Wastewater Treatment Plants: Bacteriological Testing

Effluent limitations for bacteria are included in all new municipal wastewater permits and permit renewals. Fecal coliform, Escherichia coli (E. coli) and Enterococci are used as indicator organisms to test the effectiveness of effluent disinfection in a wastewater treatment plant.
In accordance with Title 30, Texas Administrative Code, Chapter 309.3(h), Effluent limitations on bacteria, new domestic Texas Pollutant Discharge Elimination System (TPDES) permits and renewals replace the fecal coliform limit with limits for E. coli for fresh water discharges and Enterococci for saltwater discharges.

Maximum Holding Time

Samples should be analyzed immediately, preferably within 2 hours of collection. The maximum transport time to the laboratory is 6 hours, and samples should be processed within 2 hours of receipt at the laboratory.
Remote facilities in the state unable to meet the 6 hour hold time requirement have other options. For information about in-house labs or alternate sampling and laboratory testing methods , please contact your local Small Business and Local Government representative or call our confidential hotline number at 1-800-447-2827.

Calculating and Reporting Bacterial Test Results

• Unit of measure
  When reporting bacteria concentration (E.coli or Enterocci), the unit of measure required on the discharge monitoring report (DMR) are expressed in either Colony Forming Units (CFU) or Most Probable Number (MPN) of bacteria per 100 milliliters effluent.

• Daily average bacteria concentration
  The daily average bacteria concentration is a geometric mean of values for the effluent samples collected in a calendar month.

• 7-day average for bacteria
  If specified, the 7-day average for bacteria is the geometric mean of the values for all effluent samples collected during a calendar week.
  

  The geometric mean shall be determined by calculating the nth root of the product of all measurements made in a calendar month, where n equals the number of measurements made; or, computed as the antilogarithm of the arithmetic mean of logarithms of all measurements made in a calendar month. For any measurement of bacteria equaling zero, a substituted value of one shall be made for input into either computation method.
  
  

  Geometric Mean = (N₁ * N₂ * N₃...)^(1/Count)

For more information, review the EPA NPDES Reporting Requirements Handbook for definitions and calculations for DMRs.

Tools

• Geometric Mean Calculator for Wastewater—Interactive calculator to compute the geometric mean on a set of numbers to assist with completion of discharge monitoring reports (DMR) for domestic wastewater facilities.

Additional References

40 CFR § 136.3(e) —Table II—Required Containers, Preservation Techniques, and Holding Times

Laboratories Accredited by the State of Texas under the National Environmental Laboratory Accreditation Program (NELAP)