OVERVIEW OF ENVIRONMENTAL ISSUES
Daniel R. English
Major Environmental Issues
Organic solvents are used for a number of applications in the printing industry. Major uses include cleaning solvents for presses and other equipment, solubilizing agents in inks, and additives in fountain solutions. In addition to general concerns about volatile organic compound (VOC) emissions, some potential solvent components may be persistent in the environment or have high ozone-depleting potential.
During black-and-white and colour photographic processing, silver is released into some of the processing solutions. It is important to understand the environmental toxicology of silver so that these solutions can be properly handled and disposed of. While free silver ion is highly toxic to aquatic life, its toxicity is much lower in a complexed form as in photoprocessing effluent. Silver chloride, silver thiosulphate and silver sulphide, which are forms of silver commonly observed in photoprocessing, are over four orders of magnitude less toxic than silver nitrate. Silver has a high affinity for organic material, mud, clay and other matter found in natural environments, and this lessens its potential impact in aquatic systems. Given the extremely low level of free silver ion found in photoprocessing effluents or in natural waters, control technology appropriate to complexed silver is sufficiently protective of the environment.
Other photoprocessing effluent characteristics
The composition of photographic effluent varies, depending on the processes being run: black-and-white, colour reversal, colour negative/positive or some combination of these. Water comprises 90 to 99% of the effluent volume, with the majority of the remainder being inorganic salts that function as buffers and fixing (silver halide-solubilizing) agents, iron chelates, such as FeEthylene diamine tetra-acetic acid, and organic molecules that serve as developing agents and anti-oxidants. Iron and silver are the significant metals present.
Every component of the printing, photography and reproduction industries generates solid waste. This can consist of packaging waste such as cardboard and plastics, consumables such as toner cartridges or waste material from operations such as scrap paper or film. Increasing pressure on industrial generators of solid waste has led businesses to examine carefully options to lower solid waste through reduction, reuse or recycling.
Equipment plays an obvious role in determining the environmental impact of the processes used in the printing, photography and reproduction industries. Beyond this, scrutiny is increasing on other aspects of equipment. One example is energy efficiency, which relates back to the environmental impact of the energy generation. Another example is “takeback legislation”, which requires the manufacturers to receive equipment back for proper disposal after its useful commercial life.
The effectiveness of a given control methodology can be quite dependent on the specific operating processes of a facility, the size of that facility and the necessary level of control.
Solvent control technologies
Solvent use can be reduced in several ways. More volatile components, such as isopropyl alcohol, can be replaced with compounds having lower vapour pressure. In some situations, solvent-based inks and washes can be replaced with water-based materials. Many printing applications need improvements in water-based options to compete effectively with solvent-based materials. High-solids ink technology can also result in reduction of organic solvent use.
Solvent emissions can be lowered by reducing the temperature of dampening or fountain solutions. In limited applications, solvents can be captured on adsorptive materials such as activated carbon, and reused. In other instances, windows of operation are too strict to allow captured solvents to be reused directly, but they may be recaptured for recycling offsite. Solvent emissions may be concentrated in condenser systems. These systems consist of heat exchangers followed by a filter or electrostatic precipitator. The condensate passes through an oil-water separator before ultimate disposal.
In larger operations, incinerators (sometimes called afterburners) can be used to destroy emitted solvents. Platinum or other precious metal materials may be used to catalyze the thermal process. Non-catalyzed systems must operate at higher temperatures but are not sensitive to processes that can poison catalysts. Heat recovery is generally necessary to make non-catalyzed systems cost effective.
Silver recovery technologies
The level of silver recovery from photoeffluent is controlled by the economics of recovery and/or by solution discharge regulations. Major silver recovery techniques include electrolysis, precipitation, metallic replacement and ion exchange.
