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The basic sediment and water (often referred to in a shortened form, such as BS&W, BSW) is the material which collects in the bottom of storage tanks and is usually composed of oil, water, and foreign matter, also called bottoms or bottom settlings. The material is also known as bottoms, bottom sediment, bottom settlings, sediment, and water.

The particulate matter is known as sediment or mud. The water content can vary greatly from fuel to fuel (conventional or alternate). The bulk of the water and sediment is usually separated as soon as possible after production to minimize the quantity that needs to be transported and/or transferred to subsequent processing units. Further, the residual content of these unwanted impurities is measured as bottom sediment and water (BS&W, ASTM D4007).

Thus, basic sediment and water are both a technical specification of certain impurities in any fuel and the method for measuring it. In many raw fuels (including fuels produced from biomass), the crude oil will contain some amount of water and suspended solid from the starting material. The particulate matter and the water content can vary greatly from feedstock to feedstock and from process to process. The bulk of the water and sediment is usually separated at the earliest possible convenience to minimize the quantity that needs to be transported further as well as any adverse effects on the process and process equipment.

Upon production of a fuel from a renewable source, the fuel typically passes through facilities that allow gas and water to separate from the crude and solids to settle out. These facilities may comprise a simple gathering tank or a series of specialized separation vessels, and may involve the addition of chemicals to the crude to aid in the separation of various contaminants from the crude oil and natural gas.

The BS&W test has been used to determine composition of the fuel from many sources and should be tested frequently to ensure it meets the prescribed quality requirements. There are two tests conducted, a bottom sediment and water test (BS&W) and a visual test. The BS&W test is done at the beginning, mid-point, and end of the fuel production train. Visual tests are conducted every fifteen minutes during the evolution. The visual criteria is simply that the fuel be clear and bright and the test is a quick and easy check of the fuel quality.

See also: Refining.