SUBSURFACE DRIPLINE FOR ONSITE WASTEWATER DISPERSAL AND REUSE
12" SPACING IDEAS FOR DESIGN
Geoflow now offers two new products with 12" spacing between emitters.
WF-16-4-12: WASTEFLOW Classic 12" between emitters and 1 gph flow rateand
WFPC-16-2-12 WASTEFLOW PC 12" between emitters and ½ gph flow rate
Geoflow’s decision to use 1 gph for the Classic and ½ gph for the PC was based upon tests by the Center of Irrigation Technology to ensure that Geoflow WASTEFLOW continues to be the product with outstanding resistance to clogging compared to all other competitive products. These test results are available upon request.
SOME IDEAS ON WHEN TO USE 12" SPACING
We stress that the following are ideas obtained by observation of many field trials.
Conditions from one location to another will vary significantly and we strongly urge that any design based upon these ideas be reviewed by a competent soil scientist.
We must consider each point of emission as an application point of bacteria and must not overload the soil in the immediate vicinity of the dripper. Overloading the soil can create a biological mat around the emitter causing the effluent to erupt, like a volcano, along the path of least resistance to the surface.
We currently use the following table for BOD < 20 mg/l
Table 1. Minimum surface area required to dispose of 100 gpd using WASTEFLOW dripline with 24" between drippers and 24" between lines
|Soil absorption rates||Design||Total|
|type||Perc. rate||Conductivity||Loading rate||requiredft2 / 100gal|
|min/in||in/hr||gal / ft2-day||per day|
|Clay - swell||<90-120||0.1-0.2||0.1||1040|
- Area for the ability of the soil to percolate the volume of water, for evapotranspiration to reduce deep percolation or seepage, and to meet the present expectations of the regulators
- Number of drippers to handle the BOD which has (for practical purposes) a linear relationship to the BOD. That is, if we increase the BOD above 20 mg/l for the same volume of effluent we do not need more area but we do need more drippers in the same area.
For example we wish to dispose of 300 gpd of effluent with a BOD = 60 mg/l on a clay loam.
From the Table 1 above the area required is 175 sq ft per 100 gpd -> 525 sq. ft.
The BOD of 60 mg/l requires 3 times as many drippers as does 20 mg/l, hence either use 24" dripline and place the driplines 8" apart instead of 24" apart, or use dripline with 12" between drippers and space the lines 16" apart. This latter solution requires about half the labor to install as the first solution, the total cost of WASTEFLOW is lower, and the drippers are spaced more uniformly through the area.
Table 2 in combination with Table 1 below allows the soil scientist to determine the optimum area and number of emitter for the site
Minimum surface area required to dispose of 100 gpd
Minimum number of drippers in the 100 gpd area
|Soil absorption rates||Design|
|type||Perc. rate||Conducivity||Loading rate||requiredft2 / 100gal||BOD (mg/l)|
|min/in||in/hr||gal/ft2 /day||per day||#||#||#||#|
|Clay - swell||90-120||0.1-0.2||0.1||1040||260||390||780||1560|
HEAVY CLAY SOILS
Heavy clay soils requiring a hydraulic loading rate of <0.1 gallons/sq.ft./day do not use the full area with drip irrigation when the drippers are spaced 24" x 24" because the water is unable to move more than about 6" away from the dripper. This is evidenced by the growth pattern of the grass growing on top of such a system. A reduction of the spacing to 12" x 12" would allow a substantial reduction in total area required. In theory this may well be a 4 fold reduction. Under these circumstances we would recommend using the WASTEFLOW PC with 12" spacing and the ½ gph emitters and reduce the volume of each dose to as small as is practical in order to reduce the instantaneous application rate to as low as possible. Of course in the case of heavy clay soils and BOD > 20 mg/l one may have to use the full area as set out in Table 1.
It is not practical to apply the logic for clay soils to sandy soils to reduce the area required because the application rates are already well up to a practical limit and are beyond evapotranspiration and the long term ability of the plants to absorb the nitrates and phosphates may be exceeded. However if the client wishes to obtain a perfect turf cover and/or 100% absorption of nitrates is important then a design such as 12" between drippers and 12" - 18" between driplines may be suitable
There may be unacceptable risks in disposing of high BOD effluent in conditions of high water tables or in single family home systems which are not adequately controlled and/or maintained. Conditions from one location to another will vary significantly and we strongly urge that any design based upon these ideas be reviewed by a competent soil scientist.