Septic tank arrangements


If the house cannot be connected to any main drains, for waste water or sewage, then you need to have tankage for the sewer outflow – the so-called black water. The other water that from the sinks, showers and bath, from the washing machine and the dishwasher can more easily be dealt with either being re-used on the garden, or via a soakaway system. It is important not to add this grey water with the black sewer water before it exits the house, because this will massively increase the flow of water into your tank system.

Yes…the main part of the black water disposal is the septic tank. This clever invention takes the water and solids from your toilets, and through settlement and subsequent biological action over time, the water will flow through the tankage and into a run-off and ultimately, a drain field or other soakaway location. During the cause of the biological breakdown the solids will also decompose into a substance that is able to mix with the water and join the run-off process.

If the flow rate and input volumes (and resultant septic tank size) have been worked out correctly, one might never need to empty the septic tank or, one might long-term occasionally have to pump out the ‘sludge’ that has accumulated, but if the system is under-sized one can expect overloaded and blocked drain fields / soakaways, solid material backing up in the pipes and excessive sludge-pumping. That is why it is so important to match the septic system to the house size, number of toilets and expected number of people living in the house.

For this build, the calculations were done at the start, and this resulted in TD Towers having…three septic tank systems! These corresponded to the toilets in the three building zones, so one tank looked after the toilets in bedroom 1 and the office/bed 5, a second septic tank handled bedroom 2 and the up and downstairs closets, while the third tank dealt with the toilets in bedroom 3 and 4, and the gym/swimming pool changing room toilet.

Although there were 3 toilets connected to both tank 2 and tank 3, the actual usage, which was factored in, was much less so the volumes into each tank were broadly the same. Each tank system was easily able to ‘digest’ the solid material entering the tank, and efficiently break it down before the resultant liquid flowed through the connecting pipework into the initial soakaway before any residue continued down to the final soakaway/drain field. It was only at that final location that all 3 flows from the initial septic tank / soakaway arrangements came together, but the bulk of the soakaway action took place at the first three initial soakaway arrangements.

As mentioned earlier, once again forward planning and calculation will ensure you get the right number and size of septic tank. The soakaway arrangements correspond to the tankage you settle on and obviously the connecting pipework is what it is, based on those initial calculations.

Of course, it is always good to have that ‘back up plan’ for if your tank err… backs up! By that I mean, make sure the access to your septic tank(s) is sufficient for a sludge-gulper to connect up and empty the tanks if that really does become necessary.

Once the tanks are in place in the ground, they are of course buried. The tank system is sealed to the pipework, the only other connection is the vent pipe. This allows any gas build up to escape from the tank and dissipate into the air. In our case, the builder took a small pipe and lead it away from the tank location, underground, then placed an upright at 2m high against the house or boundary wall. It was of course well away from doors and windows. By having a continuous escape for any gas in the septic system, there is no pressure build up or sudden release of a lot of gas.

Ground-floor pipework: water in – waste out


Around about this time, we needed to think about the plumbing lines – water in, and waste water out. In our build, this was further complicated by splitting the grey and black waste water, so that the septic tanks did not fill up every time we took showers or a bath!

Again… this is planning ahead. If the issue arises during the build, you are prepared for it. In this case, we had a plan in the future to use the grey water for irrigation. The septic tanks, all three of them, were for toilet waste only (see the separate post on tankage).

Our builder had experience in laying plumbing lines to get the correct slopes for efficient run off so no specialist was needed. As with most house builds, our actual build was 50cm above the surrounding, raised-up ground level. This made it a lot easier to get those ‘falls’. As it was detailed on the approved plans, we all knew the exact location of every shower, wash basin, sink, toilet and tap. Therefore, it was easy for the builder to mark that location on the ground, and lay the required pipework from each point to the external wall. This was of course carried out before the floor slab was cast.

As part of the installation and where needed, the builder included the ‘P-traps’ in the line to prevent noxious odours coming back up the pipe and into the bathrooms. This another area that can be overlooked – make sure, if you are building in the future that this requirement is stated in the contract. You will have plenty of time, sitting in your new home and with a smelly drain, to regret for not planning this one!

In addition, we also fitted 40Litre grease traps to the outflow. This was necessary to ‘hold’ the oils / greases, etc. that were washed from plates and pans, from entering the grey water disposal area (field drain). If these traps were not fitted, the greases travelling down the line would congeal and ultimately block the drain field function. It’s an easier matter to periodically clear out those traps and they are much more manageable and accessible.

Termite control pipelines were also added at this time. In fact, as mentioned in our contract, I had forgotten to include termite control in the document! However, our builder was a professional. He knew it was a requirement and he had already built its inclusion into the costings. The pipe network and spray nozzles, terminating at various points around the base of the house, and all under the ground floor slab were placed, so when I came along, panicking at my omission, he had already laid those all-important termite lines under the ground floor slab!

The placement of all the plumbing and pest control concluded this part of the ground floor build. See the separate post on Termites elsewhere on this site.

Next we need to cast the ground floor slab.