Not to be Neglected

Filters are the heart of any water treatment plant. Periodic inspection can verify that they remain in good condition — or tell you when they need special attention.
Not to be Neglected
The filters are the heartbeat of your water plant.

Interested in Filtration?

Get Filtration articles, news and videos right in your inbox! Sign up now.

Filtration + Get Alerts

Without a doubt, the most important part of water treatment plant operation is filter performance. Filters are usually the last physical defense against impurities entering the finished water supply — they are literally the heart of the water plant.

Every treatment plant has a plan to address preventive, corrective and emergency maintenance, but often the filters themselves are left unchecked until a problem develops. At that point, the correction is likely to be time-consuming and costly.

Most experts recommend performing a general filter survey annually. If any irregularities are found during normal filter operation or during the backwash cycle, the inspection should be moved up. Such abnormalities would include water boils on the filter surface or excessive air escaping from the filter when no air scour is being performed.

Getting started

Ideally, filters should receive a thorough periodic inspection and evaluation to assess their overall condition and help in planning repairs, upgrades or media replacement. New media of course gets inspected during installation to ensure compliance with design specifications. After that, inspections at regular intervals are recommended to track and document media condition and operational performance.

A proper filter inspection should be performed by a qualified firm specializing in such work. You should be able to get a recommendation from your consulting engineer, the AWWA or your state Rural Water Association. Once selected, the inspection firm will need some basic information about your plant’s filters, such as the number of units in service, the dimensions, the media depth, and how many filters can be taken out of service at any one time.

Unless you have the luxury of shutting down your plant for an extended period or have numerous filters, you may have to take only one filter at a time out of service. If your system is storage-limited, it may be advisable to fill all tanks and storage reservoirs to full capacity to allow the filter downtime needed to perform the inspection.

Essential steps

At a minimum, a comprehensive filter inspection and evaluation should include:


  • Observation of filter backwash
  • Visual inspection of filters and filter media
  • Inspection of surface wash sweeps
  • Inspection of the underdrain and plenum from the manway in the filter pipe gallery
  • Determination of media depth
  • Filter media sampling and analysis
  • Report of findings, conclusions and recommendations


Observation of filter backwash

A backwash process should be witnessed from beginning to end. During this time, note the general condition and performance of the filter and media. Document the surface of the media and note if it is irregular with depressions or mounding up in some areas, as this would indicate uneven media distribution.

During backwash, the media should be adequately raised and mixed so as to scrub out impurities. This movement should be uniform and even. Any media surface unevenness after backwash may indicate that the media is being displaced through excessive air release or uneven backwash distribution. Look for any unusual boils during backwash that may indicate a problem.

Visual inspection of filters and filter media

During the observation of the backwash process, note any evidence of media being lost into the waste stream. A minimal amount of loss is normal over the course of filter operation, but any unusual amounts are cause for concern.

Inspection of surface wash sweeps

The filter’s surface wash arms should have free and unrestricted movement without signs of seizing up. When inspecting the surface wash arms, check for any media clinging to or clogging their movement. Also check the condition of the bearings to make sure they are turning freely. 

Surface wash arms that offer limited or no movement will not contribute to the uniform lifting and washing of the media. If the wash arms do not rotate properly, make plans to inspect further and to repair or replace as necessary.

One common occurrence that can limit surface wash arm effectiveness is clogging of the nozzles with minute particles of filter material and calcium deposits from hard water. If you find clogged nozzles during the inspection, plan to repair or replace as necessary.

Inspection of the underdrain and plenum from the manway in the filter pipe gallery

Observations at this point will depend on the type of underdrain. For a filter containing a bottom with Wheeler balls, check the underdrain and placement of the porcelain balls in the hoppers from the plenum chamber. Look especially for flat spots on the balls that have developed from wear and tear, as this may cause uneven settling of the balls at the end of the backwash process. Also check for media in the plenum, which can indicate disruption of the support gravel.

The plenum chamber is the open space between the physical bottom of the filter and where the filtered water enters the clearwell or storage reservoir. This is considered a confined space, and  proper entry and monitoring procedures must be followed throughout the inspection. While the plenum is accessible, visually inspect the structural integrity of the underdrain system and identify any missing or worn concrete.

