Origins of On-site Wastewater Design Irrigation Rates

Following the February 2019 post on the origins of on-site wastewater design loading rates a correspondent asked if On-Site NewZ would provide a similar paper setting out the origins of design irrigation rates. This task has now been completed and the resulting paper can be accessed from the Pages Sidebar of this Blog under the title “Design Irrigation Rates – Origins and Development in On-site Wastewater Manuals and Standards”.

The paper commences with information from the NZ TP58 design manual 1989, then covers TP58 2nd edition 1994, the Australian Standard AS 1547-1994, the Joint Australia-NZ Standard AS/NZS 1547:2000, TP58 3rd edition 2004, the revised and updated AS/NZS 1547:2012, and finally the draft GD006 2018 on-site wastewater management guideline from Auckland Council which when published will replace TP58. A plot of DIR values versus soil texture and structure categories is provided as a summary of the DIR values from the above sources.

The second part of the paper looks at the development of drip dispersal hydraulic loading rates in the US where the use of drip emitters was employed to distribute household wastewater effluent into the soil for subsurface infiltration rather than irrigation. The objective was to maximise inflitration into the soil throughout the year rather than meet the agronomic needs of crops/plants. Hence the hydraulic loading rate values for specific soils are very much higher than the DIR values for the same soils in Australia-NZ on-site wastewater practice.

Drip dispersal hydraulic loading rates are tablulated and plotted from some 10 sources in the US – research agencies, commercial suppliers of dripline and state administrations. Over time the high initial design values in the US have been scaled down closer to our local values.

A final summary plot of US values vs Australia-NZ values compares the high non-conservative values in the US with conservative US values and our local DIR values over all soil/structure categories.