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Surface Water Treatment Rule 101

A Drop of Knowledge E-Newsletter

Surface Water Treatment Rule 101

What you should know to keep your surface water system in compliance


by Brian Day, Technical Assistance Provider with the Midwest Assistance Program (Midwest RCAP)


In 1989, the Surface Water Treatment Rule (SWTR) was established to require water systems that use surface water or groundwater under the direct influence of surface water (GWUDI) to prevent waterborne diseases caused by viruses, Legionella, and Giardia lamblia. These disease-causing microbes are present at varying concentrations in most surface waters. The rule required 99.9% (3-log) removal/inactivation of Giardia lamblia through filtration and disinfection and 99.99% (4-log) removal/inactivation of viruses through disinfection.  The rule also requires that the residual disinfectant must be higher or equal to 0.2 mg/L at the entrance to the distribution center and detectable throughout the distribution system.  Finally, the SWTR requires that turbidity is monitored and must be less than or equal to 0.5 NTU in the combined filter effluent in at least 95% of the measurements taken each month.


Since the initial passing of the SWTR, the rule has been enhanced to help control other microbial contaminants and to improve performance standards.  The Interim Enhanced Surface Water Treatment Rule (IESWTR) and Long-term 1 Enhanced Surface Water Treatment Rule (LT1ESWTR) require that all water systems have periodic sanitary surveys, address Cryptosporidium, include individual filter monitoring, and that finished water storage facilities be covered. The Long-term 2 Enhanced Surface Water Treatment Rule (LT2ESWTR) supplements existing regulations for Cryptosporidium treatment requirements to higher risk systems.


Here are a few surface water treatment tips you should take into consideration:

  • Monthly reporting is required for surface water systems. Operators of surface water treatment plants must document the performance of their plants and report the information every month to their primacy agency (some states now allow for or require electronic submission). It is recommended that operators circle the 10th of the month on the calendar and send the required report to the primacy agency. Make sure you double check the forms for missing data or errors and make sure that all entries are legible. The responsible certified operator must sign the report. If the treatment plant is off-line for a month, make a notation on the reporting form describing its status. Operators should also familiarize themselves with the Public Notification (PN) Rule to determine when a treatment technique violation must be reported within 24 hours of the event.
  • It’s important to get in the habit of doing chlorine contact time (CT) calculations and recording them every day for chlorinated systems. Failure to meet the inactivation ratio of 1.0 for more than one day in a calendar month is a federal treatment technique violation. Under the PN Rule, an operator is required to report this violation within 24 hours to their primacy agency. Sometimes operators mistakenly wait until the end of the month to calculate all of the daily CT values, only to find that violations they should have known about occurred earlier in the month. If you have a computer spreadsheet, enter the raw data as soon as you have it and review the inactivation ratio. Do this every day.
  • Daily pH readings must be rounded up to the highest unit or tenth of a unit. Daily temperature readings should be rounded down to the next full unit expressed in degrees centigrade. The inactivation ratio should be reported to the nearest tenth.  Be sure there is an entry each day for disinfectant residual entering the distribution system. Every time a coliform sample is taken from the distribution system, a disinfectant residual is required.

For additional information on SWTR, visit the EPA website at or contact your local state drinking water representative.