3 Keys to Proper Media Filter Installation

3 Keys to Proper Media Filter Installation

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Many of the media filters used in today’s wastewater industry use proprietary technology or components and come from the manufacturer as pre-assembled modules. Other media filters, including sand and gravel filters, are generic technologies and usually require full installer assembly.

Proper installation of the treatment train components is essential to media filters working effectively and efficiently over a long life span. Watertight media filter enclosures are crucial to proper system function and longevity, so it is important that the installer pay careful attention to this issue and keeping extraneous water out of the filters. Some filters, such as recirculating sand/gravel filters and buried single pass filters, are designed to handle rainwater that may infiltrate its surface. Attention to diverting stormwater away from media filter footprints should be practiced to protect them from additional hydraulic loading. Likewise, the installer should have a conversation with the system owner about water use and conservation within the home and the effect of excess water on system performance and longevity.    

Manufacturers have specific installation steps that should be followed and provide vendor-based training for those wishing to install their technology. Many manufacturers have produced good-quality training documents, and this training may be required by the regulatory jurisdiction or the manufacturer. Even though they are all a little different, three key steps for media installation are covered below that are common with all: 

1. Excavation 

The correct location for the media filter needs to be pinpointed on the building site by transferring measurements noted on the design to establish fixed points of reference on the actual ground surface. Installers may choose to temporarily locate the position of components on the ground surface by using landscape marking paint/chalk or wooden stakes. The elevation of the base of the media filter (or required bedding material) should also be transferred using the benchmark noted on the design. As excavating proceeds, elevations should be checked often by a worker with surveying equipment to ensure that the hole is not over-dug. It is always best to keep the base of the filter on firm, undisturbed ground as it will not likely experience any additional settling nor require additional compaction for a stable base. The grade person should also guide the heavy equipment operator to ensure that the excavation is graded to specifications.

The excavation should be graded to design or manufacturer specifications. In most cases the filter base will be set level. However, some proprietary technologies may require a slight pitch or slope to the excavation bottom to facilitate drainage within the filter enclosure. The excavation bottom should be stable. If bedding material is required, the proper material and amount should be added and compacted to the manufacturer’s specifications to ensure a firm and stable base. 

2. Container/Liner 

Many pre-engineered modular media filters come with the media pre-installed in a container or are placed into a tank on site, whereas generic technology such as sand filters are installed into liners.

The installer should follow manufacturer recommendations for rigging and lifting modular filters; this is essential for safety reasons and to ensure that filter containers are not damaged.  Many of the proprietary media filters have a manufacturer-assembled modular container housing the media. These are typically composed of fiberglass, plastic/poly or composite material that has the desired structural strength. 

Some media filters may be installed in concrete tanks, closely resembling septic tanks but modified accordingly. Concrete units come from the manufacturer as modular units. They are pre-packaged and pre-assembled in tanks at the factory or assembled on location. Some filters may also be installed as manufacturer-assembled, pre-plumbed, above-ground units in wood or vinyl sheds or in polyethylene tanks housed in an above-ground structure. Regardless of the type of material that the container is made from, all containers need to be structurally sound and watertight so that effluent does not leak from the structures and groundwater doesn’t infiltrate them. 

Any pipe penetrations through liner walls, media filter containers, or their access manholes need to be sealed properly with vendor-approved materials and methods. These may include PVC boots, pipe primer/cleaner and glue fittings, grommets or risers to grade secured to the tank top with sealant, glue or precast assemblies. Most vendors selling these products also include installation instructions to help ensure that watertight seals are achieved.  

Sidewall insulation may be required for some media filters, especially in cold climates where the filter protrudes above the ground surface. Typically, 2-inch-thick rigid foam insulation is used for this purpose. Under extreme weather conditions, two layers of foam board insulation with overlapping joints may be needed.   

