Chemical Treatment

Chemical treatment includes (a) the use of flocculants and coagulants to augment the sedimentation or flotation process; (b) carbon adsorption; (c) ion exchange; and (d) chlorination for disinfection. The following is a brief description of each of these processes.

Flocculants are chemicals that facilitate the massing together of particles into a clump known as a floc. Flocs tend to settle faster than the individual particles, thereby increasing sedimentation efficiency. Flocculants can be either organic or inorganic in origin, and they include activated silica, certain clays, fine sands, and trade products such as Magnafloc, Purifloc, and Superfloc.

Coagulation is the process by which colloidal particles are destabilized. Normally, colloidal particles are charged and tend to repel each other, thereby maintaining themselves in suspension. Through the use of a coagulant, the electrical charge of the particles can be neutralized. Once neutralized, the particles will no longer repel one another and begin to coagulate/flocculate and settle (precipitate). Coagulates include such compounds as aluminum sulfate, ferric chloride, ferric sulfate, and copper sulfate.

Figure 14 Trickling filter (from Ref. 46).

Activated carbon has the ability to adsorb contaminants from the stormwater. This adsorptive property lends itself to the removal of soluble BOD, organics, and toxic substances. Carbon can be used in powdered or granular form. In the powdered or granular form, it may be added to the contaminated stormwater directly and in a mixed contacting environment remove impurities. The granular form can also be used in packed bed or as a filtration media, whereby the contaminated stormwater may be passed through and impurities adsorbed. Activated carbon is also somewhat effective in removing various inorganic contaminants, such as copper, lead, cyanide, and chromium. Ion exchange is only suitable for use after all particulate matter has been removed from the contaminated stormwater. This is due to the subsequent plugging of the ion exchange media that would result if this matter is not removed. Ion exchange media come in two types: cationic and anionic. Cationic media are typically used to remove positively charged ions, whereas anionic media remove negatively charged ionic substances from the water. Generally, contaminants from electroplating and metal-finishing industries may be effectively removed using the ion exchange process (e.g., cadmium, chromium, and nickel).

Chlorination is but one means for disinfecting contaminated stormwater. The object of the disinfecting process is basically to kill pathogenic bacteria. This disinfecting method has been known to be the most common and economical method of choice in the wastewater treatment field. However, methods should be investigated that may, on a case-by-case basis, offer some advantage in certain instances. Conventionally, a 15 mm contact time has been used to facilitate the required kill; however, the contact time can be decreased by increasing the intensity of mixing as well as the dosage of chlorine.

Common varieties of chlorine used for chlorination are chlorine gas, chlorine dioxide, and sodium or calcium hypochlorite. Chlorine gas, though a better disinfectant, is hazardous and, therefore, not the first choice. Both chlorine dioxide and sodium hypochlorite have found wider use in chlorination.

Sand filter structures, similar to those used in potable water and industrial treatment, have recently been introduced in urban runoff management. They differ from those described under infiltration practices by being installed in a structural box and having a surface effluent, instead of being a soil amendment with underdrain system. The structures have been used intensively in the urban environment due to the lack of spaces for large basin treatments. The interested reader can access several web sites, such as http://www.epa.gov/owm/mtb/sandñtr.pdfa.nd http://

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Figure 15 Rotating biological contactors (from Ref. 43).

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Figure 15 Rotating biological contactors (from Ref. 43).

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