Building Advice: Treating your waste water

You must give this issue careful consideration and seek professional engineering advice
Building Advice: Treating your waste water

In a rural setting, wastewater is sent to either a treatment unit or a septic tank.

Hi Kieran, I am looking at a few sites for a potential home build, but wanted your advice on wastewater treatment.

One of the sites has no connection to the wastewater network. Does that mean we need a septic tank?

What are the various systems that we can use and what is the difference?

Sean, Fermoy

Hi Sean and thank you for your question.

Wastewater treatment is a crucial consideration when deciding on your site location, so let’s provide a little background to what wastewater treatment is, the different options available, and how these relate to your particular site choice.

Types of wastewater

In a typical new build, you tend to have three different types of wastewater from a house — storm water, foul water, and greywater.

Storm water is essentially the run-off from your roofs, patios, driveway, etc, and this is normally sent to a soakaway in your garden, where possible in a domestic setting, as this is essentially just clean rainwater after all.

Kieran McCarthy: 'In many settings, grey water is combined with foul water for further treatment.'
Kieran McCarthy: 'In many settings, grey water is combined with foul water for further treatment.'

Foul water is the effluent from your toilets and, as such, needs a high level of treatment before being released back into the environment.

Grey water is the wastewater from your sinks, showers, and washing machines. This is not as toxic as foul water.

However, in many settings, grey water is combined with foul water for further treatment, although nowadays some people recycle grey water and use it to irrigate parts of their garden (taking into careful consideration the cleaning products they use in their home).

Given the toxicity of foul water, it needs to be carefully treated before being released back into the environment.

Septic tank or treatment unit?

In a rural setting, wastewater is sent to either a treatment unit or a septic tank.

The process here is that when you are seeking planning permission for your new home, you will need to carry out a percolation test to assess the drainage capacity of your soil. The outcome of this test will determine whether a septic tank, or indeed a treatment unit, will be suitable for your site.

This treatment system is designed by a specialist site assessment engineer. In most cases, this engineer will design a treatment unit that performs a more active and managed treatment process.

Septic tanks rely on the inherent bacteria to break down the harmful waste.
Septic tanks rely on the inherent bacteria to break down the harmful waste.

Septic tanks, which perform a passive biological breakdown of the effluent, were used exclusively in the past, and are still occasionally feasible.

As these tanks rely on the inherent bacteria to break down the harmful waste, they work best when the grey water is removed, as the chemicals in grey water can hinder this bacterial action.

It should also be noted that most rural councils still prefer to see septic tanks being employed where possible, as these are simpler systems with less to go wrong and maintain.

In an urban setting, your foul water will be sent to the local foul water sewer system for further treatment.

If one of your potential sites has no access to a sewer, the answer to your question really depends on whether it is in a rural or an urban setting.

If it is in a rural area, you could carry out a percolation test as described above, and propose your new treatment unit design to the local area engineer during the planning permission process for your new home.

All new treatment units or septic tanks require planning permission, regardless of whether you are actually building/extending a home or not.

Connecting to Irish Water

However, if you are in an urban setting and there is not immediate access to the local wastewater system, you would likely need to begin discussions with Irish Water, which now controls these waterways, to assess the viability and price of bringing a new foul sewer line to your site.

It would then be your responsibility to bring a sewer line from your new home and connect it to the new Irish Water foul manhole that has been placed at the edge of your property.

Your builder would perform this task as part of his overall build project.

So, clear as mud Sean.

However, whatever way you look at it, you must give this aspect of your site choice careful consideration and seek professional engineering advice as it will clearly determine what options are available, or indeed feasible, in the site selection for your new home.

When all is said and done and all the pipes are buried, you will wonder what all the fuss was about.

However, if you have ever seen the shock on the face of a neighbour when the local ‘drain buster’ team is called in, you will be happy you did your homework.

Civil engineer Kieran McCarthy is founder, and design and build director with KMC Homes. He is a co-presenter of the RTÉ show Cheap Irish Houses.

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