Sunshine Butterflies - Attachment D - 010816-2

                                OUR BACKYARD
                                468 MCKINNON DRIVE, COOROIBAH

7.0      ON-SITE EFFLUENT DISPOSAL
An assessment of the type and configuration of wastewater system(s) that is most suited to this development
requires:
       An estimate to be made regarding the probable wastewater flows;
       Evaluation of the capacity of the soil for long-term uptake of effluent; and
       Consideration of siting requirements for the land application (disposal) system.

7.1       Wastewater Flow Design Allowance
We are advised by the client that the ultimate site usage (peak) may comprise:
       20 children
       20 staff
       20 work for the dole personnel

All of these people will be non-resident.
Based upon wastewater flow rates given in the Code of Practice for On-Site Wastewater Management (July
2014), we have estimated the proposed total wastewater flows as:
       60 persons x 30 L/person/day = 1,800 L/day (9 equivalent persons at 200 L/day/EP)

It can be seen that the total site development of 9 EP will not trigger an Environmentally Relevant Activity
(ERA), being much less than 21 EP, and therefore does not require referral to the Department of
Environment and Heritage.

7.2       Terrain-Soil Assessment
The capacity of the soil profile together with the topography of the site is assessed so that the required area
for wastewater land application can be determined. Based upon the proposed development layout, and
proximity to sensitive receptors, the central southern part of the site (area of borehole BH3 and BH4) has
been chosen as the most suitable location for on-site effluent disposal.
The terrain across the site varies slightly, but is mostly Linear Planar across the proposed effluent disposal
area, in accordance with the slope configurations described in Figure C2 of AS1547:2012, which also
describes the relationship between the shape of the slope and surface drainage. Such slopes do not tend to
accelerate or spread run-off out.
Soil profiles encountered in boreholes BH3 and BH4 comprise loose to medium dense silty sand to depths of
0.35 m to 0.4 m BGL (Soil Category 2), then medium dense clayey sand to depths of 0.55 m to 0.7 m BGL
(Soil Category 4), overlying stiff sandy clay (Soil Category 6) to the depth of investigation. It has been
assessed that the overall soil profile across the proposed effluent disposal area comprises a highly
structured Clay Loam (Soil Category 4), in accordance with AS1547:2012.

7.3       Recommendations
7.3.1        Treatment & Land Application
It is proposed to direct all wastewater from the proposed development to an ‘All-Waste’ septic tank located
centrally on the eastern side of the proposed gardens (down slope from house). For an equivalent site
population of 9 persons, Table J1 of AS1547 recommends a 4,500 L septic tank.
AS1547:2012 recommends a Design Loading Rate (DLR) of 15 mm/day for Category 4 soil, with Primary
(septic) treatment. As shown in Text Figure 8 on the following page, it is proposed to dispose of effluent from
the new septic tank via a series of three parallel absorption beds (total 60 m in length), positioned in the
proposed orchard area. Each bed shall be 2 m wide, 20 m long, with minimum 2 m side wall to side wall
spacing. Total bed length has been calculated as follows:
L = Q (w x DLR)             L = 1,800 / (2 x 15) = 60 m




4 May 2016
Report No. 16081-001-Rev0                                 9