In unsewered areas, keeping and treating your domestic wastewater on your own property has always been your responsibility, and you can’t build a house and live in it until you have an approved method of managing wastewater. Assessing your land for wastewater, and drawing up a design for a wastewater system, is done under Australian/New Zealand Standard 1547 On-site domestic wastewater management. I’ve compiled a flowchart showing how and when the process works in Tasmania (PDF Document 13 kB).

Enter the Tasmanian On-site Wastewater Management Code. I’m not sure why this layer of bureaucracy exists, since AS/NZS1547 is sufficient. But this is the way of things these days – the Interim Planning Scheme (2015) of all Tasmanian local councils now have 22 different Codes from Signs and Scenic Landscapes, to Landslides, Biodiversity, Heritage and Wastewater.

The new Wastewater Code is fairly onerous, particularly in dictating (using “setback distances”) where a wastewater disposal area can be located on your land. If your property is small and steep, you could be in trouble: the setback from your lower boundary to the lowest part of the disposal area is set as 1.5m plus one metre for every degree of slope – irrespective of the capability of the soil to retain your wastewater, and irrespective of the amount of wastewater you might generate.

In at least one case that I’m aware of, a local council has elected to ignore the Code to accommodate an owner who purchased a small, steep plot of land before the Code was adopted last year. It would have otherwise been impossible to fit the wastewater system in.

Use the flowchart I’ve compiled to assess whether your property is exempt from the Code (PDF Document 33 kB). You may need professional advice from someone like me.

Recently I was asked by a Queensland local government authority for advice about setback distances from a watercourse for an on-site domestic wastewater system. A site assessor engaged by the property owner had recommended a minimum setback distance of 5m. The Queensland Plumbing and Wastewater Code (2011) required a 50m minimum setback, but permitted an “Alternative Solution”. As it stood, the wastewater installation could not proceed, but the authority would accept my opinion as to whether the 50m limit could be reduced. Download my pdf which discusses (with an example) a defensible way to use the viral die-off method in Trench®3.0 to address this problem.

An assessment report by a Tasmanian wastewater designer came my way recently. Here are the inputs to his system sizing

House

3 bedroom; rainwater supply tanks

Soil profile

0.1m of sand over heavy clay (massive structure)

Adopted design loading rate

8L/m²/day (ie 8mm/day)

Daily wastewater volume

600L/day (5 people at 120L/day each)

Calculated basal absorption area

80m²

His wastewater design consisted of a septic tank (primary effluent) discharging via a splitter box to two standard trenches each 20m long, 2m wide and 0.6m deep.

I was handed a sample of the clay, and can confirm its “heaviness” — I pinched out a 5mm thick coherent ribbon to 150mm. I can’t confirm its structure in a hand-sized specimen though — as Australian New Zealand Standard 1547:2012 states in Section E6, …structure shall be assessed by examining exposed soil surfaces such as in a soil observation pit. But let’s accept the designer’s statement that the clay was massively structured.

In Australia, AS/NZS1547:2012 calls this clay Category 6 (not good), says special designs are needed for wastewater disposal, and the effluent absorption rate should be based on permeability testing. I agree with the special designs part, but the last requirement is a bit of a problem. I know from experience that massive clay has a very low permeability. I can leave my Cromer permeameter overnight in a massive clay soil and come back next morning to find not even a drop has infiltrated. An on-line geotechnical database tells me that “inorganic clays of high plasticity” have permeabilities in the range 10^-10 cm/sec to 10^-7 cm/sec. Let’s be generous and take the high end of the range — it is equivalent to a permeability of 10^-5 m/day. That’s 0.00001m/day. AS/NZS1547:2012 doesn’t help you translate this into an effluent absorption rate, but Trench®3 does. The answer is zero, or close to it.

This fits with our every day, common-sense experience. Rainwater puddles in clay lie around for days in a Tasmanian winter and only lose water through evaporation. Wastewater in a trench in heavy massive clay loses wastewater almost entirely via evapotranspiration. (Soils of course are rarely uniformly structured, and secondary porosity such as shrinkage cracks or defects around peds and clods in clay will allow some wastewater to infiltrate vertically downwards.)

