4
3. DESIGN PARAMETERS.
Select area.
Select the area with careful consideration of the soil, the terrain and your State and Council regulations.
Soil Application Design
Note: This paragraph is extracted from "Subsurface Trickle Irrigation System for On-Site Wastewater Disposal and Reuse"
by B. L. Carlile and A. Sanjines. The rules in your shire council and state may vary.
"The instantaneous water application rate of the system must not exceed the water absorption capacity of the soil.
A determination of the instantaneous water absorption capacity of the soil is difficult, however, since the value varies with
the water content of the soil. As the soil approaches saturation with water, the absorption rate reduces to an equilibrium
rate called the "saturated hydraulic conductivity." Wastewater application rates should be less than 10 percent of this
saturated equilibrium
Even though the trickle irrigation system maximizes the soil absorption rate through the low rate of application, thus
keeping the soil below saturation, there will be times when the soil is at or near saturation from rainfall events. The design
must account for these periods and assume the worst case condition of soil saturation. By designing for a safety factor of
10 or 12, based on the saturated hydraulic conductivity, the system will be under-loaded most of the time but should
function without surface failure during extreme wet periods.
Using a safety factor of 12, a suitable design criteria would be to load the system at the estimated hydraulic conductivity
but apply water for only a total of 2 hours per day out of the available 24 hours. By applying wastewater for a total of 2
hour per day, particularly if applied in "pulses" or short doses several times per day near the soil surface where the soil
dries the quickest, this would keep the soil absorption rate at the highest value and minimize the potential of water
surfacing on poor soil conditions.
As stated previously, this design criteria will under-load the system at all times except when the soil is at or near saturation
from rainfall. If designing for an efficient irrigation system, the water supply may not be sufficient to meet the demands of
a lawn or landscaped area during peak water demand months. This problem can be overcome by either of two solutions:
add additional fresh-water make-up to the system during the growing season to supply the needed water for plants in
question; or split the system into two or more fields with necessary valves and only use one of the fields during the peak
water demand months and alternate the fields during winter months or extremely wet periods
Table 2 shows the recommended hydraulic loading rates for various soil conditions, using a safety factor of 12 with regard
to the equilibrium saturated hydraulic conductivity rate of the soil. These loading rates assumes a treated, disinfected
effluent with BOD and TSS values of less than 20 mg/l is produced in the pre treatment system.
Table 1. Minimum surface area required to dispose of 1000 litres per day
Soil absorption rates
Design
Total
Wasteflow
Wasteflow
Soil
Est.Soil
Hydraulic
Hydraulic
Area
tube reqd m
flow rate
type
Perc.
Conduc-
Loading
required
@ 60 x 60 cm
litres/hr
rate
Conductivity
rate
m2 / 1000 l
spacing
@ 100 kPa
min/25 mm
mm/hr
mm / m2-day
per day
4.3 l/hr
Coarse-
sand
<5
>50
81
13
22
158
Fine
sand
5-10
38-50
65
16
27
194
Sandy loam
10-20
25-38
53
19
32
230
loam
20-30
19-25
37
27
45
323
Clay
loam
30-45
12.5-19
24
42
70
502
Silt-clay loam
45-60
7.6-12.5
16
63
105
753
Clay non-swell
60-90
5-7.6
8
125
210
1510
Clay - swell
90-120
2.5-5
4
250
420
3010
Poor clay
>120
<2.5
3
334
560
4020
