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.
Depending upon the contaminant level, little makeup chemical is required. The presence of highly abrasive solids requires special pump configurations. No other special system requirements are necessary.
The solids removed from the system settle in a small system or, in larger applications, may be dewatered in a filter press. In most cases, the sludge is non-hazardous, especially when heavy metals are not present.
Rinse water quality improvements
Maintaining the wash water in a cleaner condition improves the quality of rinse water. Reduced carryover of oil, grease, and soil reduces the rinse water requirement.
The cost of rinsing parts cleaned in aqueous solutions is being scrutinized. Already, many users chose to keep the wash solution clean to reduce rinse water volumes and, therefore, the cost of purchasing, treating, and sewering.
In certain cases, however, the rinse water may still contain unacceptably high levels of metals. Treatment for removal of these may be required. In other cases, especially where steel parts are being washed, the increase in cleanliness may eliminate the need for further waste treatment before discharge.
Starting with high-quality water minimizes treatment costs. Parts washed before powder coating or other final preparations can be rinsed with untreated city water containing 200 milligrams per liter or more of dissolved solids. Normal evaporative losses increase the total dissolved solids content. However, using water with a low level of total dissolved solids in the first place improves both the quality and duration of rinses. The subsequent loading on the recovery system is correspondingly lower. It is relatively inexpensive to treat the smaller amount of makeup water (usually 10 to 20 percent of the total volume) for removal of total dissolved solids. A combination of softening, reverse osmosis, and demineralization yields the highest possible water quality.
Companies that use large quantities of water for rinsing parts for electrocoating, painting, or powder coating looking hard at the possibilities of reuse. When oil, grease, and particulate drag in is under control, then the rinse water recovery begins to make economic sense.
Ion exchange resins play a major part in water recovery for rinse applications. Gone are the days of resins used strictly for deionization, demineralization, and water softening. Resins can be used for the selective removal of metals or neutralization of acids and bases in the presence of organics such as surfactants.
Plating applications, and even more particularly in the circuit board industry, use huge amounts of high-quality rinse water. In the past, these rinse streams contained low levels of contaminants and therefore had to be treated prior to sewer discharge. From a recovery viewpoint though, these are lightly contaminated streams that offer a high recovery potential. Many such plants now use recycled water polished to high standards through a combinations of innovative technologies. Reverse osmosis together with activated carbon and resin technology remove metals, organics, and other dissolved solids. For relatively low operating costs, high recovery rates are possible with perhaps as little as 5 to 15 percent of the original volume being discharged to sewer.
Other recovery applications
There are other less common recovery applications worthy of mention. For instance, one company now recovers pickling acid and copper from a copper etching process using membranes developed for low pH applications. First, a nanofiltration membrane system removes and concentrates copper sulfate. The stream of weak acid goes to a reverse osmosis membrane system that removes and concentrates the acid for further use. The permeate (or filtrate) from the reverse osmosis membrane returns to the process for reuse.
This company no longer precipitates, coagulates, flocculates, or sends copper to the sewer.
Another application that shows promising use of contemporary technologies recovers emulsion paint washdown for reuse in a paint manufacturing operation. Conventional physical and chemical treatment used previously generated a paint sludge that could only be disposed of through conventional means. Now, an ultrafiltration membrane system concentrates the paint solids as the basis for a low cost paint product. White paint forms a large part of the total production and is segregated from colored. More paint solids are added to the concentrate from the ultrafiltration system to produce a new paint product. The water from the process is piped to a sewer without further treatment since the paint solids have been removed and only some residual surfactants remain in the water.
New processes are continuously under development in a pilot system and in benchtop work. The aim of the bulk of this work is the recovery of fluids, to mitigate the continual rise of water and sewer costs. However, in areas in which water and sewer render recovery unviable, conventional treatment operations continue to make sense. Where, for instance, sewer standards are still at 1980s levels and water and disposal costs are reasonable, there is no reason to use creative technologies. As these costs rise and either local or Federal standards require better technologies, plant and environmental managers will seek options not only for existing waste streams, but for reuse of treatment fluids and rinse water. These technologies reduce the costs of day-to-day operations and long-term plant liability by minimizing disposal from plant operations.