Modern printing is best described as a photomechanical process where text and images are manipulated in such a manner as to allow for a variety of enhancements, color separations, and/or positioning so that a plate can be made that allows for the physical transfer of text and images to a substrate. Traditional text and image preparation operations follow a specific sequence. Most printed products begin with art and copy (or text) preparation. Once the material is properly arranged, it is either photographed or directly imaged onto black and white film, which is subsequently developed to produce transparencies. These transparencies are then used to transfer the image or text to the plate. There are several different techniques used to make the printing plates.
Since printing cannot duplicate a continuous tone image as seen in a color photograph, then color separations are made to provide a single-color image or record which can be used to produce the single-color printing plate. A single plate is required for each color and in “full color” printing, a minimum of four plates (i.e., red, yellow, blue, and black) are necessary. Note that with the advances in technology, digital prepress is replacing steps in this process, including digital artwork and plates that eliminate the use of film.
The printing industry employs graphic arts photography in the reproduction of artwork and copy, using materials similar to those in other fields of photography. Waste associated with this part of the prepress operation include developed film, developer, fixer, solvents, plate coatings and developers, out-dated materials, waste fixer containing silver compounds and rinse water. Therefore, a pollution prevention program should focus on photographic chemistry management. Other options include proper material handling and storage, material substitutions, recovery of silver; wastewater reduction or even process changes to electronic technology.
Oxidation and reduction reactions are used to develop an exposed photographic emulsion. The exposed film or plate is first immersed in a developer. This converts the silver halide in the photographic emulsion to metallic silver, in proportion to the amount of exposure it has received.
In general, the developer solution contains an accelerator, preservative and restrainer. The accelerator is an alkaline material, such as sodium hydroxide, sodium carbonate or sodium tetraborate (borax), which increases the activity of the developer by neutralizing the acid formed during the development process.
The developing action is stopped by immersing the film in a fixing bath of acetic acid and sodium thiosulfate (hypo), ammonium thiosulfate, or sodium hyposulfite. The fixer has a low pH and the change in pH will immediately stop the development process.
Each time photographic film or paper is immersed in the fixing bath; a small amount of silver enters the bath from the photographic emulsion. Insoluble silver compounds begin to form after the silver concentration reaches a certain level in the fixer bath, and cannot be removed. The fixer solution must be replaced once this threshold is met.
Once the film is removed from the fixer solution, the residual fix must be removed. This is accomplished by using wash water, which is usually simple tap water. The wash water can become contaminated with some of the fix and a small amount of silver.
Best Management Practices & Pollution Prevention
The following is a list of options to consider reducing pollution from image making operations:
- Implement operational and work practice changes that can extend the life of chemical baths, reduce the amount of chemicals used and reduce waste water generation.
- Neutralize acidic or alkaline waste film processing solutions on site in to make them acceptable to discharge into the sanitary sewer. Simple neutralization treatment activities are exempt for the U.S. EPA RCRA on-site waste treatment regulations.
(This practice actually results in more silver being released to the environment.)
- Employ countercurrent (using water from previous rinsings in initial film washing stage) rather than parallel rinse techniques. This can reduce the amount of wastewater generated. In a parallel system, fresh water enters each wash tank and effluent leaves each wash tank. In countercurrent rinsing, the water from previous rinsings is used in the initial film washing stage. Fresh water enters the process only at the final rinse stage, at which point much of the contamination has already been rinsed off the film. A countercurrent system requires more space and equipment.
- Containerize process baths and keep covered/sealed to keep them from spoiling.
- Use a squeegee in non-automated processing systems to wipe excess liquid from the film and paper. This can reduce chemical carryover from one process bath to the next. Minimizing chemical contamination of process baths increases recyclability, enhances the lifetime of the process baths and reduces the amount of replenisher chemicals required. A squeegee should be used after the film image is hardened.