Most buried single-pass sand filters or recirculating sand filters are constructed of plywood walls used to temporarily support a watertight flexible 30- or 40-mil PVC liner. This liner is like a swimming pool liner. Flakeboard products such as strand, wafer or OSB boards can be substituted for plywood, if the material selected will maintain structural integrity during the filter construction. The wood support walls should be constructed to the dimensions and height required by the design. All screws used to fasten plywood sheets together at joints should face outward and their heads should be sufficiently recessed to avoid liner punctures. 

The wood support walls are expected to eventually decompose over time. The native soil on the outside and sand on the inside of the liner maintain the perimeter of the filter. The PVC liner provides a watertight vessel to prevent infiltration of water or leakage of effluent. PVC and rubber liners are heavy and often cumbersome to handle and should be hung over the plywood wall with careful attention paid to ensure that corners are tucked neatly and with enough "give" in the liner (extra liner material) so that it is not under tension when media is added.  

To install a lined media filter correctly and not distort the filter shape, make sure the level of media inside and backfill/bedding material used on the outside of the liner are added in roughly the same proportions (referred to as “lifts”). If equal pressure is applied both inside the liner and out, then the filter shape will not distort. Sand is typically “walked in” by foot to get the proper compaction. No mechanical compaction is used on the inside of the sand filter as it may limit wastewater flow, and vibration may separate sand particles. Careful compaction should occur on the exterior of the liner to avoid distorting the filter shape. To avoid contaminating the sand media, the tools and equipment used for moving backfill material should not be used to move media into the filter, unless they have been cleaned thoroughly.

3. Backfill 

Backfill or bedding material used around media filters should be whatever the the manufacturer or designer specifies. In some cases, native spoil material is suitable as backfill; however, do not use clayey soil, organic material, rocky fill or frozen soil. Remember that bedding material such as crushed stone, pea stone/gravel, and rice stone used as backfill have pore space that may preferentially accept infiltrating water and channel that moisture around the filter exterior. Unless precautions are taken, this moisture can be sufficient during high rainfall events to float buoyant filters out of the ground, even on sites with deep water tables. Careful attention should be paid to providing proper anti-floatation consistent with the manufacturer's recommendations. Some proprietary products have flanges or wing-type devices that bolt onto the exterior of the media filter enclosure. Other technologies have concrete mounting pads or "dead men.” Concrete and/or native spoil material is placed upon the flanges, pads or wings to provide ballast against floatation.   

Although media filter containers are designed to be structurally sound, they can be damaged by excessive compaction and heavy weight. Possible damage may include cracked walls (causing potential leakage) or distorted shapes that would limit proper opening and closing of access lids (and possible leakage). Consult the manufacturer's specifications for proper compaction around, and weight bearing atop, their units. 

In some cases, temporary or permanent bracing may be needed to prevent media filter container distortion from either inside or outside pressures. Materials used for permanent bracing need to be resistant to corrosion from contact with wastewater or the weather.  

If native material is used for backfilling, angular, sharp or large rocks should be removed during the backfilling process, as these can crack or puncture filter containers and liners. Backfill should be placed in 7- to 9-inch lifts and compacted properly to protect the configuration and structural integrity of the filter enclosure or liner. When utilizing a flexible PVC or rubber liner (sand filters), exterior backfill should be placed in lifts consistent with the level of media inside the liner and compacted properly. This will maintain a straight liner wall and square corners, producing a neat installation and placing less stress on the liner and wood support wall. This will also ensure that the distribution manifold and laterals have proper room and spacing within the filter. 

After installation of a media filter with the proper excavation, a watertight liner or container, and appropriate backfill the system will be off to great start at treating wastewater. 

About the author: Sara Heger, Ph.D., is a researcher and educator in the Onsite Sewage Treatment Program in the Water Resources Center at the University of Minnesota, where she also earned her degrees in agricultural and biosystems engineering and water resource science. She presents at many local and national training events regarding the design, installation and management of septic systems and related research. Heger is the President-Elect of the National Onsite Wastewater Recycling Association and she serves on the NSF International Committee on Wastewater Treatment Systems. Ask Heger questions about septic system design, installation, maintenance and operation by sending an email to kim.peterson@colepublishing.com.


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