What onsite wastewater system do you need?

In this article we cover:

• Typical scenarios
• Starting point for site investigations
• The percolation test
• Situation in Ireland and Northern Ireland
  
  

You or your designer or an engineer can look at the site and survey it in order to assess which type of system might work best for you and for the environment.

As previously mentioned, the preferred option, if no connection to a mains sewer can be made, is to seek consent for discharge to a sub-surface irrigation system. This uses the underground soil to filter and de-contaminate effluent before it reaches any natural water.

Percolation tests and a range of other site-specific criteria are assessed for the suitability of the proposed discharge system.

Where to start

A useful starting point for assessing soil condition is to look first at low-lying points in the site to check for signs of seasonal waterlogging or ponding of surface water.

Then look at the vegetation. An area of the site which is good at growing water-loving species of vegetation may not be the best location for an underground irrigation system. Such species may include rushes, reeds, marsh marigold, lesser celandine, yellow iris, meadowsweet, bog mosses, hazel, alder, downy birch or willow, etc.

Soils containing large amounts of peat or clay, or thin layers of topsoil over rock or clay strata, will not usually be suitable for filtration purposes.

The soil drainage characteristics in and around a sub-surface irrigation system therefore needs to be neither too dense nor too porous. It needs to contain the wastewater long enough to filter it properly.

If it is too dense (e.g. peat or clay) it will hold the wastewater too long and the ground will eventually become saturated, leading to a potentially serious health risk.

Conversely, if it has very good drainage potential, such as very sandy or gravelly soil, it will likewise not be suitable due to the potential for effluent to readily escape to natural waterways or water sources.

The key test of suitable soil conditions for a sub-surface irrigation system is known as the percolation test. It involves digging a few small holes and timing the rate at which a given depth of water flows out of each one, over a number of times. These flow rates will determine not only whether your soil will be suitable, but will also direct your designer in calculating the requisite dimensions for the drainage plan.

Your designer will also need to take levels and measurements to check the falls and directions for drainage pipes and the positions of the various parts of the drainage system, including the tank, manholes and inspection chambers, etc.

Northern Ireland

If your site has unsuitable soil drainage qualities, but which can be linked to a waterway which flows well, all year round and which can accept a new discharge connection that will not pose any environmental issues, you may find that the most efficient option is for discharge to the waterway.

Some waterways may have been piped and covered, so if it is intended to connect to such a system, you will need to provide evidence for its suitability, the final discharge location and the location where sampling can be carried out.

Where consent is sought for discharge to a waterway, in addition to the requirement for higher quality treatment of the effluent, the approving body will also check for other information, such as other effluent discharges already approved along the waterway concerned and whether it can accept any more, or whether the presence of protected species or habitats may require special consideration, etc. You are not required to conduct any site tests in order to apply for such a system.

The percolation test in NI

The test holes should be relatively easy to dig using hand tools. If you need an excavator, then maybe the soil is unsuitable. Soil porosity can vary across a site and the percolation test should be carried out at the intended location of the proposed system.

Avoid carrying out the test in extreme weather conditions such as drought, frost and heavy rain; and leave the test holes covered over after testing so that they may be inspected if required.

Firstly, find out from your designer where the holes should be located, then dig them to the dimensions as given below. You will need at least 108 litres of water per hole to carry out one pre-soak plus three timed tests on each hole.

Excavate at least two holes (preferably three), each 300mm square with a depth at least 300mm below the proposed invert level (i.e. the bottom) of the infiltration pipe (see diagram on the right) and space them evenly along the proposed line of the sub surface irrigation system.

Fill each hole with water to a depth of at least 300mm and allow this to seep away completely. If the water drains rapidly (within 10 minutes) the hole should be refilled up to a maximum of 10 times. If the water continues to drain away rapidly or if the water has not soaked away within 6 hours, the area is not suitable.

Record the start and finish times for the water in each hole to drop from a depth of 75 per cent full, down to 25 per cent full (i.e. 225mm from bottom, down by 150mm, to 75mm from bottom). Repeat the tests so that each hole has been tested three times. You should now have a list of six start and finish times (if testing two holes) or nine start and finish times (if testing three holes).

Your designer can then take those times to calculate the percolation value (Vp), the drainage field floor area and the required length, width and depth of the irrigation trenches. The Northern Ireland Environment Agency will only consent to the use of a sub surface irrigation system when the Vp value falls within their parameters.

Do not attempt to tweak the figures if you find that the test results do not give a workable Vp value. To do so will result in you installing a system that will give no end of bother in the future together with the associated costs that a failing system will require to put right.

If your Vp values do not look good, seek professional advice from a designer or engineer who can design a system that works for your site. The local Water Quality Unit of the Environmental Health Service (EHS) will also give sound advice to steer you in the right direction. In all my years in the construction industry, I have never seen a site in Northern Ireland where a workable solution for an effluent treatment system could not be found.

Ireland

The Environmental Protection Agency’s current Code of Practice treats the percolation test as part of a wider site suitability assessment, not a standalone exercise, so trial holes, soil depth, bedrock, groundwater and drainage conditions all have to be reviewed alongside the test itself.

In practice, the job is normally carried out by a suitably qualified site assessor; typically an engineer, environmental consultant or other experienced wastewater specialist familiar with the EPA methodology and local authority requirements.

Some councils may have preferred formats or ask for specific competence, so it’s wise to check with your county council before appointing anyone. The EPA is clear that the assessment and percolation test must be completed by a suitably qualified person.

As in NI, the assessor inspects the site and digs trial pits to examine subsoil, groundwater indicators and rock levels.

Then the actual percolation holes are prepared and tested, usually in triplicate, to measure how long water takes to drop through the soil. Depending on the site, this may involve a subsurface test (formerly the T-test) and, on more constrained or raised-system sites, a surface test (formerly the P-test).

Those results are then used to decide whether a conventional percolation area will work, or whether you’ll need a secondary treatment unit, raised polishing filter or another engineered solution. If the site fails entirely, and there’s no mains connection, planning permission can be refused.