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Photographic Processing Waste Management
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Overview Photoprocessing wastes may contain silver which is considered a toxic heavy metal by the Environmental Protection Agency. The silver is primarily present as soluble silver thiosulfate complex. Silver sulfide is present in smaller amounts. Depending on the stage from which the waste originates and the type of film processed, the silver concentration can range between 5 mg/L and 12,000 mg/L. Therefore photoprocessing solutions and spent rinse waters are classified as hazardous wastes. In addition to photoprocessing solutions and spent rinse waters, films and negatives may contain high silver concentrations and require management as hazardous wastes. It is illegal to dispose of hazardous wastes via drains, normal trash, or any other means which would result in a release to the environment or discharge to the city sewer system. Photoprocessing solutions and spent rinse waters must either be collected and sent to Environmental Health and Safety (EHS) for disposal or processed to remove the silver before drain disposal. All films and negatives must be collected and sent to EHS.
Applicability The Photographic Silver Waste Management procedure applies to Weill Cornell Medical College (WCMC) students, faculty, and staff using photoprocessing chemicals, photographic film and other silver-containing film (e.g. x-rays). Procedure Departments will identify those areas and persons within their facilities which generate hazardous silver-bearing photographic wastes. Identified generators must ensure that these hazardous wastes are managed via one of the two procedures listed as follows:
Disposal via the College’s Waste Disposal Procedures If the Generator chooses not to pursue the use of silver recovery equipment for the management of photoprocessing solutions and spent rinse waters, the Generator must ensure that this waste is containerized, handled, stored, and otherwise managed in accordance with the College’s Waste Disposal Procedures. Once the waste containers are full, the generator must submit a Request for Chemical Recycling / Disposal form to Environmental Health and Safety (EHS). EHS will collect the waste for off-site treatment and disposal.
All films
and x-rays must be containerized, handled, stored, and otherwise
managed in accordance with the College’s Waste Disposal Procedures. Silver Recovery Processing Depending on the quantity of waste generated, silver recovery processing may prove financially beneficial to a generator. Generators choosing to recover silver must procure the silver recovery equipment and associated services. However, EHS should be contacted to assist in the selection of the silver recovery equipment which matches the needs for the location being served. Use the following discussion as well as Tables 1 and 2 to help choose the best silver recovery method for your circumstances. Greymart Environmental Services (http://www.greymart.com) is a New York City-based company which is capable of providing a variety of silver recovery systems, installation, and services to match specific needs. However, other companies which provide similar equipment and services are available. 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. Table 1. Silver Recovery Methods
ABased on data from 1998
Table 2. Comparison of Silver Recovery Methods
Generator(s) A person or group at WCMC which produces hazardous chemical waste, including photoprocessing equipment operators their supervisors and those disposing of photoprocessing film.
Responsibilities Environmental Health and Safety (EHS) will ensure proper guidance for the management of hazardous silver-bearing photographic wastes which complies with local, state, and federal laws and regulations has been generated and disseminated. EHS will assist Generators and Departments in the selection of proper silver recovery equipment. EHS will ensure the proper disposal of Generator’s containerized hazardous wastes. Departments are responsible for identifying those areas where film processing is conducted and ensuring that all hazardous silver-bearing photographic waste is managed appropriately. Generator(s) shall be responsible for managing all wastes in accordance with the College’s Waste Disposal Procedures. If silver recovery equipment is utilized, then the Generator(s) shall ensure equipment is properly maintained in accordance with any equipment design specifications or other responsibilities as defined within a service agreement established between the Generator and vendor.
References 6 NYCRR Parts 370 through 374 and 376 – hazardous waste disposal 6 NYCRR 371.1(c)(7) – prior notification for scrap metal exemption NYC Department of Environmental Protection Sewer Discharge Regulations Susan M. Morgan, Erik A. Talley, Mohammed Z. Rahman and Keith E. Morgan; “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. Additional Information
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