Recycled Glass for On-site Wastewater Sand Filters - Water and Wastewater - Water Filtration and Separation
Issue: Sand filters are a common adjunct to conventional on-site septic treatment systems. When thedrain field for a septic treatment system does not meet percolation standards, a sand filter creates an areawith the correct permeability for effluent from the septic treatment system. Sand filters are part of a non-proprietary anaerobic septic treatment system in which the sand acts as a mechanical and biologicalfilter. Research suggests that using crushed recycled glass in place of sand may lower construction costsand minimize the potential of system failures.
Details
Best Practice: This best practice details the benefits of using crushed recycled glass as an alternativeto C-33 sand as a medium in septic treatment systems. For methods of reducing the recycled glass to thedesired size, refer to the Crushing and Screening Glass AggregateBest Practice. For details on the use ofcrushed glass as a medium in water filtration systems, refer to the Crushed Recycled Glass as Medium inSlow Rate Filtration Systems Best Practice.Sand filter septic treatment systems typically consist of a septic tank, a sand media filter, and a drain field.The septic tank collects the influx, allows sedimentation to occur, and transfers the effluent through the sandfilter to a drainfield, where the effluent infiltrates into the ground. The filter slows the rate of effluent flowand provides a place for microorganisms to break down organic matter and pathogens, and to convertammonia to nitrate. In the state of Washington, the specification for the filter medium has been ASTM C-33 sand, also known as cement sand, in which up to 10% by weight of the material can be finer than a No.100 sieve. The flow of effluent through the sand can cause the fines to migrate and form low-permeabilitylenses. These lenses reduce the rate of flow and encourage accumulations of biological material, called“biomats.” Biomats can cause clogging of the filter, resulting in system failure. Repair requires replacementof the filter sand. Minimizing the fines content inhibits the formation of biomats, but also increases the costof the sand.Tests conducted with in-situ residential septic treatment systems indicate that when crushed to C-33specifications, the fines in glass wash out with effluent flow easier than the fines in sand, reducing thepotential for biomat formation and the associated filter clogging. Experience in field tests indicates thatcrushed glass used as a replacement for sand starts with much higher permeability thn C-33 sand. Thehigher permeability appears to reduce the potential of filter clogging. Furthermore, the increased filterpermeability may allow greater hydraulic loading of the septic system, therefore reducing the required size ofthe filter, and thus the installation cost of the filter.In addition, research indicates that recycled crushed glass filters appear to perform as well as C-33 sand inthe treatment of sewage effluent. These treatment parameters include the reduction of five-day biochemicaloxygen demand (BOD5), fecal coliform count (FC), total suspended solids (TSS), oil and grease (O&G),and nitrates. Reducing the BOD5 and TSS may increase the permeability of drainfield soils by attractingworms and other, higher, life forms, which can reduce the area of leachfield required for a given volume ofeffluent
