The “Nz-onsite-wastewater” forum (http://nz-onsite-wastewater.freeforums.net/) created in August 2018 includes a discussion thread relating to the variation in design loading rates (DLR) for land application of treated effluent where a new forum member raises questions re differences in DLR/DIR values in TP58, AS/NZS 1547 and those in the proposed new GD006. The Forum Administrator, Grant Hammond of Tauranga, makes the following comments on this matter:
It would be nice to think that our national design standards were all based on first principles of engineering design, but unfortunately all too often they include design parameters and fudge factors that have never been proved and are often derived from an assumption that was based on a prior assumption! TP58/GD06 and NZS 1547 are all primarily based on desktop compilations of other design standards and research.
I suspect TP58 had a stronger US influence, whereas NZS 1547 obviously had a strong influenced from Aussie. I can only assume these influences contributed to the different correlations between DIR and soil categories.
Perhaps someone who was involved in the NZS 1547 standard and knows a bit of the history behind the drafting of NZS 1547 could chip in on this question
As the author of TP58 Editions 1 and 2 (1989 and 1994) and as a participant in the Joint Standards committees that produced the 2000 and 2012 versions of AS/NZS 1547, On-Site NewZ has some insight into the process of drawing up design parameters. A discussion on this topic entitled “Design Loading Rates – Origins and Development in On-site Wastewater Manuals and Standards” can be accessed from the Pages Sidebar of this Blog.