How can I recover the silver from my photo processing waste?
The Center for Environmental Health and Safety (CEHS) does not recover silver from photo processing waste. Use Tables 2 and 3 and the following discussion to help you choose the best method for your situation. (The information was taken from “Need For & Efficiency of Silver Recovery, or Silver Sampling Faux Pas & Fundamental Conclusions” presented at the 16th College and University Hazardous Waste Conference on July 20, 1998 in New Orleans, LA. The authors are Susan M. Morgan (Assistant Professor of Civil Engineering, SIUE), Erik A. Talley (former Associate Director of the Center for Environmental Health and Safety, SIUC), Mohammed Z. Rahman (former Graduate Assistant in Civil Engineering, SIUE), and Keith E. Morgan (former Hazardous Waste Technician in the Office of Environmental Health and Safety, SIUE).)

Electrolysis, or electrolytic recovery, and metallic replacement are the most common methods used for silver recovery from photo processing wastewaters. During electrolysis, an electric current reduces the silver-thiosulfate complex and plates almost pure silver metal onto an electrode. If the unit is placed in-line and closed-loop fixers are used, chemical use can be reduced by up to 50%. Efficiencies above 90% are easily obtainable when recovering silver from black and white processing fixers. However, while efficiencies approaching 90% are possible when recovering bleach-fix and fixer solutions from color processing, higher current densities, longer times, and pH adjustments are necessary due to iron complexes present. In addition, over-extending the electrolysis time or raising the current density can result in lower efficiencies due to sulfide precipitation on the cathode. To reduce concentrations below 5 mg/L, electrolysis must be followed by another recovery method, such as metallic replacement or ion exchange.

Metallic replacement makes use of the fact that iron is more active than silver. Silver in solution will exchange with solid iron through an oxidation-reduction reaction. Steel wool, iron particles, or iron-impregnated resin are used as the iron source. The iron is placed in a container referred to as a metallic replacement cartridge (MRC), chemical recovery cartridge (CRC), or silver recovery cartridge (SRC). One cartridge can recover more than 95% of the silver from silver-rich solutions (such as fixer and bleach-fix) while a series of two cartridges can recover more than 99%. A series arrangement will also prevent breakthrough, which occurs as small channels develop in the iron. However, for flows less than 0.5 gallons of fixer per day, one canister is adequate. Although low silver concentrations are removed with metallic replacement, the iron catalyst will be consumed more quickly due to the reduced protection from corrosion.

While precipitation, evaporation/distillation, ion exchange, and reverse osmosis are potential recovery methods that would meet many low discharge requirements, capital and operating costs preclude them from use by most small generators. Precipitation can be very efficient, generating a sludge with 99.9% or more of the silver from silver-rich solutions, but it is not a common method utilized to recover silver due to the chemicals and skilled personnel required. Evaporation/distillation can concentrate silver-rich solutions to between 8 and 30% of the original volume. However, the residue is unusable for mixing fresh developer solution, although it may be usable for making secondary replenishers (such as bleach, fixer, and stabilizers). Ion exchange works by attracting the negatively-charged silver thiosulfate complex to positively-charged sites on the resin. The resin can be regenerated with a concentrated solution or replaced. However, ion exchange only works on dilute solutions, such as wash waters (although wash baths can have concentrations as high as 200 mg/L), since high concentrations quickly saturate the resin. Reverse osmosis uses pressure and a membrane to filter solutions, removing up to 95% of salts from fixers. It results in a concentrated silver stream that could be sent to a refiner. This technology also works best on dilute solutions, achieving up to 90% efficiency. Electrowinning (used in the plating industry) is also not used for silver recovery from photographic solutions because it can decompose processing chemicals, resulting in fouled equipment and hazardous odors.

How can I reduce the amount of silver waste I have to manage?
One option to eliminate silver waste in the photographic process is to convert to electronic, or digital, technology. The SIUC Department of Cinema and Photography eliminated approximately 5% of their waste stream by using digital cameras, and the SIUC Office of Printing and Duplicating reduced approximately 25% of their waste. The SIUE University Graphics and Photographic Service also does some digital work while the SIUE Print Shop changed 100% to digital the summer of 1997.

Silver waste can also be reduced by using silver-free film, such as diazo, vesicular, photopolymer, electrostatic, and selenium-based.

Other recommendations made by the EPA include to:

• follow manufacturers’ recommendations for pH levels and stop bath use

• replenish the fixer strength by adding ammonium thiosulfate when necessary

• keep the fixer covered when not being used to prevent oxidation and minimize emissions

• use floating covers on solution tanks

• add ammonium thiosulfate to silver-contaminated baths to extend the allowable buildup of silver

• improve the efficiency of the wash bath by keeping the water at 80 0F, adding ammonia to keep the pH above 4.9, and adding washing aids (salts)

• recirculate the effluent from ion exchange units

• use countercurrent rinsing

• collect and reuse wash water

• use squeegees to minimize chemical carryover from stabilization

• collect and reuse cleaners

• control inventory to avoid disposal of off-specification film and chemicals

• store unused chemicals and paper away from heat and light.