Sitting pretty: covering in the septic tank

Services – Septic

Andrew Kerrby author page

2 comments Construction

Monday October 19 / Tuesday October 20, Friday October 23

Flushed away

This post is about the not so glamorous but oh so important topic of dealing with the stuff that gets flushed down the toilet. If you’re close enough to connect to town sewer then happy days – hook it up, flush and forget. Flowerpot is 6km away from the nearest treatment plant so we have to deal with all of our waste onsite.


Thar she flows: all of the plumbing coming out of the rear of the house

Thar she flows: all of the plumbing coming out of the rear of the house

What are the options?

Several factors determine which is the best method to treat wastewater. These include the expected typical usage (how many people per day) and the site conditions – soil type, slope and area available. For wastewater the key site condition is the soil type – how much moisture the soil is capable of absorbing. The soil type will influence which system is suitable for the site and how large it needs to be.


Septic tanks are a water-tight tank made from either plastic or concrete, installed underground and separate the solid wastes from the liquid. The liquid (effluent) contains bacteria and flows out of the septic tank into absorption trenches where it is dispersed into the soil. Bacteria break down the solids within the tank. Septic tanks are reliable and long lasting, with only occasional pumping required.



Delivery service: the septic tank being moved off the truck

Delivery service: the septic tank being moved off the truck


AWTS stands for Aerated Wastewater Treatment Systems. An AWTS works in a similar manner to a septic system (a tank with multiple chambers) but uses aeration to further treat the wastewater so the effluent is clean enough for surface irrigation. Because an AWTS doesn’t need any additional subsoil absorption it can be selected for sites that aren’t suitable for a septic and absorption trenches. AWTS cost more than septic systems – both to purchase and also to maintain and service (an ongoing annual fee for the life of the system).


With both a septic and an AWTS all of the plumbing waste (toilet, shower, hand basin, kitchen sink, laundry) ends up in the same place and gets treated the same way. A third option is a composting toilet that deals only with the toilet (black water) and a grey water treatment system that deals with everything else (bathroom, kitchen and laundry).


I did consider installing a composting toilet and grey water treatment system because it would use the least amount of water. The main design implication of a composting toilet is the required clearance beneath the toilet and access to the collection chamber (usually around 750mm). Selecting a concrete slab for thermal performance made it difficult to provide sufficient clearance under the toilet.

Take a walk

I also considered having the archetypal shack toilet – the outside dunny. Locating the toilet outside of the house meant the floor level could be whatever it needed to be. Walking to the toilet and engaging with nature before communing with it was not such a bad prospect. Unless it’s raining. Or cold. Or in the middle of a cold, rainy night. 60k House will occasionally be available on air b’n’b (yes – once this place is finished you can come and stay!) I figured an inside job would be more comfortable for all involved.

Doctor dirt

I commissioned a soil test from engineers – the same soil test was used for the design of the wastewater system and also the footings for 60k House (I will talk about this more later when I explain the various consultants). Having evaluated the site and the soil profile the ‘soil scientist’ recommended a septic system with absorption trenches would be the best and most cost effective system.


Location location: the hole for the septic tank

Location location: the hole for the septic tank


Selecting the location for the septic tank and the absorption trenches is important because you need sufficient flow downhill. I ordered a 3000 litre dual purpose concrete septic tank – large enough to deal with the waste from 60k House and a possible future 2 bedroom house that could also be connected to the same system. The outlet of the septic tank needs to be lower than the inlet so the effluent can flow out. I was under the strictest instructions from the plumber to ensure the hole for the septic tank was level, and when the tank was delivered it needed to be level or slightly sloping downhill. The first time the tank was lowered into the hole it sloped slightly uphill. My plumber’s instructions rang through my head: “If it’s not right make him take it out and do it again!” A scoop of blue metal and a scrape around with the excavator fixed that and the tank was fine the second time.