In electrolytic recovery, current is passed through the silver-bearing solution and silver metal is plated on the cathode, usually a stainless steel plate. The silver flake is harvested by flexing, chipping or scraping and sent to a refiner for reuse. Attempting to lower the residual solution silver level significantly below 200 mg/l is inefficient and can result in formation of undesired silver sulphide or noxious sulphurous byproducts. Packed-bed cells are capable of reducing silver to lower levels but are more complex and expensive than cells with two-dimensional electrodes.
Silver may be recovered from solution by precipitation with some material that forms an insoluble silver salt. The most common precipitating agents are trisodium trimercaptotriazine (TMT) and various sulphide salts. If a sulphide salt is used, care must be taken to avoid generation of highly toxic hydrogen sulphide. TMT is an inherently safer alternative recently introduced to the photoprocessing industry. Precipitation has a recovery efficiency of greater than 99%.
Metallic replacement cartridges (MRCs) allow the flow of the silver-bearing solution over a filamentous deposit of iron metal. Silver ion is reduced to silver metal as iron is oxidized to ionic soluble species. The metallic silver sludge settles to the bottom of the cartridge. MRCs are not appropriate in areas where iron in the effluent is a concern. This method has a recovery efficiency of greater than 95%.
In ion exchange, anionic silver thiosulphate complexes exchange with other anions on a resin bed. When the capacity of the resin bed is exhausted, additional capacity is regenerated by stripping the silver with a concentrated thiosulphate solution or converting the silver to silver sulphide under acidic conditions. Under well-controlled conditions, this technique can lower silver below 1 mg/l. However, ion-exchange can be used only on solutions dilute in silver and thiosulphate. The column is extremely sensitive to stripping if the thiosulphate concentration of the influent is too high. Also, the technique is very labour- and equipment-intensive, making it expensive in practice.
Other photoeffluent control technologies
The most cost-efficient method to handle photographic effluent is via biological treatment at a secondary waste treatment plant (often referred to as a publicly owned treatment works, or POTW). Several constituents or parameters of photographic effluent may be regulated by sewer discharge permits. In addition to silver, other common regulated parameters include pH, chemical oxygen demand, biological oxygen demand and total dissolved solids. Multiple studies have demonstrated that photoprocessing wastes (including the small amount of silver remaining after reasonable silver recovery) following biological treatment are not expected to have an adverse effect on the receiving waters.
Other technologies have been applied to photoprocessing wastes. Haul-away for treatment in incinerators, cement kilns or other ultimate disposal is practised in some regions of the world. Some laboratories reduce the volume of solution to be hauled away through evaporation or distillation. Other oxidative techniques such as ozonation, electrolysis, chemical oxidation and wet air oxidation have been applied to photoprocessing effluents.
Another major source of reduced environmental burden is through source reduction. The level of silver coated per square metre in sensitized goods is steadily decreasing as new generations of products enter the marketplace. As the silver levels in media decrease, the amount of chemicals necessary to process a given area of film or paper has also decreased. Regeneration and reuse of solution overflows have also resulted in less environmental burden per image. For example, the amount of colour developing agent required to process a square metre of colour paper in 1996 is less than 20% of that required in 1980.
The desire to minimize solid waste is encouraging efforts to recycle and reuse materials rather than dispose of them in landfills. Recycling programmes exist for toner cartridges, film cassettes, single-use cameras and so on. Recycling and reuse of packaging is becoming more prevalent as well. More packaging and equipment parts are being labelled appropriately to allow more efficient material recycling programmes.
Life cycle analysis design for the environment
All of the issues discussed above have resulted in increasing consideration of the entire life cycle of a product, from procuring of natural resources to creating the products, to dealing with end-of-life issues for these products. Two related analytic tools, life cycle analysis and design for the environment, are being used to incorporate environmental issues into the decision-making process in product design, development and sales. Life cycle analysis takes into consideration all of the inputs and material flows for a product or process and attempts to quantitatively measure the impact on the environment of different options. Design for the environment brings into consideration various aspects of product design such as recyclability, reworkability and so on to minimize the impact on the environment of the production or disposal of the equipment in question.