Pay Attention To Filters on Drip Distribution Systems

Preventing damaging infiltration and promoting appropriate effluent flow requires diligent operations and maintenance compliance

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We have heard questions from time to time about operation and maintenance of drip filters. These discussions prompted comments from some service providers as well as a manufacturer. We wanted to share the information we learned from these comments in hopes that it helps you think about the drip system filters you install or service.

In previous articles, we’ve described three possible filtering devices for drip systems: sand filter, screen filter and disc filter. Screen and disc filters are the two most common types. These filters are intended to catch and remove larger particles remaining in the wastewater before it is delivered to the dripfield. Over time, depending on how “clean” the wastewater is that’s going into the filter, they will plug and require replacement or cleaning. Most of the systems today have an automatic backwash for the filters that’s either set to operate when there is a pressure differential from one side of the filter to the other, indicating plugging, or at some set time interval.

One of the first operation and maintenance procedures when servicing a drip system is to check the filter holder to make sure the filters are still there. A common homeowner fix is to remove the filters if they plug frequently, and once they figure out that the filter is the problem, they remove it.

FILTER SWAP

The filters should be removed and cleaned manually during each service visit or at the interval specified by the manufacturer. This can be done using water with a disinfectant. Since it is usually easier to do this cleaning back at the shop, most service providers swap out the filters and take the dirty ones for cleaning. At the same time, the automatic backwash feature should be tested for proper operation. Filters that are manually cleaned have a valve on the bottom that allows removal of larger particles and grit. After cleaning, make sure the valve is in the proper position.

Service providers with extensive drip system experience have observed that organics that are commonly suspended in treated wastewater are soft enough to be pushed through screen filters. As a filter clogs, the differential pressure across the screen media increases and can be enough to push these soft organics completely through the screen and off into the drip tubing. Even if these organics are not pushed all the way through the screen, they can get pushed partially through and become embedded in the mesh so tightly as to be difficult to scour free in any flush cycle or manually clean with brushes or high-pressure water. Many manufacturers now recommend controlling the differential pressure to a lower value than previously recommended.

In many cases, the flushing design of manual screen filters is only intended to release hard materials (like sand) from the filter basin. It’s not intended — and therefore not capable — for clearing materials that get trapped in the screen mesh. Of the manual screen filters that do intend to clean the screen mesh of embedded particles, cleaning is dependent on the shearing action of the water flowing by the inner surface of the screen as it exits the basin through the open flush port and valve. Many screen manufacturers publish values for minimum flow that’s required through their filter to accomplish flushing. One item to check is whether this level of flow is achieved through the flushing operation.

DISC FILTER RECOMMENDATION

Most manufacturers recommend disc filters for wastewater application in drip systems. They do a better job of trapping soft, malleable particles and, therefore, a better job of protecting the downstream components, such as valves and emitters. One problem with disc filters is that between cycles bacteria and other organisms grow between the discs. Over time, this organic material will contribute to plugging the filter.

This type of growth probably occurs in screen filters as well. Reversing the flow through the filter to remove particles trapped inside the filter media — depending on frequency and flow — can reduce the amount of materials that migrate into and get trapped in the disc filter media. If performed properly, this surface flush can extend the time between a needed manual opening of the filter to clean between each disc. Manufacturers again provide values for the flow necessary to clean the filter.

At least one manufacturer addressed the growth of organic material between the discs by adding a bactericide to the plastic of the disc. This has shown good results in reducing the growth and prolonging the period between manually cleaning the discs.

If you have other observations about operation and maintenance issues for drip distribution systems, we would like to hear from you.



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