One of the most common additions to an onsite system is sandfilter. Sandfilters are often divided into two varieties; single pass and recirculating. Shown here are recirculating sandfilters which typically send two-thirds to three-fourths of the wastewater that passes through it back for another trip through the filter. Contrary to popular opinion, engineers are not trying to confuse the contaminants out of the wastewater, even if it does seem that way.
The filter media can consist of sand, peat, foam, fly ash or other media with the appropriate size and uniformity coefficient. The media serves as structure that encourages biological growth and allows the applied effluent to "ooze" slowly through the filter giving the bacteria and other organisms time to digest the "food" (which we consider pollutants) in the wastewater. By passing the effluent through the filter several times, more "food" is eaten by the microbes than would happen with a single pass filter. Recirculating filters generally improve treatment efficiency with organic material (BOD), solids, and pathogens. Depending upon the design, the systems can provide for nitrogen reduction, too.
Probably the two most common questions are what do they cost and what do they look like. The photos here can help show what the systems look like. Our observation is they can range from too ugly for us to want one to attractive enough to wonder when Better Homes and Gardens will come by for photos. Aesthetic qualities are individual and we're sure you can decide for yourself. As you can see recirculating sandfilters come in a variety of designs and there is no single look to the unit.
In terms of cost, prices vary depending on location, availability of sand (or other media), and how engineering and contractor costs. We have observed that initially prices can be steep but over time, as the contractors and engineers become more familiar with the technology, the prices drop. The range of costs we've typically encountered have been from under $5,000 to $15,000. We've seen a few cheaper and a few more expensive but most seem to run around $10,000. In time, we believe that price should drop significantly.
Rich Piluk explains design of a prepackaged recirculating sandfilter used in Ann Arundel County Maryland to Don Alexander and Patricia Miller (photo by Anish Jantrania). These units are small enough to deliver as one would ship a septic tank. As a general rule, most residential applications in Virginia would take two filters of this size. The cost to the contractor for each unit is under $2,000. This does not include the recirculation chamber, pumps, or installation.
Above left, Rich Piluk explains some of the fine points of an operational filter to George Tchebonegalos and Don Alexander. (photo by Anish Jantrania) This filter is like the pre-packaged one shown above and can also be seen in the top right photo at the top of the page. On the right is a photo showing effluent quality at different points in the system. The container on the left has an effluent sample taken from the septic tank, in the middle is effluent from the recirculation chamber, and on the right you can see the final effluent that's discharged to the drainfield.
The photos above show close ups of how wastewater can be applied to a filter. There are many ways to apply the effluent to a sandfilter. The goal is do spread the effluent evenly over the top. In this example, Rich has designed a series of four small diameter pressure distribution pipes about one inch in diameter. He points the holes up and covers them with a four in pipe cut in half lengthwise. If you look closely you can see the one inch pipes under the two center four inch pipe halves. On the right is a gravel covered sandfilter that uses a special drip emitter to distribute effluent.
Recirculating sandfilters can also be used to serve community sized systems as these photos show. These pictures were taken in a community in West Virginia. The interesting thing here is the "sand" in these rather unusual "sand filters". The media (sand) is called "Black Beauty" and is made from the coal burning by-product known as fly ash. This community treats its wastewater and returns it to nature using a recycled media to achieve treatment.
These filters use low pressure pipe to distribute effluent, too. On the right, the splash cover is removed to show how the effluent is distributed evenly along the pipe. Under normal operation the splash cover is in place and the effluent splashes back on the "black beauty" media. Some may wonder why the distribution pipes are not directed downward so that the splash plate can be done away with entirely. Doing so causes the spray to erode the sand and encourages channelized flow through the media. The pipe reduces the problem of channelized flow and the poorer treatment that occurs as a result.