Following last week’s Blog Post mentioning the delay in release of the Auckland Council on-site wastewater management guideline document GD06, correspondent Joe Whitehead of Cardiff NSW has advised On-Site NewZ of the availability of the latest version (2019) of the Australian design manual entitled “WaterNSW Current Recommended Practice: Designing and Installing On-Site Wastewater Systems” (access via link below).
The original recommended practice manual issued in 2012 was based on a report to the Sydney Catchment Authority (now WaterNSW) by Joe’s consultancy (Whitehead & Associates Environmental Consultants Pty Ltd) together with Kerry Flanagan Wastewater of Grose Vale NSW. Joe was also closely involved in preparation of the November 2019 revision
Notable changes between 2012 and 2019 versions
(a) Linear Loading Rates: The concept of design sizing for land application systems based around linear loading rates (LLR) came out of work by Dr Jerry Tyler (et al) of the Small Scale Waste Management Project, University of Wisconsin-Madison, USA. Recommended LLR values are specified in litres per metre length of installed system along the contour. It relates to any type of land application system – in current NZ practice it is only applied as a toe loading limit in sizing Wisconsin Mound systems.
When effluent is applied to soil it will move vertically and horizontally away from the infiltration system until it meets a hydraulically limiting layer. The LLR is set at a limit to ensure that the effluent will not break out to the surface below the land application area as it moves downslope after reaching the limiting layer. It is used to size and configure the land application system in conjunction with effluent design loading rate (DLR or DIR) for the proposed system. LLR values are set according to soil depth and type along with ground slope. Highest LLR values are associated with deep and courser sandy soils on the steeper slopes, thus resulting in shorter system length along the contour. Lowest LLR values are associated with shallow clayey soils with flat to negligible slope, thus resulting in longer system length along the contour. The LLR is applicable for any land application system and any effluent quality (septic effluent or AWTS effluent).
The Pages Sidebar of this Blog under the title “Design Irrigation Rates – Origins and Development in On-site Wastewater Manuals and Standards” provides an example on application of LLR values in design on pages 14/15 of the downloadable report.
In respect of the 2019 WaterNSW manual there has been a relaxing of the limits imposed by LLR rates on system sizing with the rates being lifted between 25% to 30% (in some instances 50%). The original 2012 version of the manual used a table of LLR values from British Columbia (2006). The British Columbia Standard Practice manual design values were subsequently (2007) increased to those proposed by Tyler (2001) in order to provide more flexibility in design with a less conservative approach to establishing system length along the contour. The WaterNSW 2019 manual has also adopted the higher (therefore less conservative) Tyler values.
(b) Buffer distances: Some changes have been made to minimum buffer distances (clearances or setbacks) to site features, including a tightening up on separation distance from base of land application area to limiting layer. The 2012 version allowed a variation of 300mm to 600mm clearance to limiting layer – this has been amended to “should be a minimum of 600mm for all systems”.
(c) Emerging Technology: The 2012 manual provided information on dosing syphons, floating outlet (flood loading) units and solar power. Solar power was not accepted for treatment systems requiring continuous electricity supply, such as AWTS (aerated wastewater treatment systems). It could, in exceptional circumstances, be utilised for intermittent use such as sump pump-outs.
In the 2019 manual WaterNSW accepts solar power for systems “where continuous power is needed for normal operation, such as an AWTS, only when a detailed analysis is provided as part of the application” for approval.
(d) Supplementary Technology: The “emerging technology” section of 2012 has now been replaced by an expanded “supplementary technology” section. The covers an overview (without design details) of the following systems:
• constructed wetland treatment
• membrane treatment
• textile filter treatment
• sand filters
• subsurface media treatment (the AES system)
• buffering/balance tanks
• dosing mechanisms.
Link to the 2019 manual: