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a USPHS, Manual of Septic-Tank Practice, USPHS Pub. 526, HEW, Washington, DC, 1967. Increase leaching area by 20 percent where a garbage grinder is installed and by additional 40 percent where a home laundry machine is installed. The required length of the absorption field may be reduced by 20 percent if 12 in. of gravel is placed under the distribution lateral, or by 40 percent if 24 in. of gravel is used, provided the bottom of the trench is at least 24 in. above the highest groundwater level.

bDesign Manual, Onsite Wastewater Treatment and Disposal Systems, U.S. EPA, Cincinnati, OH, October 1980. Soils with percolation rates < 1 min/in. can be used if the soil is replaced with a suitably thick (>2ft) layer of loamy sand or sand. Use 6 to 15 min/in. percolation rate. Reduce application rate where applied BOD and TSS is higher than domestic sewage. Additional area credit may be given for sidewall trench area if more than 6 in. of gravel is placed below the distributor. The EPA and GLUMR application rates are lower than the U.S. PHS rates. The former recognize the importance of settled sewage retention in the unsaturated zone to obtain maximum purification before it reaches the groundwater and results in a larger disposal system.

c GLUMR, Recommended Standards for Individual Sewage Disposal Systems, Great Lakes-Upper Mississippi River Board of State Sanitary Engineers, 1980 Edition. Absorption trench or bed shall not be constructed in soils having a percolation rate slower than 60 min/in., or where rapid percolation may result in contamination of water-bearing formation or surface water. The percolation rate is for trench bottom area. For absorption bed, use application rate of 0.6 gpd/ft2 for percolation rate up to 6min/in., then use 0.45 gpd/ft2. Trench or bed bottom, or seepage pit bottom, should not be less than 3 ft above highest groundwater level. Maximum trench width credit shall be 24 in. for design purposes, even if trench is wider.

dHenry Ryon, Notes on Sanitary Engineering, New York State, Albany, 1924, p. 33. These and the U.S. PHS rates are given for historical perspective.

e Reduce rate to 2.0 gpd/ft2 where a well or spring water supply is downgrade; increase protective distance, and place 6 to 8 in. sandy soil on trench bottom below gravel and between gravel and sidewalls. f Soil not suitable.

gSee Small Wastewater Disposal Systems for Unsuitable Soils or Sites, this chapter.

Many variations and refinements of the soil percolation test, including the use of a float gauge, inverted carboy as in a water cooler, and permeability test, have been proposed.15 Whatever the case, enough tests should be conducted to give information representative of the soil, as indicated by a relatively constant rate of water drop in the test hole. This data should make it possible to determine an average percolation rate that can be used in design the septic tank system. A typical layout for such a system is shown in Figures 3.1 and 3.2.

Where a small subdivision is under consideration, at least three holes per acre should be tested. More holes should be tested if the percolation results vary widely, say by more than 20 percent. If rock, clay, hardpan, or groundwater is encountered within 4 feet of the ground surface, the property should be considered unsuitable for the disposal of sewage by means of conventional subsurface absorption fields.

Septic systems should not be constructed on filled-in ground until it has been thoroughly settled or otherwise stabilized. Percolation tests should be made in fill after at least a six-month settling period. Soil tests in fill often are not reliable as the soil structure, texture, moisture, and density will be quite variable and other disposal systems (see "Small Wastewater Disposal Systems for Unsuitable Soils or Sites" later in this chapter) should be considered.

When septic tank systems do fail, the cause is usually either improperly performed and interpreted soil percolation tests, high groundwater, poor construction, lack of maintenance, abuse of the system, or use of septic tanks where they were never intended. Inadequate design, lack of inspection by regulatory agencies, and failure to consider soil color, texture, and structure may also contribute to the problem.

Sewage Flow Estimates

The sewage flow to be expected from various establishments can vary, depending on the day of the week, season of the year, habits of the people, water pressure, type and number of plumbing fixtures, and type of place or business. For that reason, septic system design should be based on the average maximum flow rate to ensure adequate capacity. In the absence of actual figures, the per capita or unit estimated water flow given in Chapter 1 may be used as a guide. Alternatively, a fixture basis (see Table 3.5) can be used for estimating sewage flow rates. This approach assumes that all water used finds its way to the sewage disposal or treatment system. After adjusting for lawn watering and car washing, the total number of fixtures is multiplied by the flow from each fixture to obtain a rough estimate of the probable flow in gallons per day.

The design flow and sewage application rate for subsurface absorption systems also should take into consideration the strength of the septic tank effluent (BOD and TSS) in addition to the hydraulic loading rates already presented.

FIGURE 3.1 Typical private water supply and septic tank disposal systems. (Notes: (1) Watertight footing drain within 25 ft of well. (2) Tile field to be 50 ft or more from any lake, swamp, ditch, or watercourse and 10 ft or more from any waterline under pressure. (3) Cast-iron pipe, lead caulked joints within 50 ft of any well. (4) Discharge footing, roof, and cellar drainage away from sewerage system and well. (5) Grade lot to drain surface runoff away from the subsurface absorption system.)

FIGURE 3.1 Typical private water supply and septic tank disposal systems. (Notes: (1) Watertight footing drain within 25 ft of well. (2) Tile field to be 50 ft or more from any lake, swamp, ditch, or watercourse and 10 ft or more from any waterline under pressure. (3) Cast-iron pipe, lead caulked joints within 50 ft of any well. (4) Discharge footing, roof, and cellar drainage away from sewerage system and well. (5) Grade lot to drain surface runoff away from the subsurface absorption system.)

FIGURE 3.2 Cross-section of septic tank disposal system.

Septic Tank

A septic tank is a watertight tank designed to slow down the movement of raw sewage passing through it so that solids can settle out and be broken down by liquefaction and anaerobic bacterial action. Septic tanks do not purify the sewage, eliminate odors, or destroy all solid matter, but rather, simply condition sewage so that it can be disposed of using a subsurface absorption system. Suspended

TABLE 3.5 A Fixture Basis of Estimating Sewage Flow

Type of

Gallons per

Gallons per

Gallons per Hour per

Fixture

Day per Fixture,

Hour per

Fixture (Average),

Renewable Energy Eco Friendly

Renewable Energy Eco Friendly

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.

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