Porous media graded to standard specifications can be employed in a number of applications for water and wastewater treatment including:
1.) Removal of non-settleable solids.
2.) Tertiary filtration of treated wastewater effluent.
3.) Biological reactors or contractors.
4.) Dewatering and air drying of treatment residuals.
Although silica sand is typically used as a filter media, peat (sphagnum) has been successfully utilized as a biological treatment media. The large specific area of peat available for growth of fungus and bacteria (ratio of 6:1 to 6:3) result in efficient BOD, TSS and Nitrogen reductions. Activated carbon media can be used in downflow or upflow filters to remove organic substances.
Conventional filtration is usually preceded by chemical clarification in which coagulants and coagulant aids are thoroughly mixed with the influent to flocculation and settling basins. Coagulants are typically metal salts such as aluminum sulfate (Alum) or ferric chloride. Coagulant aids are typically large molecular weight compounds called polymers. Conventional tertiary filtration is to be accomplished at a uniform downflow hydraulic loading of several gallons per minute per square foot of filter surface through filter media consisting of a specified depth of granular and graded materials, either as a single media type or as a combination of multiple types of media. Normally flocculated colloidal matter (chemically or Biologically accumulated) is adsorbed, settled and screened for removal within the upper layers of the media. When the accumulated solids begin to impede flow down through the filter media, the flow is reversed and the media cleaned by backwashing. The backwash waste flow is removed for later treatment. Filter media may consist of:
1.) Sand.
2.) Anthracite.
3.) Mineral aggregate.
4.) Other approved materials.
The sand and anthracite media are specified at depths totaling 24 to 30 inches (deep bed), with effective sizes ranging from 0.35 millimeters to 0.80 millimeters with uniformity coefficients not greater than 1.7 in value. Several layers of gravel from two (2) to eight inches in size are provided as media support. Nonconventional shallow bed tertiary filters with continuous backwashing typically provide less than 12 inches of media depth. Upflow and continuously backwashed moving-bed filters usually contain a uniformly sized sand media exceeding 24 inches in depth.
Properly loaded and operated filters can effectively remove flocculated suspended solids at removal efficiencies of 50 percent or more. Filtered wastewater effluent should contain less than 10 mg/l of suspended solids.
Membrane filter technology is now gaining acceptance as a cost effective method of producing good quality wastewater effluent. Hollow membrane tubes are utilized to withdraw filtered effluent directly from reactors. Woven fabric filter cloth and laser beam piercing of metal sheets can be used as ultra and micro filtration methods to remove particles below the size of microorganisms (1 to 10 microns). The use of pressure exerted in membrane covered tubes, such as used in reverse osmosis, can remove even smaller particulates.
Primary effluent or septic tank effluent may be applied to biological contactors referred to as filters. Biological filters using sand media have been used for small flow wastewater treatment for more than 50 years. Recently, peat moss has been used as a biological filter media. Wastewater can be applied to biological filters by gravity feed or low pressure dosing. The recommended hydraulic loading is 1 gallon/day/square foot of media surface (4.1 cm/day) or less to accomplish secondary treatment.
A layer of clean, washed, 3/8" (1 cm) crushed rock of 6 inches or more is placed at the bottom of the media to dewater the media and ensure that aerobic conditions exist in the system. A typical size for a peat bed used for residential on-site treatment would have a width of 20 feet and a length of 40 feet with a depth of 24 inches below the perforated influent distribution piping which is bedded in a small amount of clean, washed crushed rock beneath and to the sides of the pipes. The influent lines should be placed level in the bed and spaced about 2.5 feet on center and from the outer limits of the peat. The perforated pipes are connected at each end with solid pipe. An on-site peat treatment system may consist of several smaller beds receiving split flow in a parallel configuration. The treated flows would be recombined for disinfection prior to surface discharge.
For optimum on-site treatment the peat is uniformly compacted to a bulk density of 6.2 to 9.4 pounds/cubic foot (0.1 to 0.15 g/cubic cm) on a dry weight basis. If the peat is undercompacted, or unevenly compacted, channeling or short circuiting will occur. If it is overcompacted the septic tank effluent will break out at the surface. Compaction is accomplished by placing the peat in the bed in layers and compacting each layer by walking on it while wearing snow shoes. The thickness of the layers will vary depending upon the moisture content of the peat at the time of installation.
No soil is placed on the surface of the peat. The surface may be planted to lawn grasses and mowed during the growing season with a walk-behind power mower. No vehicular traffic should pass over the peat. Alternately, the surface may be planted to a shallow rooted ground cover that does not require mowing. Deep rooted vegetation that takes root in the peat should be removed.