Definitions Used in Percolation


Activated sludge treatment:

Activated sludge is a process in sewage treatment in which air or oxygen is forced into
sewage liquor to develop a biological floc, which reduces the organic content of the


Any stratum or combination of strata that stores or transmits groundwater.


The solid rock beneath the soil and superficial rock. A general term for solid rock that lies
beneath soil, loose sediments, or other unconsolidated material (subsoil).

Biochemical oxygen demand (BOD):

BOD is a measure of the rate at which micro-organisms use dissolved oxygen in the
biochemical breakdown of organic matter in wastewaters under aerobic conditions. The
BOD5 test indicates the organic strength of a wastewater and is determined by measuring
the dissolved oxygen concentration before and after the incubation of a sample at 20°C for
5 days in the dark. An inhibitor may be added to prevent nitrification from occurring.


A thin layer of micro-organisms and organic polymers attached to a medium such as soil,
sand, peat, and inert plastic material.

Biological aerated filter (BAF):

A treatment system normally consisting of a primary settlement tank, an aerated biofilm
and, possibly, a secondary settlement tank. The system is similar to the percolating filter
system except that the media are commonly submerged (termed SAF) and forced air is


A biologically active layer that covers the bottom and sides of percolation trenches and
penetrates a short distance into the percolation soil. It includes complex bacterial
polysaccharides and accumulated organic substances as well as micro-organisms.

Chemical oxygen demand (COD):

COD is a measure of the amount of oxygen consumed from a chemical oxidising agent
under controlled conditions. The COD is greater than the BOD as the chemical oxidising
agent will often oxidise more compounds than micro-organisms.

Collection chamber:

A chamber receiving treated wastewater from the collection layer and discharging through
the pipe to an outfall or polishing filter/tertiary treatment system.

Collection pipe:

A perforated pipe placed at the bottom of a trench, within the collection layer connected to
the collection chamber.

Competent person:

A person with the necessary training, skills and practical experience to enable the required
work (i.e. site characterisation or system installation or maintenance) to be carried out.

Constructed wetlands (CW):

A wetland system supporting vegetation, which provides secondary treatment by physical
and biological means to effluent from a primary treatment step. Constructed wetlands may
also be used for tertiary treatment.


The uniformity co-efficient is a measure of the particle size range. Cu < 5 – very uniform;
Cu = 5 – medium uniform; Cu > 5 – non-uniform.

Distribution box/device:

A chamber between the septic tank and the percolation area, arranged to distribute the tank
wastewater in approximately equal quantities through all the percolation pipes leading from

Distribution layer:

A layer of the system composed of granular fill material in which pretreated effluent from the
septic tank is discharged through infiltration pipes.

Distribution pipe:

A non-perforated pipe used to connect the distribution box to an infiltration pipe.

Extended aeration:

An activated sludge process where a long aeration phase enables reduction of organic
material in the sludge.


Man-made fabric, which is permeable to liquid and air but prevents solid particles from
passing through it and is resistant to decomposition.

Groundwater protection response:

Control measures, conditions or precautions recommended as a response to the
acceptability of an activity within a groundwater protection zone as set out in the
GSI/EPA/DoELG document Groundwater Protection Responses for On-Site Systems for
Single Houses.


Code of Practice: Wastewater Treatment and Disposal Systems Serving Single Houses (p.e. ≤ 10)

Groundwater protection scheme

A scheme comprising two main components: a land surface zoning map which
encompasses the hydrogeological elements of risk and a groundwater protection response
for different activities.

Hydraulic conductivity:

The volume of water will move in a porous medium in unit time under a unit hydraulic
gradient through a unit area measured at right angles to the direction of flow. In contrast to
permeability, it is a function of the properties of the liquid as well as of the porous medium.

Infiltration system:

Comprises percolation areas and polishing filters that discharge partially treated and treated
effluent into the ground.


The occurrence of reddish/brown spots or streaks in a matrix of dark grey soil; the
reddish/brown spots or streaks are due to intermittent aeration and the grey colours may be
due to anaerobic conditions.

Nutrient-sensitive locations:

These are locations, which include rivers designated as nutrient sensitive under the Urban
Waste Water Treatment Regulations and groundwater bodies, where a programme of
measures are needed to achieve the objectives of the Water Framework Directive.

Organic matter:

Mainly composed of proteins, carbohydrates and fats. Most of the organic matter in
domestic wastewater is biodegradable. A measure of the biodegradable organic matter can
be obtained using the BOD test.


Ortho-phosphorus is soluble reactive phosphorus and is readily available for biological

Pathogenic organisms:

Those potential disease-producing micro-organisms which can be found in domestic
wastewaters. Organisms, such as Escherichia coli, and faecal streptococci, with the same
enteric origin as the pathogens are used to indicate whether pathogens may be present or
not in the wastewater.

