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
I honestly wonder how significant is vertical ET from a conventionally designed/installed absorption trench/bed in real life?
Because a large volume of the excavation is occupied by blue metal, a very coarse material with limited capillary rise, unlike finer grained sandy/silty soils.
Capillary movement towards the surface from the middle of the bed will only commence once the effluent level is above the top of the distribution aggregate layer, by which time, the bed is effectively half full anyway.
Out in the field, you’ll often see this where the top of the bed is still quite dry but with a fringe of long, green grass growth outlining the bed along both sides.
My own view is that if you want to use ET as a factor in your water balance model, you really should be using an ETA bed which has a very shallow distribution bed which quickly grades into sand and topsoil to maximise vertical capilliary movement into the root zone from whence we can make a start on ET.
I’ve seen similar mistakes with designers trying to use a water balance method to design a mound system; they treat the top of the mound as part of the total evaporative area, however there is very little evaporation from the top of a properly functioning mound, simply because the predominant direction of effluent movement from the distribution bed is mainly straight downwards.
One of the biggest problems with mound systems is actually getting anything to grow on top; this is another reason why I prefer BSFs; you can just accept that it’s going to be dry and simply finish the top of the bed with ornamental pebbles or else plant vegetation which is resistant to arid conditions; however this will take some months/years to establish and flourish, in the meantime, in a mound, your topsoil cap can be blowing/washing away whilst this is going on, whereas the framed sides of an ET bed will retain the top cover whilst the vegetation gets a start.
All this indicates to me that there ain;t no ET happening, simply because there ain’t no water to play ET with.
Granted, ET from the sloping sides of a mound appears to be a significant mechanism for removal of water but really, if you’re using a water balance design, you need to realistically make your model show where the ET is happening in real life, where the rubber hits the road so to speak.
Richard, thanks for your comments about the relative importance of evapotranspiration (ET) as an “effluent loss mechanism” in on-site wastewater systems.
It’s true that vertically upwards ET is negligible in a conventional trench or bed with screened coarse gravel as a distribution aggregate. The capillary rise from this grade of material is a few millimetres at best (see graph).
My Note “Designing for Failure” about the importance of ET remains valid for massive (unstructured) clayey, sandy clay and gravelly clay soils. Effluent loss from beds or trenches in these materials can only occur through ET (and of course, surface overflow on failure). This in turn means that in these beds or trenches, the effluent level would be above the top of the aggregate and in the usually thin topsoil much of the time, so ET starts working. (Also, some self-supporting arch trenches have a minimal gravel layer at their base, or no gravel at all. Most or the entire arch is soil-surrounded, so again, ET may become important depending on the texture of the “topsoil” backfill and the depth of the trench.)
As you point out, it’s common in the field to see what appears to be dry-topped trenches and beds with a green fringe of grass along their perimeter. This would be particularly true where the top of the bed is mounded, and effluent levels are close to the surface. The perimeter soil is moister than the higher centre of the bed, so that grass grows quicker. Nevertheless, ET may be occurring all over the bed footprint but not be noticeable at the ground surface – on either bare or grass covered soil.
ET should be incorporated in all full water balance models (eg in Trench®3.0) for all types of disposal systems because ET also acts on soil moisture from infiltrating rain before net rain infiltrates to the effluent level. If ET is neglected in such balances, unrealistically high rainfall volume is added to the trench or bed, resulting in oversizing.
Of course, these issues do not arise at all in the sizing approach for most trenches and beds in Section L4 of Australian/New Zealand Standard 1547:2012 On-site domestic wastewater management. The Standard simplifies real-life conditions by ignoring rainfall, ET and site slope angle.