Water and Wastewater
Water Filtration Media
Oil and Grease (O&G) are a common occurrence in wastewater. An EPA commissioned study recently concluded that O&G is an indicator of the presence of numerous other organics in a wastewater, the types that partition into oil. The most famous one is PCB, which partitions into transformer oil.
The EPA is debating new rules that would set specific limits for O&G content in wastewater, thereby encompassing an entire range of organics that partition into oil. Such organics also would include solvents such as benzene and phenols.
Removing oil and grease from wastewater is relatively simple and cheap. Process industries should take the lead in addressing these questions, so that they are ready when the law takes effect. Furthermore, if they design their system such that the wastewater can be recycled, they actually can use this new law to save costs and lessen public concern about discharge of contaminated wastewater.
Effective removal of O&G requires an understanding of emulsions, mechanical versus chemical. Furthermore, the operator must know how to break emulsions and how to test effective treatment methods in the laboratory. He then must know how to remove the now mechanically emulsified oil most economically (i.e., how to coalesce the oil droplets effectively and how to reduce the O&G content to non-detect) so that recycling of the wastewater is feasible. This means being familiar with post-polishing techniques, particularly the use of organically modified clays (organoclays). Organoclays remove oil and grease from water at seven times the rate of activated carbon.
Oil appears in wastewater in a number of different forms including free oil and grease (FOG), mechanically emulsified oil, oil wet solids, chemically emulsified oil and dissolved oil.
Free Oil and Grease
Free oil rises rapidly to the surface of the water tank under calm conditions. The droplet size is *150 microns. This oil can be removed by an overflow weir in the tank and a skimmer. The traces can be removed by passing the wastewater through an adsorber tank (such as a carbon tank) filled with organoclay.
Mechanically Emulsified Oil
These oil droplets range in size from 20-150 microns. Mechanically emulsified oil is stabilized by electrical charges and other forces that result in the coating of suspended solids. Such oils mix with water due to shear that can result from the wastewater traveling through a pump, wastewater splashing into a tank and anything that will break up and disperse larger oil droplets.