Peat filter:

A filter system consisting of peat used to treat wastewater from a primary settlement tank
(usually a septic tank) by biological and physical means.

Perched water table:

Unconfined groundwater separated from an underlying body of groundwater by an
impervious or perching layer.

Percolating filter system:

A wastewater treatment system consisting of primary settlement and biological treatment
(effected by distributing the settled liquid onto a suitable inert medium to which a biofilm
attaches) followed by secondary settlement.

Percolation area:

A system consisting of trenches with pipes and gravel aggregates, installed for the purpose
of receiving wastewater from a septic tank or other treatment device and transmitting it into
soil for final treatment and disposal. This system is also called a soil infiltration system
(EN 12566), drain field, seepage field or bed, distribution field, subsurface disposal area, or
the treatment and disposal field.

Percolation pipe:

A perforated pipe through which the pretreated effluent from the septic tank is discharged to
the filtration trench or bed.

Polishing filter:

A polishing filter is a type of infiltration system and can reduce micro-organisms and
phosphorus (depending on soil type) in otherwise high quality wastewater effluents.

Population equivalent (p.e.):

Population equivalent, conversion value which aims at evaluating non-domestic pollution in
reference to domestic pollution fixed by EEC directive (Council Directive 91/271/EEC
concerning Urban Waste Water Treatment) at 60 g/day related to BOD 5.

Population total (PT):

Sum of population and population equivalent (p.e.).

Preferential flow:

A generic term used to describe the process whereby water movement follows favoured
routes through a porous medium bypassing other parts of the medium. Examples include,
pores formed by soil fauna, plant root channels, weathering cracks, fissures and/or

Pretreated effluent:

Wastewater that has undergone at least primary treatment.

Primary treatment:

The primary treatment stage of treatment removes material that will either float or readily
settle out by gravity. It includes the physical processes of screening, comminution, grit
removal and sedimentation.


Raised percolation area:

This is a term used to describe a percolation area where the percolation pipes are laid at a
depth between 800 mm below ground surface and the ground surface itself. The in situ soil
and subsoil are used to treat the effluent and material is brought in to provide protection for
the pipework.

Reed bed:

An open filter system planted with macrophytes (reeds).

Rotating biological contactor (RBC):

A contactor consisting of inert media modules mounted in the form of a cylinder on a
horizontal rotating shaft. Biological wastewater treatment is effected by biofilms that attach
to the modules. The biological contactor is normally preceded by primary settlement and
followed by secondary settlement.

Sand filter:

A filter system consisting of sand used to treat wastewater from a primary settlement tank
(usually a septic tank) by biological and physical means.

Secondary treatment:

The secondary treatment stage of treatment by biological processes, such as activated
sludge or other (even non-biological) processes giving equivalent results.

Septic tank system:

A wastewater treatment system that includes a septic tank mainly for primary treatment,
followed by a percolation system in the soil providing secondary and tertiary treatment.


The solids that settle in the bottom of the primary/secondary settlement tank.

Soil structure:

The combination or arrangement of individual soil particles into definable aggregates, or
peds, which are characterised and classified on the basis of size, shape, and degree of

Soil texture:

The relative proportion of various soil components, including sands, silts, and clays, that
make up the soil layers at a site.

Soil (topsoil):

The upper layer of soil in which plants grow.

Submerged aerated filter (SAF)

See biological aerated filter (BAF).


The soil material beneath the topsoil and above bedrock.

Suspended solids (SS):

Includes all suspended matter, both organic and inorganic. Along with the BOD
concentration, SS is commonly used to quantify the quality of a wastewater.

Swallow hole:

A depression in the ground communicating with a subterranean passage (normally in karst
limestone) formed by solution or by collapse of a cavern roof.

Tertiary treatment:

Tertiary treatment (advanced treatment) additional treatment processes which result in
further purification than that obtained by applying primary and secondary treatment.

Total nitrogen:

Mass concentration of the sum of Kjeldahl (organic and ammonium nitrogen), nitrate and
nitrite nitrogen.

Total phosphorus:

Mass concentration of the sum of organic and inorganic phosphorus.


Also referred to as a percolation trench, means a ditch into which a single percolation pipe
is laid, underlain and surrounded by gravel. The top layer of gravel is covered by soil.

Unsaturated soil:

A soil in which some pores are not filled with water; these contain air.


The discharge from sanitary appliances, e.g. toilets, bathroom fittings, kitchen sinks,
washing machines, dishwashers, showers, etc.

Water table:

The position of the surface of the groundwater in a trial hole or other test hole.

Researched from the EPA

O.S.C Environmental Engineering and Percolation Testing