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From Missouri Rural Water Association - MRWA:
The Missouri Rural Water Association, MRWA, Percent Efficiency Calculator will allow an individual to simply enter the influent and effluent laboratory results for any parameter and the calculator will determine how efficiently the process removed the parameter. Wastewater Biochemical Oxygen Demand (BOD) and Total Suspended Solids (TSS) are the most common parameters that are required by permits. The calculator can also be used to determine the efficiency of individual processes throughout a facility. An individual must only know what went in and what went out .