Draining It Right

Effective media filter performance depends on taking great care when designing and installing a watertight underdrain system

Media filter drainage systems or underdrains are an important part of filter design and operation. If they are not installed properly, the filter will not meet the expected treatment level.

Filter systems collect and clean effluent and deliver it to the next part of the treatment system. Drainage systems are sometimes made by the manufacturer as a part of the product. Other times, the installer constructs the system. In either case, the drainage system must be watertight so that the effluent leaves the filter and flows onward without leaking into the soil, and so that the filter does not collect additional water.

Handling the piping

With manufactured drainage systems, installers only need to make the proper watertight connections. This means using the proper PVC pipe primer and glue to make solvent welds that will withstand the pressures and stresses placed on the pipe.

When constructing a drainage system for a filter you are building, it is important to check the plans, specifications and state and local requirements first. Typically, a 4-inch Schedule 40 pipe is used. The pipe has 1/4-inch slots spaced about4 inches apart and oriented at the 12 o’clock position.

Sometimes regular 4-inch sewer pipe is used with the 1/2-inch holes at 4 and 8 o’clock. At times, 3-inch pipe is used. If slots or holes are larger than 1/4-inch, you must use stone larger than pea rock to prevent the rock from entering the pipe.

It is recommended to avoid using a filter fabric around the drainage pipe. This is to prevent the pipe from being clogged with sand fines, bacterial growth, or both. If that happens, it limits drainage from the filter. That in turn causes water to back up into the filter, creating anaerobic conditions and leading to hydraulic failure.

The end of the pipe opposite the outlet should be capped. To prevent fines from entering the bottom drainage system on sand filters, 6 inches of washed stone should be placed around the pipe. Over this area, place 2 to 3 inches of washed pea rock. This pea rock also should be placed over the entire remaining area of the sand filter base. Avoid using sharp stones, since they could puncture the liner membrane.

Dealing with pumps

Depending on the technology, a basin may be contained within the media filter to house a pump and float controls. The pump delivers the effluent back to a recirculation tank or to the final soil treatment area.

Pumps may be positioned at the very bottom, raised above the base, or placed in the mid-zone of the filter. It is important to know the purpose of the pump and what the pump requirements are for that particular use.

Single-pass filters often use an internal pump basin that is formed as a part of the excavation and flexible liner installation. The pump basin bottom has to be at the correct elevation, level and plumb. It is important not to push the underdrain piping too far into the pump basin, as it could then interfere with pump float operation.

The pump on float elevation must be set below the top of the underdrain pipe. This ensures that the liquid level inside the filter does not come in contact with the sand media. If it does, it will interfere with oxygen exchange in the media. That in turn will result in reduced treatment and possibly growth of a biomat in the anaerobic conditions created at the bottom of the filter. The ultimate result will be hydraulic failure.

One way to keep track of where the components should be placed is to keep track of the elevations required inside the pump basin. You can then use these markings in the future when you perform maintenance or troubleshooting.

Underdrain piping

The underdrain pipe has to be watertight where it leaves the box. If the container has rigid walls, a proper watertight glued or threaded PVC fitting is best.

As we discussed in our previous article on installing liners, flexible boots are used. The most complicated of these boots to install is on recirculating filters. Good installation requires that you determine the proper invert elevation. The liner is then cut and glue applied to both the liner and the boot.

Preparation and the gluing are much like the procedure used in repairing an inner tube. A putty seal and stainless steel band clamp is applied on the exterior of the outlet pipe, and as it is tightened, the putty material makes contact with the interior surface of the boot. When done correctly, this provides a mechanical watertight seal.

Where sub-aeration piping is included in the design, it is usually installed at the interface of the pea rock and the stone. The purpose of this pipe is to enable introduction of air into the system to break down any biomat and to help the filter recover.

The pipe will be small in diameter and may be either PVC pipe or perforated polyethylene tubing. A vertical PVC or poly standpipe connects to the sub-aeration system and ends at grade, where it is capped. It is placed within a lawn irrigation-type valve box for access and is finished with the cover flush to grade. It is important, as always, to follow the specifications for pipe size, orifice size and orifice spacing.

Following these procedures will ensure that the underdrain system will function the way it was designed to give the customer long-term performance with only routine maintenance.


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