Soil defects like these tend to close when the clay expands on wetting. But let’s be generous and allow a long-term vertically downward effluent absorption rate of (say) 1L/m²/day for our “massive” heavy clay. Then, if our system permits evapotranspiration, add 1 – 2L/m²/day for vertically upwards wastewater evapotranspiration loss in a Tasmanian winter, and perhaps 6 – 7L/m²/day in summer. These are figures for Hobart Airport, and in the accompanying graph, I’ve plotted the daily evapotranspiration and added daily rain for good measure. (They will of course vary from place to place. The design I’m talking about is close to the airport, and it’s valid to use these data.)

So, the effluent application rate (in Australia we call it the Design Loading Rate, or DLR) for massive clay soil near Hobart Airport varies seasonally, and ought to be around 7 – 8L/m²/day in summer, decreasing to about 2 – 3L/m²/day in winter. But our designer adopted a DLR of 8L/m²/day all year round, which in winter is (say) 5L/m²/day (5mm/day) too much. But the trench is not open space – let’s assume it is 50% voids. So 5mm of effluent raises the wastewater depth in the trench by 10mm each day (5mm divided by 0.5 = 10mm). Both trenches are bound to discharge horizontally (overflow!) in winter – and we haven’t added rain yet.

I never adopt a DLR for massive clay higher than 3L/m²/day. And to size wastewater systems I always do a full water balance which includes two inputs (rain and wastewater) and two outputs (evapotranspiration upwards and infiltration downwards) and trench storage volume. Trench®3 handles this easily.

Graph plotting daily evapotranspiration and rain for Hobart Airport, Tasmania

An assessor from Syracuse in the US noticed my discussion paper on bottomless sand filters (BSFs) and was asking about filter sand specifications and maximum slope angles for such installations. Richard Mason from Sorell Council has done more work on BSFs than anyone in Tasmania and I asked him to join the on-line discussion.

The filter sand specification was not an issue, but the slope angle side of things got me thinking – I’ve not seen any guidance on slope of a site and BSF suitability. Richard made the point that “the NYS regulations seem to be saying  that maximum permissible slope for raised beds and mounds are 15% (8.5⁰) and 12% (just under 7⁰) respectively. (See “Alternative Systems – clauses (b)(2)(111) and (c)(2)(iv)”. I’m happy, for example, to place my nonconventional beds on almost any slope angle. The basic module for all system needs to be built level, and for the BSF this can easily be achieved by starting with a level base of filter sand on any slope. Treated effluent from a BSF moving through soil downslope and away from a basic module does three things:

• it evapotranspires vertically upwards,
• it infiltrates vertically downwards, and
• it moves parallel to the slope in the apron soil.

The first two ought to dominate over the third if the system has been sized and designed properly. But the steeper the slope below the module, the faster the treated effluent moves parallel to the slope through the apron soil, and the less chance it has to evapotranspire and infiltrate. So, there is more chance of leakage from the lower, outer limit of the apron.

So, we should perhaps lengthen the flow path.  But then, we are dealing with highly treated effluent so downslope leakage should not be too much of a problem.  Perhaps one answer is to install a cut-off drain along the lower edge of the apron.

Read what the US EPA has to say about BSFs and intermittent sand filters.

Recently, I became aware of the design work being done on Bottomless Sand Filters (BSFs) by a fellow wastewater designer – Richard Mason – here in southern Tasmania.

In 2004 I released a discussion and guidance paper for what I called Nonconventional Beds (NCBs) – above-ground, on-site, domestic-sized wastewater systems similar to Wisconsin Mounds.  The paper was aimed mainly at designers, installers and regulators of such systems in Tasmania, but it had broad, even universal application.

Nonconventional beds have now been installed in southern Tasmania without incident for more than a decade.

BSFs provide a high level of wastewater treatment in a relatively small area, and are best suited, like NCBs, to sites with limiting features like clay soils or high water tables.  Accordingly, I recommended BSFs systems for a small township with limiting soils and mainly small lots, which led to the need for a discussion and guidance paper for stakeholders. Richard kindly offered advice, plans and drawings, and Chris Lewis Plumbing, installer of such systems, was forthcoming with site photographs and further advice.

The result is Bottomless Sand Filters: Notes for designers, installers and regulators, released in July 2013 accompanied by an updated version of the 2004 NCB paper.  Subject to the requirements of local or national jurisdictions, the principles of wastewater management and the generic system designs described in them ought to be globally applicable.

Feel free to download copies of each. Feedback would be good.