simon
New Member
Posts: 19
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Post by simon on Oct 4, 2018 10:03:52 GMT 12
Hi,
I've always been confused by the DIR differences between TP58 and 1547 as seen in Table 9.2 of TP58 and Table M1 of 1547. Luckily I've only worked in Auckland so normally use TP58 and usually only dealt with Cat 5or 6 soils, but where councils have a choice it really is a major discrepency in values. I see GD06 has come back in alignment with 1547 (Table 50), can anyone comment on why they were different in the first place, 1547 and GD06 does seem very conservative when permeability has increased by up to 50 times ( from Cat 5 to Cat 1) yet DIR is less than doubled, I would have thought TP58 was more realistic at about 10 times DIR
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Post by techworks on Oct 8, 2018 9:51:24 GMT 12
I was hoping someone else would chip in on this question. Perhaps someone who was involved in the NZS 1547 standard and knows a bit of the history behind the drafting of NZS 1547.
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. Yes there is some NZ research and field trials that have been used to support some design parameters such as the National OSET Trials, and the recent Opus project to monitor Marae wastewater loadings, but in general I believe the design parameters come from overseas research.
I suspect TP58 had a stronger US influence, whereas NZS 1547 obviously had a strong influenced from Ausi. I can only assume these influences contributed to the different correlations between DIR and soil categories.
Another point to consider, there does not seem to be any real qualification for the significantly lower loading rates provided for "irrigation" systems compared to "soakage" trench/bed system. Sure irrigation is designed on arial loading whereas trench/beds are basal loading, but that does not account for a 5mm/day loading versus 20mm/day. Perhaps as there is no aggregate to distribute effluent with sub surface irrigation, it relies on capillary action within the topsoil to distribute the effluent. This could be the main reason for the lower loading rates but I've not read any discussion on that point.
If you look at the Bay of Plenty Regional Council's current OSET Plan, that specifically requires sub-surface drip irrigation as the only options for AWTP systems as a permitted activity. You need a resource consent if you want to discharge to trenches or beds.
As irrigation results in a much lower loading rate on the land than trench/bed systems this rule is effectively building an extra environmental safety factor into the design for AWTP's under the BOPRC OSET Plan. I note the proposed Change 14 has now separate treatment and land application rules so you will be able to mix and match AWTP with trench/beds as a permitted activity.
Sorry Simon, I havnt actually answered your original question!
Cheers
Grant
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Post by techworks on Feb 11, 2019 8:10:35 GMT 12
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