Dilute Solutions
The discharges from continuous manufacturing processes are normally dilute solutions of compatible and sometimes nonconventional pollutants. They may be discharged to the industry’s pretreatment system or directly to the POTW without any pretreatment. Manufacturing processes such as plating bath rinses, raw food cleaning, and crude oil dewatering are all examples of dilute solutions of pollutants that may be discharged directly to a POTW sanitary sewer. If a problem occurs in the manufacturing process, a probable result is that the quality of wastewater will change; it may be more laden with pollutants. Some wastestreams from utility services, such as cooling tower and boiler blowdown, are continuous and represent the discharge of dilute solutions.
Another low‐strength wastewater is storm water runoff from chemical handling and storage areas. Products which may have spilled on the industry’s grounds are washed off during a rainstorm or during the spring thaw. The pollutant concentration is usually too dilute to require pretreatment before discharge to the sewer, but it exceeds the discharge standards for discharge to surface waters. While the strength of the storm runoff may be low, the volume that must be treated in addition to normal flow to the pretreatment system or to the POTW can cause hydraulic capacity problems. Excessive flows can be diverted to storage reservoirs or basins and then gradually discharged to the pretreatment system. A great deal of attention is presently focused on cleaning up groundwater sources that have been contaminated by leaking underground storage tanks. Cleanup projects of this nature typically involve large quantities of wastes that may contain high concentrations of solvents, fuels, heavy metals, and pesticides. Because of the public attention surrounding groundwater cleanup projects, pretreatment of the contaminated water is almost always required and the result is usually a “high‐ quality” industrial wastewater.
Concentrated Solutions
Typically, concentrated solutions are batch‐generated and the frequency of generation is usually not daily but weekly, monthly, annually, or even longer. These solutions are process chemicals or products that cannot be reconditioned or reused in the same manufacturing process. Concentrated solutions such as spent plating baths, acids, alkalies, static drag out solutions, and reject product may have concentrations of pollutants hundreds or thousands of times higher than the discharge limits of the POTW or higher than can be adequately treated by the pretreatment system if discharged all at once. Time has to be taken to examine and understand each manufacturing process, then identify these concentrated solutions and take the necessary steps to prevent damage to the treatment facilities.
Some wastes may be considered concentrated by the POTW but not by the industry. For example, the 10% sulfuric acid solution used for pickling parts is considered “dilute” by comparison to the 98% or 505 stock solution that the industry uses to make up the pickling solution. When this solution is spent or can no longer be used as a pickling solution, proper treatment and disposal are required. From the industrial manufacturer’s point of view, the solution is spent and no longer concentrated. However, from a wastewater treatment point of view, the solution is concentrated since it contains high concentrations of acid (pH < 1.0) and heavy metals (1000 mg/l) compared to the normal pH of 1.0–4.0 and heavy metal concentrations of less than 100 mg/l (IWT 1999). Another source of concentrated solutions is the wastewater from equipment cleanup. While the amount of material in the process chemical bath may be considered dilute by industry standards, it forms a concentrated wastestream when discharged during the cleanup of manufacturing equipment. Cleanup wastestreams contain a high concentration of the product during the first washing of the tank, pipe, or pump. This discharge of concentrated waste is followed by successive rinses which contain less and less pollutants. If cleanup flow concentrations are not equalized, the cleanup cycle can cause problems in the industrial waste treatment system (IWTS). Spills of process chemicals to the floor, if not contained, can flow directly to the floor drain and the pretreatment or sewer system. The adverse effects on the pretreatment system and POTW are the same as those of any other concentrated solutions. This is why chemical containment areas must not have drains.
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