Transvap Sewer Systems

121 and greater

Clay loam, clay

See Ref. 14.

aApproximate vertical hydraulic conductivity.

bVery approximate horizontal hydraulic conductivity. Make field or laboratory determination. cReduce to 0.8 gpd/ft2 if groundwater supplies may be affected.

dFletcher G. Driscoll, Groundwater and Wells, Johnson Division, St. Paul, MN, 1986, p. 78. Coarse sand 0.84 to 1.17 mm size; fine sand 0.2 to 0.3 mm size.

aApproximate vertical hydraulic conductivity.

bVery approximate horizontal hydraulic conductivity. Make field or laboratory determination. cReduce to 0.8 gpd/ft2 if groundwater supplies may be affected.

dFletcher G. Driscoll, Groundwater and Wells, Johnson Division, St. Paul, MN, 1986, p. 78. Coarse sand 0.84 to 1.17 mm size; fine sand 0.2 to 0.3 mm size.

Example 1

Design of a residential mound system on level ground for a flow of 300 gpd. Natural soil has a percolation of 1 inch in 120 minutes, or 0.2 gpd/ft2.

Sand infiltration rate 1.2

This area can be provided by two 2-foot wide trenches 62.5 feet long.

2. Trench and lateral spacing = space laterals 4 feet on center.* Gravel trenches may be combined into one gravel bed (4 + 1 + 1) = 6 feet wide.

3. Mound height (at center) = Sand depth + Gravel depth + Soil cap and topsoil (See Figure 3.10.) = 2 + 0.75 + 1.5 = 4.25 feet

4. Mound length = Lateral length + End barriers (mound height x 3 on 1 slope x 2 = 62.5 + (4.25 x 3) x 2 = 88 feet

5. Mound width (including topsoil) = 0.5 x Trench width x 2 + Trench spacing on center + (Mound height at edge of trench + 3 on 1 slope) x 2 = (0.5 x 2 x 2) + 4 + (3.75 x 3) x 2 = 2 + 4 + 22.5 = 28.5 feet

Provided 62.5 x 28.5 = 1,781 ft2, excluding end areas

7. Distribution system: See Table 3.11 for lateral length and diameter and corresponding hole diameter and spacing. Make manifold 2 inches in diameter for pumping.

* A 1-in. diameter pipe holds .041 gal; a 1.25-in. pipe .064 gal; a 1.5-in. pipe .092 gal; and a 2-in. pipe .164 gal

8. Pressure distribution: For pumping chamber volume, pump size, and dosing volume, see Converse29, and for siphon discharge. Include valve on pump discharge line for fine adjustment of pump head and discharge.

9. Pumping chamber = 500 gal capacity for 1-, 2-, 3-, or 4-bedroom dwelling is recommended.

10. Dose volume = 0.25 daily flow and at least 10 times lateral volume when pump is used.

11. Pump size: The pumping head is the difference in elevation between pump and lateral invert, plus friction loss in the pump discharge line, manifold, fittings, valve, laterals, orifices, plus head at end of lateral (2 feet). Pump capacity is 20 gpm for 1-bedroom dwelling (150 gpd) and 0.25-inch diameter orifice spaced 30 inches on center; 36 gpm for 2-bedroom, 54 gpm for 3-bedroom, and 70 gpm for 4-bedroom. For 7/32-inch diameter orifice, use a 15-gpm pump for 1-bedroom dwelling, 28-gpm for 2-bedroom, 41-gpm for 3-bedroom, and 54-gpm for 4-bedroom.

Based on historic experiences with sand filters, sands with effective size less than 0.2 to 0.35 mm can be expected to clog with a dosage of 1 to 1.5 gpd/ft2. Also, during construction, compaction of the sand fill and the natural soil under and around the dispersal area should be kept to a minimum.

Electric Osmosis System

In this process, septic tank effluent discharged to a conventional subsurface absorption system in a soil having a percolation slower than 1 inch in 60 minutes is disposed of by evapotranspiration. Mineral rock-filled anodes adjacent to the trench and coke-filled cathodes with graphite cores a short distance away generate a 0.7 to 1.3 V potential, causing soil water, claimed to be removed by evapotranspiration, to move to the cathodes. These systems have been used successfully in several states.

Septic Tank Evapotranspiration System

An evapotranspiration system can be used, when the available soil has no absorptive capacity or where little or no topsoil exists over clay, hardpan, or bedrock, provided that a water balance study shows the evapotranspiration plus runoff exceed precipitation infiltration plus inflow. It can also be used when the ground-water level is high, provided the system is provided with a watertight liner on the bottom and sides to exclude the groundwater from the transvap bed. If an impermeable liner is not provided, elevation of the bed or curtain drains may be necessary if seasonal high water is a problem. The design of a transvap system is based on maintenance of a favorable input-output water balance.

Evaporation from water surfaces varies from about 20 inches per year in the northeastern United States to 100 to 120 inches in some southwestern areas, and that evaporation from land areas will be approximately one-third to one-half these values. Brandes found that over a 15-month period, 58 percent of the total precipitation on a sand filter in Ontario, Canada, left the filter through evapotranspiration.30 This value is significantly higher than the average values for hydrologic water recycling given historically by McGauhey:31

Evaporation 30 percent

Evapotranspiration 40 percent (from soil mantle)

Surface runoff 20 percent

Groundwater storage 10 percent

Successful operation of a transvap system is largely dependent on runoff, surface vegetation, soil cover, capillarity, and evapotranspiration, in addition to controlled wastewater flow to maintain a favorable water balance. Plant roots can reach a depth of about 24 inches in well-developed absorption beds and take up wastewater. Maintenance of a permeable soil structure and microbial and macroscopic organisms is essential to minimize system clogging and failure, as previously explained.

Figure 3.11 provides design and construction details of a transvap disposal system, which uses sand and gravel beds to provide storage during the periods when transpiration and evaporation is low or zero. The sand ridges and sand bed are essential to provide capillarity. Soil evaporation can average one-third to one-half of lake evaporation for 6 months of a year in which average lake

Oil Grit Separator Unloading Areas

FIGURE 3.11 Transvap sewage disposal system in tight soil. (Raise bed as necessary if groundwater or bedrock is a problem.) Clean washed sand, 0.1 mm effective size for up to 12 to 16 in. gravel and sand depth, and 0.05 mm sand for up to 24 in. gravel and sand depth. Sand ridges are necessary to obtain capillarity and promote evapotranspiration. Permeable geotextile fabric is recommended over the sand ridges and in place of the 6 in. of sand over the gravel. Add 6- or 8-in.diameter perforated risers in and to bottom of gravel bed for inspection and emergency pump-out. Pressure distribution is usually required. Silt in sand will increase capillary rise.

FIGURE 3.11 Transvap sewage disposal system in tight soil. (Raise bed as necessary if groundwater or bedrock is a problem.) Clean washed sand, 0.1 mm effective size for up to 12 to 16 in. gravel and sand depth, and 0.05 mm sand for up to 24 in. gravel and sand depth. Sand ridges are necessary to obtain capillarity and promote evapotranspiration. Permeable geotextile fabric is recommended over the sand ridges and in place of the 6 in. of sand over the gravel. Add 6- or 8-in.diameter perforated risers in and to bottom of gravel bed for inspection and emergency pump-out. Pressure distribution is usually required. Silt in sand will increase capillary rise.

evaporation is 30 inches/year. Sublimation during the snow-covered nongrowing season, although small, can contribute to moisture removal from the system.

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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