sewage treatment in cork and munster
 

Percolation Tests

A percolation (permeability) test assesses the hydraulic assimilation capacity of the subsoil i.e. the length of time for the water level in the percolation hole to drop by a specified amount. The objective of the percolation test is to determine the ability of the subsoil to hydraulically transmit the treated effluent from the treatment system, through the subsoil to groundwater. The test also gives an indication of the likely residence time of the treated effluent in the upper subsoil layers and therefore it provides an indication of the ability of the subsoil to treat the residual pollutants contained in the treated effluent.

There are two basic types of percolation test: the T-test and the P-test.

The result of the percolation test is expressed as either the “T” value or the “P” value.

A minimum of two test holes should be excavated and tested at each site.

The T-Test:

The T-test is used to test the suitability of the subsoil, beneath the invert of the proposed percolation pipe or polishing filter distribution system to hydraulically transmit the treated effluent from the treatment system. The precise depth at which the percolation pipe will be located (and, by consequence, the top of the T-Test percolation test hole) will depend on the most suitable subsoil layer for treatment and disposal and the depth of topsoil at the site but will normally be at least 450 mm below the ground level, to provide adequate protection for the percolation pipe work and to ensure that the percolation pipe is discharging into the subsoil layer. The assessor will decide the actual depth at which the percolation pipe will be located, based on the results of the visual assessment and the trial hole investigation. This in turn will dictate the depth from ground surface to the top of the T-test percolation hole.

A T-test should be conducted at all sites because if a T-test is in excess of 90 then irrespective of the P-test result the site is unsuitable for discharge of treated effluent to ground as it will ultimately result in ponding due to the impervious nature of the underlying subsoil (or bedrock).

The P-Test:

The P-test is carried out at ground level to establish a percolation value for soils that are being considered to be used for constructing a mounded percolation area or a polishing filter discharging at ground surface. Situations where a P-Test might be considered include:

  • Where the T-test shows that the site is not suitable for treating effluent from a conventional septic tank (50 ≤ T ≤ 90) and consideration is being given to an alternative treatment system which would discharge to ground through a polishing filter, and,

Where the visual assessment and trial hole investigation indicate limiting factors for installation of a conventional septic tank such as a high water table or shallow bedrock, and an alternative treatment system that would discharge to ground through a polishing filter is being considered.

Location of Test Holes:

Percolation test holes should be located adjacent to; but not within, the proposed percolation area. It is important to note that the top of the percolation hole should be located as accurately as possible to the same level of the invert of the percolation pipe. Further, attention should be given to the impact of slope and subsoil layering on the location of the invert of the percolation pipe. Where unsaturated subsoil depth is limiting, it may be possible to choose a percolation pipe invert level, which is near or at the ground surface, in order to fully exploit the available subsoil depth. In such cases it will be necessary to provide protection for the percolation pipe-work, when installed, by placing soil over the pipe-work in sufficient quantities (minimum of 150mm gravel and 300mm topsoil) to ensure that damage due to activities on the surface does not occur.

In the case where there is a high water table present then it is critical to assess the subsoil layer just above the water table by carrying out a percolation test, thus determining whether or not the water table is due to a low permeability subsoil or a
naturally high water table due to the site’s hydrological location.

In situations, where the T-test is in excess of 90 then irrespective of the P-test result the site is unsuitable for discharge of treated effluent to ground as it is likely ultimately to result in ponding due to the impervious nature of the underlying subsoil (or bedrock).

Where experience indicates that the site may be borderline, then both tests should be carried out at the same time.

The subsoil classifications from the trial hole should be confirmed by the percolation test results. If there is not a good correlation then further examination should be undertaken to determine which assessment provides the accurate assessment of the suitability of the site to treat and dispose of the effluent. An integrated approach is required when carrying out the assessment.

INTERPRETATION OF PERCOLATION TEST RESULTS

  • T <1 : Retention time in the subsoil insufficient to provide satisfactory treatment. Site is unsuitable for conventional wastewater treatment system. P-test should be undertaken to determine whether the site is suitable for a Secondary Treatment System with a polishing filter at ground surface or over ground. Sites may be suitable for discharge to surface water in accordance with Water Pollution Act licence.
  • 1 ≤ T ≤ 50  Site is suitable for the development of a conventional septic tank system or a Secondary Treatment System discharging to groundwater.
  • T value between 50 -75   Wastewater from a conventional septic tank system is likely to cause ponding at the surface of the percolation area. Not suitable for a conventional septic tank system. May be suitable for a secondary treatment system with a polishing filter at the depth of the T-Test hole.
  • 75 ≤ T ≤ 90 Wastewater from a conventional septic tank system is likely to cause ponding at the surface of the percolation area. Not suitable for a conventional septic tank system. Site unsuitable for polishing filter at the depth of the T-Test hole. P-Test should be undertaken to determine whether the site is suitable for a Secondary Treatment System with polishing filter, i.e., 1≤ P ≤ 75, at ground surface or over ground.
  • T > 90 Site is unsuitable for development of any wastewater treatment system discharging to ground. Site may be suitable for treatment system discharging to surface water in accordance with Water Pollution Act licence.
  • T<1 and P <1 Retention time in the subsoil insufficient to provide satisfactory treatment. Site is unsuitable for any treatment system without carrying out significant site improvement works.
  • P >75 and 75<T< 90 : Site is unsuitable for development of a wastewater treatment system discharging to ground.
  • P > 75 and T>90 Site is unsuitable for development of a wastewater treatment system discharging to ground. Site may be suitable for a discharge to surface water in accordance with Water Pollution Act licence.