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Filter Surveillance - The Most Important SOP for Your Plant

Jim DeWolfe PE, BCEE, CWO - Hazen and Sawyer

Last Modified Nov 30, 2023


Since 1972, turbidity regulations form filter effluent water and the water delivered to the public has decreased. Prior to 1962, a turbidity value of 10 NTU was acceptable. From 2003 to the present, 0.30 NTU in 95% of the combined filter effluent samples is required. The Partnership for Safe Water program requires 0.10 NTU in 95% of the combined filter effluent samples in order to participate. This seemingly drastic reduction is largely driven by the direct correlation of turbidity with the presence of pathogens. Therefore, lower values have been regulated over the years to improve the protection of public health from water borne pathogens.

However, granular filter media filter design has not changed much since its inception over 60 years ago. New underdrains systems and air scour have been introduced, but the granular media portion is largely unchanged. This places a substantial responsibility on utilities to ensure that overall filter operations are optimized to the maximum extent possible. Assuming pretreatment prior to filtration is optimized, the filter components - especially the media – must be routinely monitored to help assure maximum protection from pathogens. This can be accomplished by a formal filter surveillance program.


This presentation will provide an overview of filter surveillance and convey the value of a implementing this maintenance activity. Not only will filter surveillance reveal the condition of media and evaluate filter backwash procedures, it will help utilities to better understand what changes may be required to improve and optimize treatment. Many times, operational variables can be changed without the need for capital resource intensive efforts. A major benefit of this activity will be to improve the consistency of filtered water quality through operational changes. Other goals include a prioritized sequence of recommendations, in the event capital resource intensive improvements are required.


The following aspects of filter surveillance will be presented:

  • Interview Plant Staff - discussions with plant staff on recent and historical operations to help gain additional insight into filtered effluent quality excursions. This includes a review of the existing filter backwash protocol and visually observing backwashes.
  • Filter component visual observations – evaluation of the visual physical condition, included but not limited to control valves, air scour blowers, backwash pumps, and backwash collections troughs.
  • Media depth and depth to gravel measurements – verifies the depth of media and the depth to the gravel across the filter area. The media depth will be compared to original values, while the depth to gravel will determine if the gravel has become displaced in any area of the filter.
  • Bed expansion measurement – this is conducted during backwash to evaluate if the media is being adequately expanded to remove particles stored during filtration.
  • Floc retention analyses - before and after a backwash – This involves collecting samples of media throughout the media depth and assessing where solids are stored (before backwash) and the effectiveness of the backwash process (after backwash).
  • Spent filter backwash turbidity profile – this helps to gage the effectiveness of the backwash process, and shows the amount of solids removed throughout the backwash process.
  • Sample collections for sieve analyses – this will be conducted to determine the effective size (ES) and uniformity coefficient (UC) of the existing media. We also recommend conducting acid solubility to gage the amount of precipitates that may have accumulated on the media in 20 years of operation. These analyses will be performed by a laboratory that has routinely performed these analyses on filter media in accordance with the latest edition of AWWA B100 Standards.