Replenishing system
From Silvergrain Labs
Replenishment is a general strategy to make most processing capacity out a processing chemical, thus minimizing the volume of the chemical waste. Different strategy is used in film developer, stop bath, bleach, fixer, and perhaps other auxiliary processing baths.
The need to minimize the volume of waste solution has increased considerably due to rising concern for the environment and limited resource. One major regulatory step in this trend was London dumping convention (1990s) that prohibited dumping of waste solutions from commercial photographic processing to ocean.
In small scale darkrooms, replenishment method is rarely used, but results of highest quality can be readily achieved with the bleed replenishing method with suitably designed developers.
Contents |
Film developer
On average, each 80 square inch of pictorial b&w negative film will consume a small amount of developing agents, and release small amount of oxidized developing agent and about 40 to 60mg of bromide (potassium bromide equivalent) when developed to a normal contrast. In a general purpose film developer like Kodak D-76 and Ilford Ilfotec DD-X, the amount of developing agents and other agents are sufficiently large so that the small loss of developing agents can be easily supplemented to a good accuracy. However, the problem comes from the increasing amount of bromide present in the developer solution each time the solution is reused. Therefore, the main problem in replenishing film developers is to counteract the increasing bromide concentration. There are three major ways to deal with this problem.
Top-up replenishment
In top-up replenishment, the rate of replenishment is about 20ml per 80 square inch, which is about the same as the fluid loss due to carryover. In other words, each time film is developed, the fluid loss is topped up with the replenishing solution.
In the top-up method, the replenisher contains considerably larger amount of developing agents and alkaline agents compared to the base developer. Increase in the bromide concentration is thus compensated with increased amount of developing agent(s), higher pH, or usually both.
Loss of developer activity due to increased bromide concentration is compensated with increased developer concentration and increased pH.
Since one mechanism is compensated with another mechanism, maintenance of constant developer activity requires frequent quality control and adjustment. Yet, the image quality, such as granularity and shadow speed may fluctuate over time. Therefore, top-up developers are usually discarded after an amount of replenisher used reaches the volume of the original developer. (E.g., the bath is totally replaced when 1 liter of replenisher is used for each 1 liter of starting developer, allowing processing of 50 rolls of film per liter of replenisher.)
Because of the difficulty in maintaining constantly high image quality, this method is not recommended for modern use.
Bleed replenishment
In bleed replenishment, a portion of developer is discarded after each roll is developed, and the developer is replenished with a much larger rate than in top-up system. For example, for each film, about 50ml of developer may be discarded (20ml fluid loss and 30ml bleed) and 50ml of replenisher is added. The replenishing rate is 50ml per roll per liter of the developer.
In bleed replenishment, bromide released by the film is removed by bleeding. For example, the film developer contains 1.0g KBr per liter. Developing one film would make this to 1.05g KBr. Removing 50ml of this developer takes 53mg of KBr with it, and a replenisher containing no bromide is added to make one liter. Therefore, the replenished developer will contain 1.0g KBr per liter, the same as the initial developer. The amount of bromide in the bath reused many times is determined by an iterative calculation, but the degree of error can be maintained so small that no photographically significant change can be observed.
Because of this mechanism, the replenishing rate is directly related to the bromide concentration to be maintained in the developer. The higher the bromide concentration, the lower the replenishing rate. For this reason, replenishable developers usually contain 0.5 to 2.0g KBr per liter.
Loss of developer activity due to increased bromide concentration is compensated by removing the bromide, without changing the developer concentration or pH.
The composition of the replenisher is very similar (or identical) to the developer, except no bromide is included. In some cases, the replenisher may contain slightly more of the developing agents and the alkaline agents to make up for the loss of these agents during development. Well designed system can practically run forever, since the system compensates increased bromide with removal of bromide, maintaining steady state.
Using the figures from the above example, one liter of replenisher can develop 20 rolls of films in this system. This is less economical than the top-up method, but it is far easier to maintain excellent image quality and the chemical waste is far smaller than one-shot processing.
Rejuvenation method
Rejuvenation method is conceptually very similar to the bleed method. Using the numbers used to illustrate the bleed method above, after each roll is processed, 50ml of the developer is taken from the tank and passed through an ion exchange resin to remove bromide ion. This rejuvenated developer is added back to the original developer tank, thus maintaining the constant bromide concentration.
Bromide released from development is removed by ion exchange resin rather than bleeding, minimizing the amount of chemical waste.
Of course, there is some loss of developing agents and alkaline agents as many runs are made. This loss is easily compensated by adding a small amount of replenisher, which is slightly more concentrated, as in the bleed system. It is sometimes necessary to remove a part of the developer to accomodate the replenisher solution. However, in some modern minilab machines, the replenisher is administered as solid tablet directly into the tank, to further minimize the handling of solution.
