One important criterion for evaluating the possible effect of waste disposal into surface waters is the amount of floatable material in the waste. Two general types of floating matter are found: particulate matter that includes grease and liquid components capable of spreading as a thin, highly visible film over large areas. Floatable material in wastewaters is important because
• it accumulates on the surface,
• is often highly visible,
• is subject to wind-induced transport,
• may contain pathogenic bacteria and viruses associated with individual particles, and
• can significantly concentrate metals and chlorinated hydrocarbons such as pesticides and PCBs.
Colloidally dispersed oil and grease behave like other dispersed organic matter and are included in the material measured by the COD, BOD, and TOC tests. The floatable oil test indicates the readily separable fraction. The results are useful in designing oil and grease separators, in ascertaining the efficiency of operating separators, and in monitoring raw and treated wastewater streams. Many cities and districts have specified floatable oil and grease limits for waste-water discharged to sewers.