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septic system types


We’ve found that a lot of people’s understanding of the different septic system types is murky to say the least, and as part of our transparent no-fuss approach to things, we want to try and answer the regular question we get “what are the different types of septic systems?”


Of course, we presume that if you’re interested in this seemingly mundane subject, then your block of land is rural – or at least semi-rural – which means that there’s no town sewer available so you have to get a septic system installed.


As a brief overview, there are three categories of septic systems –

  1. Primary
  2. Secondary
  3. Other Alternatives


Before we explore these three in a bit more detail let’s just have a look at the basic function of a septic system. One definition is that it treats the wastewater from your home (also classified as black water and grey water, incorporating the waste water from the toilets, showers, sinks, troughs, basins, dishwashers and washing machines) so that it can be disposed of into the soil without detrimentally affecting the health and safety of people and the flora and fauna.


Within that definition there’s a lot of other detail such as the level of safety of the treated water (ie, what is considered a ‘safe’ level of toxins?) and the varying method treatments to get to the desired outcome. This blog is not about the nitty-gritty detail of the science of wastewater treatment, but more about a broad overview of the type of treatment to get to the outcome.


Another key point to raise is why there are different types of treatment systems available. There are 2 main reasons –

  1. Differing site conditions require different treatment types. We’ll drill into this in a minute.
  2. Customer demand for more environmentally friendly or perceived better outcomes.



So the way a ‘Primary’ septic system works is that all the wastewater runs through pipes under your home across and down to a treatment tank (about 3500 litres, and stored underground) where bacteria slowly break down the sludge and scum that’s collected. The bulk of the broken down material runs out into trenches near the tank, and eventually dissipate into the surrounding soil. The trenches are typically about a metre deep and a metre wide and depending on how many bedrooms in the home, about 3 or 4 trenches at 30 metres long. The sludge in the tank that can’t be fully broken down, gradually builds up and has to be periodically pumped out.


A couple of things to point out – a primary treatment system is widely considered to be the lowest maintenance of the different types and also the cheapest to construct. But it also requires a fair amount of free land space, and doesn’t work with some challenging soil types. So if you’ve got plenty of land and the soil is reasonable then it’s likely that a primary system is your best bet.



But if not, that’s where the ‘secondary’ system comes in. This works by starting off the same way as a primary system, but then instead of the broken down material feeding into trenches, it feeds into another couple of tanks or ‘chambers’ which are specially designed to break down the material a lot more. And then instead of sending it out to trenches, it’s fed into an irrigation system that sits just below the surface, and is drip fed out into the soil. This irrigation area needs to be somewhere between 300 to 500metres square on average, depending on a few different factors.


One of the key things about a secondary system is that the final output is far less toxic or dangerous than a primary system. And here’s a few reasons why it’s recommended instead of a primary system –

  1. Close to waterways, creeks, dams, rivers and lakes. In a primary system, the trenches are used for continuing to break down the septic material and it’s been proven that some of this can seep through the soil for quite a way. This is fine if there’s plenty of room, but it’s not good to get that into waterways.
  2. Limited space or format of the space. A primary systems trenches have to spaced at least a metre apart and have to be long trenches to work. But an irrigation area can take whatever shape you want it to, and can even be split into multiple areas – down the side of a home and around the back of a home or shed for example. Of course there are some limitations to this, but as a general rule, it’s a lot more flexible.
  3. Difficult soil. One of the secrets of the primary system is the soils ability to allow the broken-down material to seep out and away year after year. If the soil is very stiff clay or a lot of rock then this doesn’t work very well – if at all. Some clay’s are just too thick and sticky and it just doesn’t have the permeability needed for a primary treatment plant and so a secondary system is needed.


Other Alternatives

And then we have the third category of alternative systems. We’re not going to go into much detail here. The two main alternatives are a worm farm system, and the mound system. The theory behind the worm farm is that the worms break down the material to the extent of a secondary treatment system. One of its apparent advantages is that it’s supposed to function with less smell and less maintenance. And the mound system is for really tight spots. The theory is that more soil is built up so there’s more soil for the broken down material to dissipate through before eventually going back into the original soil.


Four other interesting factors come into septic system types. The geotechnical engineer, the health officer, the plumber, and the government authority – the Environmental Protection Agency (EPA). They all have a key role in determining what septic system is going to be right for your site. Here’s what generally happens –

  1. A geotechnical engineer visits the site and takes some soil samples to test back at the laboratory. A report is then drawn up based on the results, and also based on the EPA rules and regs. The report recommends a particular type and size of septic system for that particular site, home, soil type, area, etc.
  2. The plumber then visits the site and also looks at the report, and then makes their own informed decision about what sort of system will go well at that spot. A plumber often draws on his experience of how various systems have fared – they will sometimes get called back to a site to attend to the maintenance of a system and have first hand experience of what works and what doesn’t.
  3. Then the health officer gets involved when a permit is lodged for a septic system. Once again, they take the geotech report into consideration, but they’re not bound by it. It happens periodically where a health officer overrides both the engineer and the plumber to recommend a different system. This may be because of local rules by the water authority. Or it might also be because of experience once again. And of course, all of them are bouncing everything off the EPA guidelines.


We’re not here to give an opinion on either the systems or the processes, but one thing’s for sure – there’s no one size that fits all. For many sites, it’s a very uncertain iterative process that can be quite frustrating for everyone involved.


Of course, a good outcome is the ultimate goal here – a bad outcome can really stink. So work with various parties involved and try and understand their thinking and you’ll get the best set-and-forget outcome that’s possible for your site.


Disclaimer. This blog is our opinion only. The information provided in our blogs is accurate and true to the best of our knowledge, but there may be omissions, errors or mistakes. The information presented in our blogs is for informational purposes only and we are not professionals, so the content we provide shouldn’t be taken as legal advice. We strongly recommend consulting with a professional before taking any sort of action. We reserve the right to change how we manage our blog and we may change the focus or content at any time.

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