THE TRADITIONAL TECHNICAL PROCESS THE OTHERS USE
Industrial wastewater contains a mixture of suspended of colloidal solids, dissolved metals and ions, and organic contaminants. The art of treating this type of wastewater is based on the concept of precipitating the solids out into an aggregated mass, that either floats on the surface to be skimmed off or sinks to the bottom to be removed by filtration or decantation. This process is called flocculation and the solids that are formed are called a floc (short for floccule).
The mechanism of flocculation is rather complex, but it involves the presence of extremely tiny, less than 1-micron, colloidal particles and dissolved ions.
All particles exert positive or negative forces, and both attract and repel each other. For colloidal particles, the attractive forces are much weaker than the repulsive forces; that fact, coupled with their extremely small size, allows them to remain suspended in solution. However, under the right conditions, the attractive forces can be strengthened, and the repulsive forces shielded so that a floc can be formed.
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The traditional approach to treating industrial wastewater containing particulates, oils, and dissolved metals, involves flocculating these contaminants and successfully adjusting the wastewater conditions to the point where each particular class of contaminant will become insoluble and agglomerate into a mass that can be removed from the much larger volume of purified water.
The first step usually consists of adjusting the wastewater to acidic conditions to break any oil emulsions that may exist. Most of the oil will float up to the top of the tank where it can be removed by skimming.
A cationic polymer or flocculent is typically added at this point to attract negatively charged particles in the water, as well as any remaining oil. When the polymer performs properly, it flocculates into a mass that can be removed from the water by decantation and filtration.
The water must then be tested to make sure that enough oil has been removed to achieve the appropriate standards. If it does not pass this test, the procedure is repeated until it does. This process is often time consuming as a certain amount of trial and error is frequently required to find the right polymer and conditions for generating a floc that is dense enough to separate the oil.
Often the polymer/oil interaction is not particularly strong and since oil and water don’t mix (like materials that readily solubilize each other) the oil tends to associate with the polymer chains because they are more similar to the oil than the water is.
Since the polymer itself is polar enough to be soluble in water, it becomes only slightly more desirable as a partner for the oil than the water. If enough water washes through the polymer/oil floc, the two will separate. Such a floc is unlikely to pass the leachability tests for hazardous materials.
Once the water has passed the tests for residual organics, it is taken to another holding tank and made basic to precipitate any dissolved heavy metals. This step is also time consuming and uses a lot of base material. Anionic polymers are now added to collect the metals and flocculate them into a separable solid. The polymers available for this task are usually quite good, and for heavy metals under the proper circumstances, claims of non-leachability are probably valid.
Again, the time and monetary investment are considerable. Finally, the water is separated and tested to ensure that the metal species are eliminated. If this test is passed, the water is neutralized and released.
Unfortunately, the sludge’s left from such operations are enriched with heavy metals, oils and greases and are typically hazardous.
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