Installing Media Filters

It’s important to follow the proper procedures, from preparing the excavation to placing and covering the filter media
Installing Media Filters
A pressure distribution system over a sand filter before backfilling. Effluent is applied to the rock at the surface.

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Many kinds of media filters are used for additional pretreatment of septic tank effluent. The media can consist of sand, peat, fabric, foam and other materials. Many makers of proprietary media filters have moved to building and shipping modular units with the components pre-assembled. This obviously greatly simplifies the installer’s role.

Still, there are times and places where the installer must deal with a generic media filter that requires on-site assembly. Generic media include sand, gravel and sometimes peat. Let’s walk through the basics of media filters. As always with proprietary products, recognize that you must follow the installation instructions – or void the warranty.

 

Basic configurations

There are many flow configurations for media filters; but they fall into two broad categories: single-pass and recirculating. In a media filter, effluent is distributed over the media surface usually using pressure distribution. A major exception is the Canadian peat filter that uses a tipping tray to distribute effluent by gravity over the peat surface.

As the septic tank effluent moves through the media, it comes in contact with surfaces where microorganisms grow. These are aerobic treatment processes: If a media filter goes anaerobic, it is failing. The treated wastewater collects in the bottom drain piping at the base of the filter. From there it is sent back to the recirculation tank or to the soil dispersal area.

Generic single-pass or recirculating sand filters involve excavating and burying the filters. The area for the filter is excavated to the desired depth and lined with a 30-mm PVC liner. Depending on the filter design, the bottom will be set level or provided with a slight slope to allow free drainage. The bottom should be sloped from the outside of the filter to the point of underdrain collection. The slope should be one inch per foot of run.

Typically plywood support walls are constructed so that the liner can be placed in the excavation with vertical sidewalls. The plywood is typically 1/2 inch and untreated – its only function is to make placement of the liner easier, and it will decompose over time. When constructing the walls, make sure there are no nails or screws sticking out where they can puncture the liner material.

The liners themselves are heavy and difficult to work with. They are custom made and folded for the dimensions of the filter. Always follow the directions for unfolding and placement. Hang the liner over the plywood, making sure the corners are tucked in so that the material is not under tension when the media is added.

In backfilling around the outside of a liner, it is important to coordinate with the placement of the underdrain material and filter media. The level of material inside and outside should be roughly the same to ensure that the filter shape will not be distorted.

 

Keep it watertight

Most proprietary units have a manufactured modular housing made of fiberglass, plastic/poly or composite. Some models use concrete tanks modified for use as media filters. Most peat, foam and textile filters are marketed this way.

It is important to follow all manufacturers’ recommendations when lifting and setting these components. This is for safety reasons and to avoid damage to the housing, which would make them no longer watertight and void the warranty.

Regardless of the material, all modular containers need to be watertight. Any pipe penetrations, access manholes or covers need to be sealed properly with manufacturer-approved materials. These may include PVC boots, certain pipe primers, and glue. Any risers attached to tanks must be sealed to make them watertight, just as in installing septic tanks or pump tanks.

In a generic sand filter, once the liner has been laid, the underdrain system must be installed. The underdrain piping is typically a 4-inch schedule 40 PVC pipe with 4-inch slot spacing with 1/4-inch slots, laid with the slots on top. Around the piping, 4 to 6 inches of 1/2- to 1-inch washed stone is placed with 2 to 3 inches of 3/8-inch pea gravel over the top. This keeps fine sand particles from washing down through the rock and plugging the underdrain.

Sharp angular stone should not be used for either layer, since it could puncture the liner. If it is necessary to pump from the filter for recirculation, or to the final dispersal area, the liner can be formed to accommodate the pump, controls and piping.

The pump basin bottom needs to be level, at the correct elevation, and large enough to accommodate the pump, piping and floats. If there will be gravity flow to other system components, the underdrain piping must exit the container, and the penetration must be watertight. If using a flexible liner, this means using flexible boot. Follow the directions for installing the boot, using proper gluing procedures and a stainless steel band to provide a mechanical watertight seal.

 

Placing the media

Next, place the proper depth of media. In a sand filter, this is usually about 24 inches. The design should specify the sand gradation to be used for the application. It is important to follow those specifications.

In some applications we have worked on, the sand material is actually a fine gravel specified as number 2 bird grit. Typically, the “sand” must be coarse and clean to allow relatively high loading rates and to make sure the media does not plug easily.

In a generic sand filter, pea rock or gravel would be placed over the filter material, and the pressure distribution manifold and laterals would be covered by the rock. Then a filter fabric is placed on top of the rock and 6 to 12 inches of sandy loam topsoil is placed over the top. Access points should be provided to the pressure distribution system to allow for maintenance.

Obviously, there is a lot more to filter installation than is described here, but this gives you a good idea of the processes. If you have never installed a media filter, make sure you understand the product being used and its unique requirements. Follow the manufacturer’s or designer’s specifications and instructions and the installation will go well. Adding these systems to the list of options you provide to your customers will be good for business.



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