Anyone keeping abreast of industrial waste water treatment has noticed significant developments. The first is regulatory driven, as the business always has been. The tightening of discharge requirements, either from pressure by state and Federal regulators or by the introduction of new Federal <I>categorical<I> standards, may mean that conventional treatment no longer meets discharge requirements. Aggressive advances in technology and the innovative use of existing technologies provide treatment that meets the more stringent standards.
The other significant development now being felt by more industries is the increasing costs of purchasing water and, more significantly, of discharging to sewers. These combined costs are currently $5 to as much as $10 per 1,000 gallons. This increasing cost encourages the reuse of water and, where possible, other reductions in the consumption of water. Doing both is feasible in many cases, especially if the water is only lightly contaminated and if, as perhaps in the past, the water was sent to a sewer without treatment. New sewer restrictions require water discharged to a sewer to first pass through a treatment process. In certain areas like Washington, DC, larger users of city water and sewer discharge face a sliding scale whereby greater usage generates higher rates, not lower as would be the case with normal market forces. This is an obvious effort on the part of municipalities to reduce loadings on overburdened treatment systems by making water reuse and reduced consumption an attractive alternative. This trend is likely to become stronger as water and sewer plants reach capacity.
In the metalworking industries, there have been several important advances. For one, the increasing use of synthetic machining coolants allows for recovery on a more or less continuous basis. Because coolant chemicals are water solubile, fine filtration can remove contaminants that previously could not have been removed from hydrocarbon-based coolants that form an oil/water emulsion.
Using microfiltration or ultrafiltration to remove contaminants like tramp oil strips most of the coolant from the water. The droplets of tramp oil and coolant are so similar in size that a membrane cannot discriminate between them. The only answer is to remove as much of the <I>filterable<I> solids as possible. When the coolant becomes rancid or otherwise unusable, there is no practical alternative but either sending the coolant to a contractor or removing the oil on site and sending the extracted water to a sewer.
Even this becomes a difficult proposition at times, since the metal content often means that water must be run through several processes before discharge. Microfiltration, or more commonly ultrafiltration, removes all tramp oil, all solids, and a high percentage of bacteria in synthetic coolants. This permits almost continuous reuse of the coolant. The small amounts of contaminant that remain after treatment normally can be disposed of to a waste oil recovery company.
Aqueous cleaning solutions
The move from solvent to aqueous cleaning of metal parts caused a change in the thinking of production and environmental managers. In many cases, the change is easy from a production point of view since the degree of cleanliness derived from aqueous cleaners matches that of solvent cleaning. Solvents are recovered easily through a simple distillation process. The residue is small both in terms of the initial volume of solvent used and the number of parts cleaned.
In many aqueous cleaning applications, the volume of waste is significantly larger than that generated by solvent cleaning. The aqueous waste may not be hazardous like a typical solvent residue, but disposal volumes significantly increase total disposal costs.
The normal way to get good cleaning performance from a wash solution is to regularly dispose of it. To avoid enduring this expense, some plants increase the concentration of chemicals and the operating temperature of the wash tank in an effort to make the solution last longer. The objective is to generate less liquid for disposal. This is, at best, a compromise. The practice increases reject rates and lengthens the wash cycles, both factors that increase production costs.
Membrane technology--in the form of ultrafiltration or microfiltration--removes the oil and soil from these solutions on a continuous basis to allow returning the solution for reuse. This process is straightforward. In its simplest form, it consists of a process tank and a small membrane system that operates continuously in a dialysis mode. It removes the contaminants and returns clean solution to the wash tank.
Waste material accumulates in the process tank. A wide-channel membrane configuration easily achieves an oil concentration of between 12 and 20 percent. In some cases it produces concentrations as high as 40 percent. Periodically the concentrated contaminants must be drained from the tank for disposal.
A cleaning solution is normally discarded when the concentration of oil reaches 0.5 percent. A membrane filtration that achieves an oil concentration of 15 percent yields a 30:1 reduction in waste volume. It also reduces disposal costs by this same ratio!
In addition, it maintains the cleaning solution in clean condition. This ensures quality parts with fewer rejects and often shorter cycle times at lower temperatures.
Mass finishing relies on an abrasive media in a water-based compound to smooth and remove sharp edges from parts after machining, stamping, or some other mechanical operation. The solution is usually a detergent that keeps solids in suspension. This fluid also may contain rust preventive.
Membrane technology also removes solids and oil from mass finishing fluids. Wide channel ultrafiltration removes the abrasive solids and returns the fluid for reuse.