Reference Document - OPW25 0085 (AS Published)

7.           Wastewater Treatment and Disposal
An assessment of the type and configuration of wastewater system that may be suitable for a proposed
development requires:
    Assumptions to be made regarding the typical building/visitor configuration and probable
     wastewater flow; and
    Evaluation of the terrain-soil capacity to accept treated wastewater; and
    Consideration of NC wastewater treatment and disposal system construction and siting
     requirements.


7.1          Wastewater Flow Design Allowance
For a typical four-bedroom house (or three bedrooms plus study), we have estimated a peak occupancy
of seven people. This assumption is consistent with current NC development standards and has been
used to demonstrate feasibility of on-site effluent disposal. Should the proposed future residences
exceed this size, it is the owner’s responsibility to demonstrate feasibility of the design, meeting all
relevant regulations.
For each occupant, a typical wastewater flow allowance of 120 L/person/day has been taken from
Appendix H of AS1547:2012. This represents the flow allowance for households with standard water
reduction fixtures (including duel flush toilets, shower flow restrictors, aerator faucets and water
conserving automatic washing machines) serviced by rainwater tank supply. We therefore estimate a
typical daily household wastewater flow rate of 840 L/day (7 x 120 L/person/day).
It should be noted that wastewater flow rates may be further reduced by the adoption of full water
reduction facilities (e.g. reduced flush 6/3 L toilets, shower flow restrictors, aerator faucets, front-load
washing machines and flow/pressure control valves on all water-use outlets, etc.). Conversely, the use of
extra wastewater producing fixtures (e.g. spa baths) has the potential to increase the household
wastewater flow rate.
Final wastewater flow rates must be calculated at the time of detailed design of the wastewater system,
at building stage once the house design is known.


7.2          Terrain-Soil Assessment
The terrain-soil combination(s) encountered at a site are assessed so that an Expected Available Area
(EAA) can be delineated for suitable methods of wastewater treatment and land application.
It is anticipated that residential building areas will be suitably positioned near the crest, or across
elevated consistent slopes, within each respective lot. The terrain across the likely effluent disposal areas
associated with probable residential dwellings may vary, however it is expected that typically Linear
Planar areas available, with low to moderate potential for diverging or converging terrain. Such slopes
have less effective natural drainage as distance from the crest increases, with no spreading or
acceleration of run-off as it moves down slope.
It has been assessed from the results of the investigation that the soil profile across the likely building
and effluent disposal areas are consistently silty and sandy clay soils across most of the site, with some
sandier soils encountered on proposed lots 1 and 2. Based on this understanding a conservative
characterisation has been applied to all sites equivalent to Light Clay (Soil Category 5). This



Geo-Environmental Investigation for Proposed Subdivision
7 February 2025 | 24386-001-Rev0                                                                              23