Sand Filters (See endnote 9)
Sand filters are used to clean up the effluent from the septic tank by using physical, chemical, and biological processes. Suspended solids are removed by mechanical straining and sedimentation. Bacteria on the sand grains help remove suspended solids. At the upper part of the sand filter, there are aerobic conditions and the microorganisms assist in removing the BOD and in the conversion of ammonia to nitrates. The nitrates in the lower part of the sand filter are converted by anaerobic bacteria to nitrogen. The effluent which has percolated down through the sand goes into an underdrain and is dispersed into the effluent dispersal field. The sand filter usually has two sections and each section is dosed individually allowing the other one to rest and process the effluent. There are three types of sand filters used after the septic tank: buried, intermittent, and recirculating. The advantages of these systems are that they reduce the contaminants in the effluent considerably in a consistent well-understood manner and have less problems with clogging of soil, they are relatively compact, and they can be used on soils that have high groundwater and poor permeability where other systems have failed. Problems with these systems include: there may be too little land available to add the system; there has to be a sufficient amount of pressure or head to make the filters work properly; odors can occur from the anaerobic parts of the single-pass filtering system; power outages; pumping and distribution unit failures; poor maintenance; and allowing water from other areas to cross the sand filter area. Aesthetically especially in suburban communities, the sand filter looks like an Indian burial mound and may not be very appealing to the neighbors. The recirculating sand filter sends part of the effluent back into the new effluent which and are then processed together thereby reducing potential odor problems.
Best Practices in Use and Maintenance of Sand Filters (See endnotes 21, 22)
- • When installing the system make sure that the design recommendations for the specific size of sand are followed closely.
- • Use a professional maintenance group to clean the filters every 6-12 months or earlier if required.
- • Make sure that the septic tank is watertight.
- • Install appropriate electricity and a back-up electricity source in the event of electrical failure, as well as an alarm system if the filter shuts down.
- • If the effluent is released to the surface under the condition it is allowed by local and state laws, it must be chlorinated and regularly tested for contaminants including E. coli.
- • If problems occur, the system must be taken off-line until the sand is cleaned or replaced.
- • If a filter is exposed to sunlight it may develop algae mats. This can be corrected by shading the surface.
- • Remove all weeds at the surface above ground filters.
- • Use recirculating sand filters instead of the single-pass sand filter to eliminate odors and increase the oxygen content of the filter bed. The advantages are that a very good effluent is produced with a 95% removal of BOD in total suspended solids as well as a significant reduction in nitrogen. Less land is needed than for the single-pass system. Weekly maintenance is required for the media, pumps, and controls. The system may not function very well in extremely cold weather.
- • Use intermittent sand filters instead of the single-pass system in order for one bed of sand to rest while the other one is being dosed and working. This also provides a high quality effluent and requires minimal maintenance but more land for treatment and disposal.