18/09/2026
Designing & building a home on a lifestyle block in Southern Tasmania... Tip #2 - WASTEWATER!
It's a delicate subject, we know! But, if you've fallen in love with that lifestyle block with the fantastic view and the hopefully short driveway (see tip #1 in our feed), then you are going to have to deal with wastewater. Here are some answers to the most common septic FAQs:
1. What systems are available? The main two systems we install are (a) Good old septic tanks with absorption beds. These consist of a septic tank with a dividing baffle. Effluent enters the tank and the solids stay in the first chamber and the liquids move to the second chamber and then are dispersed, by gravity, into underground trenches. The untreated liquid is cleansed by the soil. (b) Option #2 is the Aerated Wastewater Treatment System (AWTS). These are basically mini sewerage treatment plants. The effluent enters the treatment tank where air and chlorine/UV are used to clean the effluent up. That treated effluent is then sent to a dispersal field which might be below ground or, if you are lucky, above ground.
There are other systems - like sand filters, Wisconsin mounds, reed beds and so on. We can install them, but they are quite rare and a whole other post.
2. Do we need a septic if we have composting toilets? Unfortunately, you do as effluent is not only generated by using the loo but also by showers, washing machines, sinks and so on. It all has to be treated irrespective of whether it is black or grey water. Council will not let you spray untreated effluent on the garden. A composting toilet may allow you to install a smaller system though.
Stormwater from your gutters, by the way, does not enter the septic system. We generally keep that to drink and wash with! Any overflow goes into a creek, dam or a dispersal bed.
3. Which is the better system? The regular septic is cheaper to install, requires no power supply, and is more cost effective to run as the only maintenance required is a pump out every 3 to 5 years. Neglect that and your trenches will fail. The downsides to the OG is that they are only suited to dispersive soil types and generally rely on gravity. The AWTS is more expensive to install, and you will pay about $150 per quarter for the mandatory service. You will also need power to the tank. The massive upside is that they can be made to work on pretty much any soil profile. Having said that, if you are on heavy clay for example, you may also have to import a lot of sandy loam to cover the dispersal pipes. They also have the benefit of being able to pump upslope to the dispersal beds.
4. Who decides which system we get? Firstly, we will organise a soil test for you - which we need for both structural and wastewater design reasons. Then our wastewater consultant will suggest what she feels is the best system bearing in mind the soil profile, available area for dispersal, any site constraints like proximity to waterways and overall cost effectiveness. We then send that report to Council with your building approval paperwork. They have the ultimate say in the matter.
One piece of advice I will give you is not to engage a wastewater consultant before you have, at the very least, a set of conceptual plans - including a site plan. There is little point paying for a wastewater design that places the beds where the house will ultimately go! The house and septic design are best run in tandem.
5. We are extending and there is an existing septic. Surely, we can just use that? Unfortunately, it is quite rare for an older system to be still fit for purpose. The standards have increased massively over the last decade or so. It is also not normally possible, nor cost effective, to try and "expand/improve" an existing system to meet the present standards. When the original system was installed, the tank and beds would have been sized for the house. The size of the new system will be based on the new number of bedrooms/users. Extensions will invariably add extra bedrooms - meaning a new, suitably sized system will be required. If we can, we will use or improve the existing system but it is quite an uncommon occurrence. Sorry.
By the way, the number of toilets in your home has no bearing on the size of the system. Size is based on bedrooms which dictate the number of users. And, no, Council will not fall for the old trick of calling a new bedroom an "office" or "store". It isn't their first rodeo!
6. How much is this going to cost? It will depend on the system and the soil profile, but you should budget for anywhere between $20,000 (if you have a good soil profile and flat ground) and $45,000 (if you choose a steep block with a poor profile which will require bed excavations, retaining walls and imported loam). Time for a stiff drink either way!
So, there you have it! The lesson to be learnt here is to speak to an experienced design and build company, like yours truly, before buying your block. Septics are one of the many things we will consider when looking at your block.
I think I'll write about something a bit nicer - and less expensive - for tip #3 next week.
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And don't hesitate to call us on 0458 441 711 if we can be of any help at all.
