Trench®, born in 1999 and created using Microsoft’s Excel, was designed loosely around Australian Standard 1547:1994 Disposal Systems for effluent from domestic premises. If you are assessing sites for domestic wastewater in Tasmania, you will know the software has become an industry standard here. It’s used in some mainland states, too – notably Queensland, where it is also a teaching tool – and in New Zealand.
AS1547, in conjunction with our New Zealand counterparts, has changed twice in the intervening years (in 2000 and 2012), and some regulators and wastewater practitioners ask me when Trench® is going to catch up.
There’s no need. It’s true the software could be massaged to better suit the newer versions of Excel, but its fundamentals remain as valid as they did 14 years ago. That’s because it was deliberately designed to be independent of any particular Standard or protocol, and in fact has global application. Its site capability and environmental sensitivity factors, coupled with the very flexible ranking system, means Trench® is as equally valuable sizing a wastewater system in Dubai as it is in Glasgow. The assessor in Dubai, and the assessor in Glasgow, separately tweak the ranks of each factor to suit his or her local Standard and protocols.
I may tinker with Trench® a bit, though……
Hi Bill,
There’s a lot of truth in what you say, however given the greater emphasis in later versions (2000 & 2012) of AS1547 on using textural soil classification (soil category & structure), largely independent of site-specific water balance calculations which rely on measured permeability and climatic conditions, I’d like to see some clarification on whether it’s appropriate to simply plug in the assigned DLR into the adopted LTAR box in the system sizing module of Trench or perhaps see Trench updated to include both approaches.
Hi Richard
Thanks for the comment.
If you enter a permeability value of a soil (whether or not you actually measured it in the field), Trench will spit out an LTAR. You then have a choice: you can (a) use this LTAR (adjusted downwards to be your DLR) in system sizing, or (b) ignore it and plug in any value you choose. So Trench “includes both approaches”, and again, as I keep on saying, the software stands separate from any and all standards because it gives you absolute flexibility to enter any parameters you like – and generate any wastewater system size you want. GIGO.
While we’re on the subject of DLRs, I will be uploading a separate post about wastewater systems on Category 5 and 6 soils. I think some designers in Tasmania are not being conservative enough in system design or wastewater application rates.
One other issue; this regards viral die-off method used in Trench. This is a wee bit embarrassing because I think may actually have been me that put you and others onto the Beavers Gardner approach following some reading I did on this back in the mid to late 1990s; at the time and even until quite recently, I thought it was pretty handy.
The issue that has recently come to my notice is the selection of appropriate groundwater temperature; As you’ll be aware, this method doesn’t work all that well for groundwater temperatures much less than 12 deg C.
This was considered to be OK because average monthly max temperatures rarely fall below this level in the lowland areas of Tasmania where most people live and certainly I’d always understood that soil/rock temps average out at this figure and a trip to Hastings Caves in mid-summer confirms this.
I think this model works fine where we are discussing deep (ie 10-20m) groundwaters but recent experience has led me to conclude that this perhaps doesn’t work so well for the shallow groundwater situation you’d see in a typical duplex soil profile in southern or eastern Tasmania, where the limiting layer (typically clay) is usually within a metre or less of the surface.
Last August, I took the opportunity of jumping into some very recently (ie 5 mins before) excavated holes in the ground here in Sorell and used a probe thermometer to check temperatures at a depth of 0.5m to 1.2m. I found that they were all in the range of 5 to 6 deg C.
This is really the depth where the shallow perched water tables we are interested in actually sit in real life.
This has shaken my trust in using the viral die-off method for estimating safe setbacks from receiving waters etc where we are simply looking at a trench in duplex soil close to a watercourse, beach etc. In different situations, such as deeper water supply bores where soil/rock/water temps probably become more uniform with depth, I have no problem with it.