If your filters have not been inspected for a long time, be prepared to cut off bolts or other fasteners that mount the access covers to the underside of the filters, as these may be rusted and twist off when applied with force. After the inspection, install new nuts and bolts. Be sure to inspect the gasket material that seals the plate to the wall and replace it if it is significantly worn or if the gasket is torn or cracked. Finally, run a bead of silicone or similar sealant around the access covers to provide an extra physical barrier against leaks.

If possible, take plenty of photographs or video to document the structural condition of the underdrain system. This can be a valuable training tool to familiarize operator trainees with the function of the unit. Since many licensed operators may not have seen the undersides of the filters, it is informative for the staff to observe the process and to be able to ask questions.

Determination of media depth

Believe it or not, sand can and does wear out. Through long-term filter operation, the originally rough edges on sand grains that help catch floc particles become smooth and less adept at stopping particulate matter. This can result in higher filter bottom turbidity readings and increased headloss readings, both of which will contribute to increased backwash cycles. Over time, this will contribute to increased operating costs.

Measure the media to determine whether an adequate amount is present. The recommended media depth for your plant’s filters can be found on the facility plans and specifications. Since media depths can vary with the quality and quantity of water being filtered, it is important to see if the media has changed significantly from the configuration installed during start-up.

The type of media that yields the best filtering results will vary from plant to plant and will depend somewhat on water volume and clarity.

Media replacement is recommended or required at the point where the media size has changed significantly (becoming smaller or larger) from the original specification. When significant changes occur, filter cleaning with an acid solution may prolong media life in some cases. If fouling continues, cleaning may have to be repeated, and eventually you will have to decide if media replacement will be more cost-effective in the long run.

In terms of media condition, look for media that is fouled. If the media has become rounded (like a pebble), then its effective filtering capacity may be diminished. Note if the media still meets the effective size originally specified. Finally, media replacement is required if the support gravel has been disrupted so that significant media has been lost through breakthrough to the filter bottom.

Filter media sampling and analysis

Collect samples of each type of filter media (sand, anthracite or others) and send them to an independent laboratory for testing. The laboratory needs to be a specialist in this type of analysis — your filter consultant should handle this aspect of the inspection. The samples are analyzed for specific gravity, solubility and effective size to determine whether the media can still yield top filtering capability.

The laboratory will perform a sieve analysis to determine the current effective media size and compare that to the original specification. This helps determine how much wear the media is experiencing. The sieve analysis will indicate the stability of the media and whether the media is breaking down (becoming smaller) or accumulating fouling (becoming larger).

Report of findings, conclusions and recommendations

After filter inspection and evaluation, your consultant should issue a written report detailing the findings, conclusions and recommendations of the inspection. You should expect a report three to four weeks after the inspections. You can use this report to plan and schedule operations, maintenance and repairs to your filters up to and including media replacement.

Filter media cleaning

In some cases, you may be able to clean the media rather than replace it. The media analysis will tell whether the media has filtering capabilities left and will recommend cleaning options.

Filters are usually cleaned by dosing with an acidic media cleaner. The dosage can be determined by using the analytical test results along with the filter dimensions and media depth. The dose will certainly depend on the overall media condition.

After being dosed with cleaning agent, the filter must rest for the required reaction time before extensive backwashing to remove all traces of cleaning chemical. After the backwashing cycle, the filter should be run to waste for an extended time before going back into service.

Keeping them running

Many factors can affect filter and media lifespan. Performance is affected by the daily hours of filter operation and the overall quantity and quality of water being treated. Since inspection is somewhat time-consuming, try to schedule the evaluation around other large operation and maintenance projects so that you can devote the necessary time to the process. Depending on the number and size of filters, allow at least several days for a complete inspection.

Finally, take this opportunity to review the general standard operating procedures (SOPs) for the filter and backwash sequence and make any necessary changes. With a little planning, your water plant filters will keep you up and running well into the future.


James E. Didawick is superintendent of Public Works for the Town of Woodstock, Va. He can be reached at


Comments on this site are submitted by users and are not endorsed by nor do they reflect the views or opinions of COLE Publishing, Inc. Comments are moderated before being posted.