New York Catalogue of Practices. Management practices are categorized as operational, or structural, depending on their purpose, function and design. Operational practices involve changes in the management or design of the system. Examples of operational practices include: public education on materials that should not be disposed of in the system, conservation measures such as low flow toilets that reduce water use, and guidelines for inspection and maintenance of the system.
Structural measures usually require engineering design. In the NY catalogue, the structural practices generally describe a treatment system.
Conventional Systems. Design specifications for the following conventional systems are included: absorption field system, gravelless absorption systems, deep and shallow absorption trenches, cut and fill systems, absorption bed systems, and seepage pits. There are slight differences in site requirements for depth of percolation test hole, minimum depth of in-situ usable soil, minimum separation between the trench bottom and groundwater, soil mottling, bedrock, or impermeable strata.
Septic tanks and standard absorption fields are the standard practice. Aerobic systems (systems with a chamber where air is injected into the wastewater to promote aerobic decomposition processes) can be used on sites where unsuitable soils or high groundwater conditions have caused traditional systems to fail. This technology may also be used on small lots. The liquid effluent from an aerobic system, which has lower concentrations of suspended solids and oxygen-demanding material, is then discharged to a standard absorption field. The maintenance requirements for an aerobic system are much higher than for a standard septic tank, due to the motors, aerators and filters.
Gravelless absorption systems are also considered conventional systems. These can be plastic or concrete chambers of various designs or large corrugated plastic pipes wrapped in geotextile fabric. This design does not overcome site limitations, and is used in areas where gravel is not economically available.
Deep absorption trenches are used in sites where a thick layer of impermeable soil overlies more suitable soil. This conventional system is excavated through the impermeable layer and backfilled with aggregate or coarse sand. Because of the depth of discharge, there is less uptake of treated wastewater by site vegetation. In contrast, shallow absorption systems (also considered conventional) are used on sites where there is at least four feet but less than four feet of usable soil and/or separation to groundwater.
Cut and fill systems refer to standard absorption trench systems installed in sites where impermeable soil overlays a permeable layer. While similar to deep absorption trenches, these systems are considered to be more effective provided that the underlying soil is not compacted during installation.
Seepage pits are considered the least preferred of the conventional treatment systems in the NY catalogue. They may be adequate for treating very small flows on sites with inadequate land resources for a standard absorption field.
Alternative Systems. There are three alternative systems listed in the NY catalogue. Raised systems are conventional absorption trench systems constructed in permeable fill placed above the ground surface. The leach field is constructed entirely in the fill. Use of this technology can enable a properly functioning wastewater disposal system on a site that could not meet horizontal or vertical separation distance to limiting conditions.
Another alternative system is the elevated sand mound. A mound system is defined as a pressure-dosed absorption system that is elevated above the original soil surface in a sand fill. The system consists of a septic tank (or aerobic tank), dosing chamber, and the elevated sand mound. Elevated sand mounds may be appropriate for sites with native subsoils that do not transmit water or insufficient depth of permeable soil above limiting conditions (high groundwater or porous bedrock). It is important to note that the elevated sand mound is not acceptable within the watersheds of New York Citys water supply.
The third alternative system is the intermittent sand filter. This is a biological and physical treatment process consisting of a filter bed of carefully graded media (commonly sand). The surface of the bed is periodically dosed with wastewater from the septic tank. Liquid passing through the sand filter is then discharged to a mound absorption system. This system can achieve high levels of pollutant removal due to the double filtration of the sand filter and the downstream mound. Best suited for large lots, the intermittent sand filter can be installed where site constraints of shallow or slowly permeable soils limit applicability of conventional systems. These systems will generally not be approved for use within the watersheds of New York Citys water supply.
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CLW IO 2004