Rejuvenation method is a very clever system that maximizes efficiency with which the processing chemical is used, while maintaining excellent image quality. However, the processing system needs extra hardware to feed developer through the ion exchange resin. This system is most suited for minilab machines and commercial photofinishers.
Print developer
On average, each 80 square inch of b&w enlarging paper will consume a small amount of developing agents, and release small amount of oxidized developing agent and about 10 to 30mg of bromide (potassium bromide equivalent) when developed to a normal contrast. In a general purpose print developer like Kodak D-72 and Ilford ID-62, the amount of developing agents and other agents are sufficiently large so that the small loss of developing agents can be easily supplemented to a good accuracy. As in the case of the film developer, bromide released from the print is one factor affecting the reused print developer. However, paper emulsions vary in terms of bromide content (some chlorobromide emulsions contain very small amount of bromide, while other enlarging papers are bromide emulsion). Also, paper emulsions vary in terms of the kind and amount of antifoggants added to them. Furthermore, the paper emulsion may take up antifoggants added to the developer bath.
In the case of print developer, important factors in image quality is development rate and image hue. The image hue is influenced by many factors, such as the antifoggant concentration.
Since print developers usually contain 1g to 4g of KBr per liter, and 80 square inches of paper may release only 20mg of bromide, a small replenishing rate is sufficient to maintain steady-state level of bromide. For example, if the print developer contains 1g/L of KBr, replenishing at the rate of 20ml per 80 square inch would keep the bromide level constant. Therefore, distinction between top-up and bleed system is usually unnecessary in replenishing print developer, as long as a matching replenisher containing no bromide is used.
It is obvious from the above that chloride emulsion containing maximum of a few percents of bromide is useful in minimizing replenishing rate. For this reason, modern color printing paper is 95+% silver chloride, with very small amount of iodide, and sometimes a few percents of bromide. This way, color developer can be reused with replenishment for a long time. (Another advantage of chloride emulsion is very rapid development, and fixation.)
On the other hand, b&w bromide printing emulsions are the toughest paper to the printing developer. Still, the required replenishing rate is within well manageable range for manual processing in ordinary darkrooms.
Stop bath
Stop bath is usually an acid bath, which exhausts due to carryover of developer solution. The exhaustion comes from the rising pH, which makes the bath too slow to arrest development reaction.
In case of buffered acid stop bath, increase in the alkaline agents carried from the developer can be compensated with an addition of a small amount of pure acid. This can prolong the working capacity of the stop bath, which is already very large in case of buffered stop bath.
Over heavy reusing of replenished stop bath, the bath accumulates oxidized developing agents, which may cause staining problems. Therefore, replenished stop bath is usually discarded when the solution is considerably darkened by the developer carryover.
In automatic processing machines, stop bath is often replaced with mechanical squeegees.
Bleach
Modern bleaching agents used in color processing is based on Fe(III) complex of polyamine polycarboxylic acids, such as PDTA. These oxidizing agents are reversible, and they can be rejuvenated by aeration. Sometimes, hydrogen peroxide may be used to rejuvenate the bleach bath.
Since Fe(III)-PDTA and similar bleaching agents do not decompose thiosulfate, bleach and fix are combined in some color processing. By combining them, the bleaching power is weakened, so blix is more often used for chloride printing paper, where the processing load is the lightest. Blix may be used for film processing but the processing time may be long, or the capacity may suffer.
Fixer
The major cause of fixer exhaustion is accumulation of silver-thiosulfate complex in the solution. There is no easy and effective way to solve this by replenishing method alone. It is best to use fixer to exhaustion and properly dispose of the waste solution.
In manual processing, another cause of fixer exhaustion is dilution due to carryover of stop bath (or rinse water).
In commercial processing, silver is removed from the fixing bath by electrolysis or other methods. The rejuvenated fixer is fed back to the fixing tank. A small amount of concentrated fixer is occasionally added to make up for the lost thiosulfate during fixation.
Washing water
Until Konica developed wash-less color minilab machines, films and prints had to be washed in water before dried. This would require plumbing of fresh water and waste water to the machine, which limited the location in which such a machine could be installed. In modern minilab machine, washing water is internally recycled by removing thiosulfate and other agents by reverse osmosis membrane and/or ion exchange resin.
Such an internally circulating water is highly susceptible to bacterial and fungal growth. Therefore, one or more biocides agents are added to the water. Calcium and magnesium are often removed from the water used for this type of washing system, as their presence can promote bacterial growth.
