Glossary of Water and Wastewater Terms

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5 random terms from our glossary

Corrosive Gases

Gases that, when dissolved in water, can oxidize materials of construction such as steel and concrete.

Time-Weighted Average (TWA)

A time-weighted average is used to calculate a workers daily exposure to a hazardous substance (such as chemicals, dusts, fumes, mists, gases, or vapors) or agent (such as occupational noise), averaged to an 8-hour workday, taking into account the average levels of the substance or agent and the time spent in the area. This is the guideline OSHA uses to determine permissible exposure limits (PELs) and is essential in assessing a worker's exposure and determining what protective measures should be taken. A time-weighted average is equal to the sum of the portion of each time period (as a decimal, such as 0.25 hour) multiplied by the levels of the substance or agent during the time period divided by the hours in the workday (usually 8 hours). Also see PERMISSIBLE EXPOSURE LIMIT (PEL).

Storm Runoff

Water that flows over the ground surface directly into streams, rivers, or lakes. Also called direct runoff.

Confining Unit

A layer of rock or soil of very low hydraulic conductivity that hampers the movement of groundwater in and out of an aquifer.

Bypassing

The act of causing all or part of a flow to be diverted from its usual channels. In a wastewater treatment plant, overload flows should be bypassed into a holding pond for future treatment.