1. Occurrence and Significance
Surfactants enter waters and wastewaters mainly by the discharge of aqueous wastes from household and industrial laundering and other cleansing operations. A surfactant combines in a single molecule, a strongly hydrophobic group with a strongly hydrophilic one. Such molecules tend to congregate at the interfaces between the aqueous medium and the other phases of the system such as air, oily liquids, and particles, thus imparting properties such as foaming, emulsification, and particle suspension.
The surfactant hydrophobic group generally is a hydrocarbon radical (R) containing about 10 to 20 carbon atoms. The hydrophilic groups are of 2 types, those that ionize in water and those that do not. Ionic surfactants are subdivided into 2 categories, differentiated by the charge. An anionic surfactant ion is negatively charged [e.g., (RSO3)–Na+], and a cationic one is positively charged [e.g., (RMe3N)+Cl–]. Nonionizing (nonionic) surfactants commonly contain a polyoxyethylene hydrophilic group (ROCH2CH2OCH2CH2……OCH2CH2OH, often abbreviated REn, where n is the average number of -OCH2CH2-units in the hydrophilic group). Hybrids of these types exist also.
In the United States, ionic surfactants amount to about two-thirds of the total surfactants used and nonionics to about one-third. Cationic surfactants amount to less than one-tenth of the ionics and are used generally for disinfecting, fabric softening, and various cosmetic purposes rather than for their detersive properties. At current detergent and water usage levels the surfactant content of raw domestic wastewater is in the range of about 1 to 20 mg/L. Most domestic wastewater surfactants are dissolved in equilibrium with proportional amounts adsorbed on particulates. Primary sludge concentrations range from 1 to 20 mg adsorbed anionic surfactant per gram dry weight.1 In environmental waters the surfactant concentration generally is less than 0.1 mg/L except in the vicinity of an outfall or other point source of entry.2