Second time lucky: lowering the septic tank into the hole for the second time

Second time lucky: lowering the septic tank into the hole for the second time


The septic tank is concrete – it’s heavy but it is hollow. Because the tank sits in a very compact hole with no drainage, there is a risk of it floating to the surface if there’s heavy rain, even after the hole is filled. The septic tank and the water tank were both delivered on the same day so a truck load of water (1500 gallons) could be split between the two tanks: to stop the septic tank floating and to stop the water tank from blowing away.


Once the hole was dug for the septic tank we marked out the two absorption trenches using a laser level (dumpy). The bottom of each trench needs to be level, following the contours of the site. A splitter box is located between the trenches and ensures even flow to each of them. The trenches are each 1.2m wide and 0.6m deep – the soil type determines this size. Blue metal is installed in the base of the trench. Instead of using arches we used 100mm pipe with holes drilled in either side to release the effluent. This method is cheaper and stronger than arches, allowing you to occasionally drive over the top of the trenches if you need. A layer of geotextile (filter cloth) is placed over the top of the pipes and blue metal before being covered in top soil.


On the level: the first absorption trench following the contours of the site

On the level: the first absorption trench following the contours of the site


Even flow: the splitter box ensures even flow to each of the absorption trenches

Even flow: the splitter box ensures even flow to each of the absorption trenches [Photo credit: Alan Behrens]

Gone: the absorption trenches, covered in topsoil, hardly noticeable except for the inspection opening pipe

Gone: the absorption trenches, covered in topsoil, hardly noticeable except for the inspection opening pipe


Costs: septic tank – $1,650; earthworks – $1,920; blue metal – $1,523; plumbing and pipework – $1600; water – $100


Disclaimer: Any advice contained within this blog is of a general nature only and cannot be relied upon. Details provided are in good faith and relate specifically to this project. Any author will not be held responsible for advice or information presented.


  1. I’m planning on a composting toilet – but I am trying to do the Glenn Murcutt approach and “tread lightly” and not having a concrete slab floor, and instead will have a raised floor on piers (stumps). For me, in a cold climate in the mountains (where it snows and often overnight frost on ground) I would need to have so much glass to allow the sun to warm up my slab that in winter, with only maybe 4-6 hours of reliable sunshine (not including overcast or wet days) that the thermal mass of my concrete slab may be a burden rather than a benefit.

    My (yet to be purchased or installed) composting toilet uses no water, I can put the compost on my garden (yes, apparently – argggh – lol) , the new designs have no odour, and I’ll have room underneath the subfloor for the composting bin. I think I need between 750mm and 1000 mm subfloor clearance, depending on the model of composting toilet (which is basically the larger the capacity, the bigger the waste bin, and the bigger the underfloor clearance required). In NSW, my Blue Mountains Council LEP require a NSW Health Department approved / certified waste water system (which includes items like on-site toilet waste treatment), but the NSW Local Government Act (or some other statute) has an exemption to requiring NSW Health Dept certification if I am designing and building it myself – means I don’t need to sign up for those commercial systems that require annual inspections, yearly fees, and high up front purchase price. Well, that’s the theory anyway – things are never as simple as they seem when I try to do something.

    1. Andrew Kerr

      Thanks for sharing your plans about your planned house in the mountains.
      If you do opt for a raised floor then you will of course need to provide sub floor insulation – it may also be beneficial to enclose the subfloor (for both bushfire and insulation purposes). Thermal mass will help to regulate and even out the diurnal range (day-time – night-time temperature difference). Even if you don’t feel a slab will receive enough direct sunlight there still may be some benefit. Alternatively you could look at including thermal mass in internal walls (such as brick) or even incorporate water which is more effective than concrete or brick in storing energy. One option I considered with 60k house was a series of water pipes within an internal wall. The water in the pipes could be warmed by a solar hot water system, or if you ever want to remove the thermal mass it would be easy to open a tap and drain them. Literally – thermal mass available on top, when you want it.
      Unfortunately there is no nationwide acceptance of regulations for composting toilets as you have discovered – good luck with your local council!

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