"Effluent wiH contain 5.2 mg/l dissolved oxygen and 17 mg/l nitrates. Source: J. A. Salvato, Jr., "Experience with Subsurface Sand Filters," Sewage Ind. Wastes, 27, 8, 909-916 (August 1955).

Wood-frame covers may also be used. Regular maintenance is essential, including raking and weeding.

Aerobic Sewage Treatment Unit

Another type of treatment unit that can be used when subsurface absorption systems are not practical is the self-contained, prefabricated aeration unit. Although effluent from such units is low in suspended solids and BOD, it will still need further treatment for other constituents, such as nitrate. Such treatment can include sand filtration and/or chlorination prior to discharge to a stream, if permit, or discharge to an oxidation pond or irrigation system. Routine maintenance and operation of the unit must be ensured by a maintenance contract or other means. Design details for extended aeration and activated sludge treatment plants are given later. Small rotating biological contractors with 2- to 4-foot diameter discs and for flows of 350 to 1,500 gpd or more are also available. Their application and limitations are similar to the above aeration units. In some locations, where tight soil exists and ample property is owned, the waste stabilization pond, irrigation, oxidation ditch, or overland flow system design principles may be adapted to small installations. Design information is given under "Sewage Works Design—Small Treatment Plants."

Septic Tank Mound System

The septic tank mound system was originally developed in North Dakota in late 1947. Numerous refinements to this system have been made over the years, including those noted by Salvato,24 Bouma, et al.,25 and Converse, et al.26 In the mound system, the absorption area is raised above the natural soil to keep the bottom of the trenches at least 2 feet above groundwater, bedrock, or relatively impermeable soil. In this respect, the system serves the same purpose as the built-up soil absorption system previously described. Where it differs is in the type and size of fill material and method used to apply septic tank effluent to the mound system (see Figure 3.10).

The texture and structure of the fill soil used (see Table 3.3) will affect its tendency to clog and its purification capacity; appropriate settled sewage application rates for the range of soil types used in mounds are listed in Table 3.11. In the mound system developed by Converse et al., a 2-foot bed of clean coarse sand was used (i.e., 0.5 to 1.0 mm effective size with less than 5 to 6 percent silt and clay and less than 15 to 16 percent fine and very fine sand).27 To provide sufficient vertical and lateral spreading of the percolating wastewater, 2 feet of sand and 1 foot of natural topsoil is typically used.28 However, experience indicates that mound systems can also be constructed (1) wholly in the natural soil, (2) partly in the natural soil, or (3) completely above the natural ground surface. Because design of a mound system is complex (i.e., involving hydraulic conductivity determinations, hydraulic analyses, pump/siphon selection and sand analyses), it should only be done by a qualified professional, preferably one with experience with mounds.

Septic tank

Pumping chamber with pump switches and high water alarm

Slope 3:1 min

Lateral o

Renewable Energy 101

Renewable Energy 101

Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable. The usage of renewable energy sources is very important when considering the sustainability of the existing energy usage of the world. While there is currently an abundance of non-renewable energy sources, such as nuclear fuels, these energy sources are depleting. In addition to being a non-renewable supply, the non-renewable energy sources release emissions into the air, which has an adverse effect on the environment.

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