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MODULE-5 Wastewater Management
Innovative and Alternative (I/A) Wastewater Technologies
Innovative and alternative systems are used to accommodate a variety of site conditions such as, high groundwater, and low-permeability soil. They may provide additional treatment. Sometimes systems are classified as alternative because, even though they make use of a conventional septic tank and drainfield, they may also utilize alternative plumbing fixtures like composting toilets. Other examples of alternative wastewater technologies include re-circulating sand filters, intermittent sand filters, trickling filters, sequencing batch reactors, aerobic treatment units and nitrate reactive media. The following examples are just some of the available alternatives available for Washington communities.
Composting Toilets
Composting toilets have been an established technology for more than 30 years. Since they require little to no water, they may be a solution to sanitation and environmental problems in unsewered, rural and suburban areas. The toilet relies on unsaturated conditions where aerobic bacteria break down waste. The resulting "humus" must be disposed of either through burial or removal by a licensed septage hauler. A typical composting toilet is a well-ventilated container that collects and composts waste (including toilet paper) in a large container installed below the toilet in a basement or in a small compartment located directly beneath the toilet. Composting toilets can be used almost anywhere a flush toilet can be used, but typically appear in seasonal homes, or homes in remote areas and recreation areas.
Re-circulating Gravel Filters
A re-circulating gravel filter (RGF) is a modified version of a single-pass open sand filter. It was designed to alleviate odor problems associated with open sand filters through recirculation, which increases the oxygen content in the effluent distributed on the filter bed. RGFs can be used on sites that have shallow soil cover, inadequate permeability, high groundwater and limited land area. The most important treatment process in a RGF is a biological one, and RSFs typically produce a high quality effluent with about 85% to 95% biological oxygen demand (BOD) and total suspended solids (TSS) removal. In addition, RGFs may also remove up to 50% of nitrogen within the treated effluent. RGFs typically serve subdivisions, mobile home parks, rural schools, small towns and other small wastewater generators such as individual family residences.
Intermittent Gravel Filters
Intermittent Gravel Filters (IGFs) typically have 24-inch deep filter beds of carefully graded media (typically sand but not always). The surface of the bed is intermittently dosed with effluent that percolates in a single pass through the sands to the bottom of the filter. After being collected from the underdrain, the treated effluent is transported to a line for further treatment or disposal. IGFs are typically built below grade in excavations 3 to 4 feet deep and lined with an impermeable membrane where required. Discharges from IGFs are usually of high quality and can be used for drip irrigation or for surface discharge after disinfection. Typical concentrations of BOD and TSS are 5 mg/l or less and nitrification of 80% or more of applied ammonia is usually achieved. IGFs serve the same users as RGFs.
Trickling Filters
Trickling filters are one form of fixed film systems. Fixed film systems are biological treatment processes that employ a medium (such as large stones or peat) that will support a biomass on its surface and within its porous structure. Other filter media include plastic forms, foam cubes, crushed glass, clay pellets, woodchips, and textile material. In a trickling filter, the medium is held in place and is stationary relative to the wastewater flow.
Aerobic Treatment Units
Aerobic treatment units provide a suitable oxygen rich environment for organisms that can reduce the organic portion of the waste into carbon dioxide and water in the presence of oxygen. Aerobic systems are similar to on-site sewage systems except that the treatment process requires oxygen, which is provided by a mechanism that injects and circulates air inside the treatment tank. Two aerobic primary treatment systems have been adapted for on-site use: suspended growth, where microorganisms are suspended within the waste stream; and fixed film, where the microorganisms are attached to an inert medium within the waste stream. Aerobic treatment units are suited for use in single family dwellings, clustered subdivisions, restaurants and other commercial applications, as well as renovation of biologically failed septic systems.