Separation of oil from refinery wastewater is carried out almost exclusively by gravity separation using flotation of the oil droplets in the water, in American Petroleum Institute (API) separators and in occasionally in large tanks. This design is not sufficient to meet the effluent requirements in most areas, (according the API study quoted in their "Design and Operation of Oil Water Separators, Publication 421") and installation of coalescing media in the existing pits is a good economic alternative to other more costly designs such as the use of dissolved air floatation or other systems. Plant Process Improvement There are dozens of places in any oil refinery where it is necessary to separate aqueous and hydrocarbon streams. These have often been mixed intentionally to facilitate a chemical reaction or may result from water phases condensing within the process. Rain water may also enter the tops of storage tanks and migrate to the bottom of the hydrocarbon phases. MSR Coalescing systems can be used almost anywhere there are two non-mixing phases to be separated and can improve process operations and increase recovery of valuable products. Once-Through Cooling Water Systems Some refineries utilize "once-through" cooling systems using river or lake water as the fluid for cooling process streams. This is an efficient and inexpensive means of cooling but can be a water contamination source if any of the heat exchangers leak. The usual design is to have the effluent water from the refinery exit the plant by one or two pits designed as rudimentary API separators. This design is not sufficient to meet the effluent requirements in most areas, so installation of coalescing media in the existing pits is a good economic alternative to other more costly designs such as the use of dissolved air floatation or cooling tower systems.