- Replace repetitive steps of photographing, editing, re-shooting and the photodeveloping process with electronic imaging (including the capability to edit images on a computer). By using computers to generate graphics and negatives, printers can skip the photographic developing stage of the process, thereby eliminating the use of darkroom chemicals.
- Develop inventory control programs that offer the advantage of reducing spoilage of photodeveloping chemicals and supplies, such as paper and film.
- Solutions should be made up only in quantities to meet realistic processing volumes.
- Floating lids should be used on developer solution tanks to prevent evaporation and loss of potency.
- Common sense safeguards, such as keeping the mixing area clean, avoiding mixing of dry chemicals where airborne particles can cause contamination of other solutions, and use of separate mixing tanks for developers will minimize contamination or errors in mixing.
- Bleach, bleach-fix, fix, developer, and wash water can be recycled and re-used.
- Non-hazardous chemicals and films can be substituted for hazardous ones.
- Photopolymer films contain carbon black as a substitute for silver. These films are processed in a weak alkaline solution that is neutralized prior to disposal. As such, they produce a non-hazardous waste.
There are several methods of silver recovery:
Metallic replacement or the use of chemical recovery cartridges (CRCs) with manufacturer-specified flow control is commonly used by small operations that need to pretreat fixer and wash water prior to discharge.
A terminal electrolytic unit followed by a chemical recovery cartridge with manufacturer-specified flow control.
For access to vendors who may supply alternative materials and equipment, see the PNEAC Vendor Directory.
The Clean Water Act strictly prohibits discharges of silver into publicly owned treatment works (POTWs) above certain levels. Generally all POTWs have established sewer codes, which specify acceptable levels of discharges including silver. Other pollutants of concern from film processors include pH, total suspended solids, and Biological Oxygen Demand (BOD) and/or Chemical Oxygen Demand (COD). All printers discharging to a POTW must obtain a copy of the local sewer code and meet its requirements for discharge limits, permits, and reporting.
In order to meet the local sewer discharge limits for silver, recovering silver from fixing baths is required and can be a money saving procedure. Many companies have found on-site recovery of spent fixer to be cost-effective, compared with the cost of disposal of the waste.
The type of silver recovery depends on the size of the facility, the type of film used, and the amount of silver-rich solution generated (volume of film processed) each day. The specific type of silver recovery will vary depending on the required silver recovery that is to be achieved.
Recently, a new concept in addressing silver discharge limits has been developed and implemented in many cities throughout the United States. This approach is called the Code of Management Practice, which eliminates numeric discharge limits and replaces them with performance standards. Additional information on the Code of Management Practice can be found at www.silvercouncil.org The performance standards are as follows:
Required Silver Recovery and Compliance Options, Based on Size of Imaging Facility.
|SIZE OF FACILITY
||SILVER-RICH SOLUTION GENERATED(gallons per day)
||REQUIRED SILVER RECOVERY
||Less than 2
||One or two CRCs with flow control ; or terminal electrolytic unit followed by a CRC with flow control; or off-site management; or alternative technology to achieve 90 percent.
||More than 2, but less than 20
||Terminal electrolytic unit followed by a CRC with flow control; or in-line electrolytic unit with a CRC; or two or more CRCs with flow control; or off-site management; or alternate technology to achieve 95 percent.
||More than 20.
||Terminal electrolytic unit followed by a CRC with flow control; or in-line electrolytic unit with a CRC; or off-site management; or alternative technology to achieve 99 percent.
Health & Safety
While film processing chemistry is not necessarily toxic, care needs to be taken in handling it due to the low and high pHs present in fix and developer respectively. As a minimum, the appropriate personnel protective equipment (e.g., gloves, goggles, splash apron, etc.) should be used during the mixing, filling, and removal of chemistry from the processor.
The act of neutralizing acidic or caustic processing bath can result in an exothermic chemical reaction (gives off heat). Proper precautions, including wearing personal protective equipment, should be taken if solutions are treated on site.