in the trials were Weisman 3-stage membranes, model MPSW-11. The configuration consists of three parallel membranes, each capable of filtering 0.5 m3 per hour. A small portion of the concentrate was diverted directly to the sewer, to avoid excessive retentate concentration.

II. Nanofiltration for ECP process

II.1 The ECP process

Two trial tests were performed using nanofiltration technology with motion picture processing machines. The trials were run in both small lab (approximately 2,000 m/hour) and large lab (approximately 12,000 m/hour) environments. The ECP process consisted of the following steps:

• Prebath
• Rem-jet removal and rinse
• Developer
• Stop
• Wash
• First fixer
• Wash
• Accelerator
• Bleach
• Wash
• Sound track developer
• Sound track spray rinse
• Second fixer
• Wash
• Final rinse
• Dryer

In this sequence, the washes remove various contaminants in order to prevent contamination from occurring in subsequent stages. This increases the concentration of salts in the washing tanks, which are continuously fed with fresh water. A counter current of fresh water is also recommended.

II.2 Experiments

The chart below summarizes the specifications of the nanofiltration trials:

Values for the following were measured:

• Silver
• Total Sulfur
• pH
• Chemical Oxygen Demand (COD)
• Biological Oxygen Demand (BOD5)
• Thiosulphate
• Sulfate
• Chloride
• Hydrogenophosphate
• Phosphorous
• Magnesium
• Calcium
• Organics

II.3. Results

1.Wash water recycling

During these trials, recycling ratios of between 80 and 95 percent were achieved. (Variations were due to changes in machine speed.) It was observed that the flow rate started at 5 m3 and decreased to 2.5 m3 as the concentration of retentate increased; however, periodically draining the tanks and/or washing the membranes enabled the initial flow rate to be restored.

2. Products retention

The results of the trials also indicated that the recovered solution was of sufficient concentration and quality to be reused in the ECP process.

Since only 2 mg/l of silver is able to pass through the nanofiltration membrane, silver retention is very high. The concentrated solution, however, must be treated to recover silver. In addition, recent experiments have indicated that where nanofiltration is used with separated wash water, it is possible to reuse the concentrate as new replenisher after some chemical adjustments have been made.

3 Economic data

* Note: Annual cost in French Francs

Base for nanofiltration:
6 hours/day X 3.5 m3 X 330 days/year = 7,000 m3/year
141 kFF* / 7,000 m3 = 20 FF/m3 versus 17.5 FF/m3 for French fresh water

III. Nanofiltration for ECN process

III.1 The ECN process

Trial tests was performed using nanofiltration technology with motion picture processing machines. The trial was run in small lab (approximately 1,800 m/hour) environments. The ECN process consisted of the following steps:

• Prebath
• Rem-jet removal and rinse
• Developer
• Stop
• Wash
• Fixer
• Wash
• UL Bleach
• Wash
• Final rinse
• Dryer

As for the ECP process, the washes remove various contaminants in order to prevent contamination from occurring in subsequent stages. This increases the concentration of salts in the washing tanks, which are continuously fed with fresh water. A counter current of fresh water is also recommended.

III.2 Experiments

The chart below summarizes the specifications of the nanofiltration trials:

III.3. Results

1.Wash water recycling

During this trial, recycling ratios near 95 percent was achieved. It was observed that the flow rate started at 3.5 m3 and decreased to 3 m3 as the concentration of retentate increased.

2. Products retention

The results of the trial also indicated that the recovered solution was of sufficient concentration and quality to be reused in the ECN process.

3. Bacteriology

We noticed a biogrowth into the tanks during test and to avoid this development it seems to be important to make continuous addition of biocide.

4. Sensitometry

During the trial, the sensitometric results were consistent and inside the specifications. Keeping test of strips do not show any differences using or not the nanofiltration.

IV. Conclusion

The nanofiltration technique is very efficient in eliminating most of the pollutants from washes, and it enables the recycling of up to 80 percent of wash water. It is especially efficient in silver recovery. Among the variables which have not been established through use is membrane life. This could have a significant impact on the final cost data of using nanofiltration in film processing. In addition, reusing recovered concentrate as replenisher as well as recycling water will bring a faster return on investment than recycling water alone.