Reference Documents

                                         PROPOSED SUBDIVISION
                                         41 TALLOWWOOD PLACE, BLACK MOUNTAIN

            7.2         Wastewater Flow Design Allowance
            For a typical four-bedroom house (or three bedrooms plus study), we have estimated a peak occupancy of
            seven persons. This assumption is consistent with current NC development standards and has been used to
            demonstrate feasibility of on-site effluent disposal. Should the proposed new residence 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.

            7.3         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.
            The terrain across the proposed new building area is categorised as Linear Planar in accordance with the
            slope configurations described in Figure C2 of AS1547:2012.
            Ground surface gradients typically range from 15° to 17° (up to ~30%) across the proposed effluent disposal
            area. Sizing of the likely wastewater irrigation area on the proposed new lot will therefore need to incorporate
            a slope reduction rate, as described in Table M2 of AS1547:2012, or shall be effectively terraced as
            necessary to reduce slope to < 10% negating the effect of natural slopes. The addition of a slope reduction
            rate will result in an increase to the land application area of approximately 50% to facilitate sloping ground
            conditions.
            It has been assessed from the results of the investigation that the soil profile across the likely effluent
            disposal area on proposed Lot 2 comprises predominantly silty clay, with gravel throughout, which is
            equivalent to Light Clay (Soil Category 5). This categorisation has been derived via the method described
            in Table M1 of AS1547:2012.
            Topsoil materials obtained from excavations or imported to site, should be used to supplement the soil profile
            across the proposed irrigation area. Such materials could be used to form narrow mounds/swales along the
            contours within the irrigation areas.

            7.4         Recommendations
            7.4.1           System Type
            It is recommended that wastewater from the new lot be directed to a suitably sized Household Sewage
            Treatment Plant (HSTP) connected to an irrigation system. The HSTP must be licensed to produce at least
            Secondary standard effluent, with Chief Executive approval from the Department of Housing and Public
            Works; however, to maximise available residential space and to reduce setback requirements, it may be
            necessary/desired to use an Advanced Secondary treatment system.

            Table M1 of AS1547:2012 recommends a Design Irrigation Rate (DIR) of 3 mm/day for Category 5 soil.
            Therefore, a minimum irrigation area of about 280 m² (840 L/day / 3mm/day) would be required. However, an
            additional 50% slope reduction rate for application areas with slopes between 20-30% has been applied in
            accordance with Table M2 of AS1547:2012. The calculated land application area for is therefore 560 m².


             22 May 2023
             Report No. 23099-001-Rev0                             20

Document Set ID: 22826802
Version: 1, Version Date: 23/02/2024