7.3.1 GENERAL
To determine the minimum requirements for a subsurface effluent disposal system it is necessary to calculate the required contact area in square metres.
The formation of the bio-mass on the soil contact surfaces within the soakage system is a limiting factor on the effluent percolation rate. The cumulative effect of the bio-mass is greater in cohesive soils such as clay, silts and fine sands, the exception being when the soil is in the limiting factor;
e.g. heavy clay or rock etc.
The effluent percolation rates shown in Chapter 11 - Table 3, take into account the effect of the biomass, and are set as the effective long term effluent percolation rates (EPR).
The EPR is expressed in litres/m2/day.
To calculate the required contact area for a subsurface soakage system use the formula:
P2 x DF
EPR = Required contact area in m2 for the total daily inflow Where...
P2 = number of persons using the system. In the case of residential/industrial or commercial premises, P2 shall be not less than 6 persons,
DF = daily inflow in litres per person per day,
EPR = effluent percolation rate (Refer to Chapter 11 - Table 3).
7.3.2 FOR SINGLE & MULTIPLE RESIDENTIAL DWELLINGS
A range of options is available to determine the minimum contact area required for the soakage system for residential dwellings (all waste system). The minimum contact area is determined as above, and calculated according to the type of water supply serving the premises as this alters the daily inflow rate (DF). (Refer to Chapter 11 - Table 4 or 5)
To determine the minimum subsurface soakage requirement proceed as follows:
• using the information obtained by either the Indirect Method or the Direct Method for determining the long term percolation capability, refer to Chapter 11 - Table 3 and obtain the appropriate effluent percolation rate (EPR),
• refer to Chapter 11 - Table 4 or 5, depending on type of system to be installed, and according to:
Type of water supply
• where the water supply is via a private or government reticulated system the daily in-flow rate is 150 litres/person/day,
• where the water supply is by means of roof catchment storage or stored carted water the daily in-flow rate is 125 litres/person/day.
C H A P T E R 7 : T H E E F F L U E N T D I S P O S A L S Y S T E M
Effluent percolation rate
• where the percolation rate is determined by the indirect method and is classified as:
-
practically impermeable - the EPR will be equal to or less than 10 litres/m2/day depending on the percolation rest result,-
poor soakage - the EPR will be 10 litres/m2/day,-
good soakage - the EPR will be 15 litres/m2/day.• where the percolation rate is determined by the direct method to be:
-
between 15 mm and 23mm - the EPR will be 10 litres/m2/day,-
greater than 23 mm - the EPR will be 15 litres/m2/day,-
between 15mm and 10mm the EPR of 10 litres/m2/day will be reduced by 1.5 litres/m2/day for each 1mm below 15mm. Refer to example in Chapter 11 - Table 3.EXAMPLE
A residential dwelling serving up to 6 persons and having a reticulated water supply and soil classified by the indirect method as having an EPR of 10 litres/m2/day would require a subsurface disposal system sized as follows:
P2 = 6 persons
DF = 150 litres/person/day EPR = 10 litres/m2/day P2 x DF
EPR = 6 x 150
10 = 90 m2 contact area
EXAMPLE
A residential dwelling serving up to 8 persons and having a rainwater supply and a soil classified by the direct method to have a percolation rate of 55mm/hr would require a subsurface disposal system sized as follows:
P2 = 8 persons
DF = 125 litres/person/day
EPR = 15 litres/m2/day (ie >23mm/hr) P2 x DF
EPR = 8 x 125
15 = 66.7 m2 contact area
EXAMPLE
A residential dwelling serving up to 6 persons and having a reticulated water supply and a soil classified by the direct method to have a percolation rate of 13mm/hr would require a subsurface disposal system sized as follows:
P2 = 6 persons
DF = 150 litres/person/day
EPR = 7 litres/m2/day (reduced by 1.5 litres/m2 for every 1mm below 15mm, see Chapter 11 - Table 3 for details).
C H A P T E R 7 : T H E E F F L U E N T D I S P O S A L S Y S T E M
NOTE: (1) Where Chapter 11 - Tables 6 to 9 show trench lengths in metres these are to be taken as the standard trench dimensions as indicated in Section 7.4.3 of this Standard.
(2) Where the direct method or indirect method indicates a soil horizon having the appropriate percolation capability it is necessary that the effluent disposal systems is installed within the nominated horizon and it has a depth of at least 500mm extending below the base of the selected disposal system.
(3) In situations where the discharge from the septic tank to the soakage trench positioned in the appropriate soil horizon cannot be achieved by gravity flow, then the effluent shall be lifted from a pump sump, with a motor operated pump, to the disposal system.
7.3.3 FOR NON-RESIDENTIAL PREMISES
To determine the minimum subsurface soakage requirements use the formula as shown in Section 7.3.1 General Provisions and proceed as follows:
• using Chapter 11 - Table 1 obtain the appropriate P2 and DF values for the intended use,
• where appropriate it may be necessary to calculate a range of use factors for a single installation, and add them to obtain the total requirements,
• using the information obtained from either the Indirect or Direct Method for determining the long term percolation capability and Section 7.3.1 General Provisions, refer to Chapter 11 -Table 3 to obtain the appropriate effluent percolation rate (EPR),
• having established the minimum contact area in square metres refer to Section 7.4 System Construction to determine the disposal system requirements.
NOTE: The minimum contact area for a subsurface soakage system for any non-residential premises shall be 30 square metres or the effective area as calculated, whichever is the greater.
EXAMPLE
A warehouse/store with four employees working only 1 shift (no shower facilities provided) and served by a reticulated water supply and having a soil classified by the indirect method to have an EPR of 10 litres/m2/day would require a subsurface disposal system sized as follows:
P2 = 6 persons (minimum number) DF = 30 litres/person/shift (1 shift only) EPR = 10 litres/m2/day
P2 x DF
EPR = 6 x 30
10 = 18 m2
As this is less than the minimum contact area, 30 m2 is required
7.3.4 TEMPORARY OCCUPATION
For caravan parks and holiday camps, not permanently occupied, one of the following formulae should be used. These formulae are only used for situations of temporary occupation - i.e. where the maximum continuous period of full occupancy does not exceed 42 days in any 120 day period.
If the period of full occupancy exceeds these criteria, then calculate the requirements as for permanent occupancy. An occupancy rate exceeding 60 per cent is considered to be full occupancy.
NOTE: These provisions do not relate to the sludge scum capacities of the septic tank.
C H A P T E R 7 : T H E E F F L U E N T D I S P O S A L S Y S T E M
Formulae
(1) where the EPR is 10 litres/m2/day:
P2 x DF
10 x 75
100 = required contact area in m2 (2) where the EPR is 15 litres/m2/day:
P2 x DF
15 x 85
100 = required contact area in m2