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.


Time to lift the game around On-site Wastewater Management

On-Site NewZ has received feedback regarding the 28 March post entitled “Time to lift the game around On-site Wastewater Treatment”.

There appears to have been some editorial confusion in reporting on the discussions at the Christchurch workshop in which the Water Journal article referred to “treatment units” being the subject of the proposed “best practice guide”.

A correspondent to On-Site NewZ noted that

“at Christchurch we were more concerned with on-site wastewater management and recognised that treatment was only one factor in this and somewhat minor compared with investigations and overall design, designers capability, Council process approvals, operation and maintenance, ongoing service contracts and reporting and so on”.

The writer indicated that

“the meeting clearly confirmed that treatment and OSET-NTP certification was only one aspect of the bigger picture of on-site wastewater management. For example Andrew Dakers was critical of Council approval of OSET certified plants in that he then found Council officers approving an overall design because it included an OSET certified plant, without considering the overall site investigation and design”.

Hence the commitment by Water New Zealand is for the development of a Best Practice Guide for On-site Wastewater Management which will focus on a “whole of system” approach.

The Pages Sidebar item of 26 March has now been replaced by an On-Site NewZ amended version of the Water Journal article which reflects the comments received above, the new title being “Time to lift the game around On-site Wastewater Management”.

Time to lift the game around On-site Wastewater Treatment

One of the outcomes of a recent one-day workshop in Christchurch to discuss on-site wastewater management practice in New Zealand was a commitment by Water New Zealand to develop a “Best Practice Guideline for On-site Wastewater Treatment Units”.

A report on the workshop by Noel Roberts, Water New Zealand Technical Manager, summarises key elements from the day’s discussions, citing concerns over high numbers of failing units and deficiencies in installation, maintenance and regulatory oversight.

Andrew Dakers of ecoEng in Christchurch presented a case for an approach that went beyond reliance on just the performance testing on treatment units as is carried out at Rotorua under the OSET-NTP (On-site Effluent Treatment National Testing Programme). Andrew advocates a whole of system approach involving evaluation and mitigation of individual site risks associated with the both the treatment unit and land application system, and including design, installation operation and maintenance.

Other issues raised at the workshop included a warrant of fitness for on-site wastewater systems, and national certification for installers and designers.

To read Noel’s report as published in the March/April 2019 issue of “WATER”, go to the Pages Sidebar of this Blog under the item “Time to lift the game around On-site Wastewater Treatment”.


Origins of On-site Wastewater Design Loading Rates

The “Nz-onsite-wastewater” forum ( 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.

Auckland Council Guideline Document: On-site Wastewater Management in the Auckland Region [GD06-2018] – Updated Draft, September 2018

Dr Gretel Silyn Roberts, Principal – Technical Guidance, Engineering and Technical Services, Infrastructure & Environmental Services at Auckland Council, has advised On-Site NewZ of the issue of an updated draft of GD06 as follows.

Thank you to all those who have provided initial feedback on GD06 – On-site Wastewater Management in the Auckland Region. We very much appreciate the effort put into the review of this draft. The deadline for feedback is October 12th, but we would much appreciate you sending through any major issues or questions prior to this date so we can address them fully.

Update of amended draft:
Based on feedback to-date, and some suggested changes, we have updated the document on the Auckland Design Manual website. Most changes are adjustments to cross references, corrections and clarifications.

The key changes we would like you to be aware of are:

1. Section D: Wastewater treatment units –

(a) Adjustment to the text around Section D1.4.3 Septic tank sizing

(b) Restructuring of Section D1.5.5 – Types of system

  • Removal of trickling filter
  • Removal of single pass packed bed filter
  • Removal of multi pass packed bed filter
  • Removal of diagrams for all “Alternative treatment systems”
  • Removal of diagrams for all “Alternative toilet systems”

2. Section G: Risk – Additional comment around the need to design for climate change

Please do not hesitate to contact our team if you have feedback.

Gretel Roberts

To access a PDF version of the September 2018 update go to the Pages sidebar and the topic heading for this Blog Item.


Following the July 2018 release of the draft of GD06, the Auckland Council “Guideline Document: On-site Wastewater Management in the Auckland Region”, Grant Hammond of Tauranga has set up a ProBoards forum entitled “Nz-onsite-wastewater” at

Grant is a Bay of Plenty Regional Council approved on-site wastewater designer and a principal of Technologyworks Consultancy. In his introduction to the forum Grant says:

“We now have national standards AS/NZS 1546:2008 and 1547:2012 for treatment systems and land application systems and a national testing program for aerated wastewater treatment systems, plus numerous design manuals and guidelines produced by Territorial Authorities and Regional Councils….yet I seem to now have more questions than ever!

OSET designers come from a range of disciplines with varied practical and theoretical training and experience. Some come from a hands-on background (drainlayers), some from an engineering design background like myself, and others from the building and manufacturing industries.

We all have strengths and weaknesses and hopefully this forum will provide a technical resource where we can share our knowledge, and collectively answer some of these questions.”

The forum discussion topics to date are:
• General Discussion (are media and family rooms counted as bedrooms; uncontrolled trickle loading).
• GD06 July 2018 Draft for Consultation (the cost of design; Appendix M1.0 LPED design examples).

Membership of the forum is free and it is easy to register and set up your forum name and login password.



Auckland Council Guideline Document: On-site Wastewater Management in the Auckland Region [GD06-2018] – Community of Practice Presentation, August 2018

The Auckland Council Engineering and Technical Services team which managed the development of the new on-site wastewater guideline to replace TP58 held a “community of practice” meeting in Auckland on Friday 17 August to discuss the July 2018 draft of the document. A wide invitation was extended to representatives of the on-site wastewater industry in Auckland to participate in a discussion on the development of the document, significant differences between TP58 (2004) and GD06 (2018), and the next steps in consultation toward production of the final document and its placement within the Auckland Unitary Plan.

The PowerPoint presentation delivered at the meeting prior to open discussion is available from the Pages side-bar of this Blog under the topic of this Blog item. The presentation topics covered an overview of the document structure, key changes from TP58 (2004), content of the main sections of the guideline and the next steps in consultation toward publication. Feedback sessions on technical detail. operation and maintenance, consents and Unitary Plan changes were held following the presentation.

Key issues identified for community feedback were:
• slopes (including maximums for various land application systems);
• setback distances from land application areas to site features; and
• design loading rates.

Although the official consultation period continues through to 12 October the team managing this project would very much like major feedback to be sent through over the next 3 to 4 weeks (by mid-September please).

Take a look at the presentation material from the Pages side-bar along with the July Draft of GD06 (also accessed from the Pages side-bar) and get your comments away to the Council using the instructions from PowerPoint Slide No.5 to access the feedback form.