What’s an On-Site System and what does it do?

The first requirement of building is proof that the site you want to develop is capable of disposing of the waste water from the dwelling. The waste water needs to be percolated into the ground at a rate and in a location where it isn’t harming either ground or surface water. This is the reason for needing a Site Suitability Assessment and Percolation test as part of your planning application.

So why are we protecting our water sources? Well our source of drinking water is the ground, so we have to be very careful that Sewage or Effluent don’t get into our drinking water source, also we have to protect our rivers and lakes which are valuable natural habitats.

Sites that have too fast drainage or sites that have very bad soakage will both fail. Sites that are too fast run the risk of untreated waste water getting straight into a water source as the subsoil beneath percolation area slows down and further treats it. If the waste water can’t go down it goes up and will sit on our lawn for your kids to play in, this is caused by clay type soils, bedrock, or a water table on the site.


Septic tank systems comprise a septic tank and a percolation area. The majority of the treatment occurs in the percolation trenches and in the underlying subsoil. These systems provide effective treatment and disposal of domestic wastewater when properly sized, sited, installed and maintained in accordance with this code of practice. These systems require greater depths of subsoil and a larger percolation area than secondary treatment systems.

The most important component of a septic tank system is the percolation area (also called an infiltration area) as it provides the majority of the treatment of the wastewater effluent. In the percolation trench, the wastewater is allowed to flow by gravity into a distribution device, which distributes the flow evenly into a minimum of four percolation pipes in the percolation trenches. The depth to the invert of the percolation trench may vary and is dependent on the T-test location, trial hole information, layering of the subsoil and any other limiting factors such as water table and depth to bedrock. Wastewater flows out through orifices in the percolation pipes into a gravel underlay, which acts both to distribute and provide a medium for initial treatment of the effluent.

The effluent then percolates into the soil/subsoil, where it undergoes further biological, physical and chemical interactions that treat the contaminants. For effective treatment, the wastewater should enter the soil; if the base or walls of the percolation trench are compacted or glazed or otherwise damaged during excavation, they should be scratched with a steel tool such as a rake to expose the natural soil surface. It is equally important that the wastewater remains long enough in the soil; the hydraulic loading and the rate of flow into the sides and base of the trench control the residence time.




Packaged wastewater systems use media and mechanical parts to enhance the treatment of domestic wastewater. They require a polishing filter to allow for further treatment of the wastewater and to convey the treated wastewater to groundwater. These systems should be certified to specific performance criteria and may be suitable in areas where a septic tank is not acceptable.

Packaged wastewater treatment systems comprise several components some of which are mechanical and/or electrical. These systems require regular monitoring and maintenance. Generally such systems produce a higher-quality effluent in terms of organics and micro-organisms as compared with septic tank systems.




Fibrous peat filters are used as intermittent open filters to treat septic tank wastewater. A peat filter typically consists of a distribution system, the peat treatment media and a drain. Septic tank wastewater is intermittently dosed evenly, via a pipe distribution network fitted with orifices, onto the top peat media. The effluent then percolates through the peat, receiving treatment by passive biofiltration processes (filtration, absorption, adsorption, ion exchange, microbial assimilation).

Peat is polar, has a high surface area and a highly porous structure. In addition, the low pH of the peat media, its trace hydrocarbons and indigenous microflora have some anti-microbial properties. Each module of a modular unit should be provided with a cover. The hydraulic loading rate on peat filters may vary depending on the type of peat employed. They require a polishing filter prior to discharge to ground.




The treated wastewater from packaged systems should be treated in a polishing filter system, the primary purpose of which is to reduce micro-organism numbers in the treated wastewater. If the packaged wastewater treatment system is poorly maintained and operated outside of optimal conditions the polishing filter may clog and fail to function properly leading to water pollution. All polishing filters should have a minimum thickness of 0.9 m of free-draining unsaturated soil or sand between the point of infiltration of the effluent and the water table and bedrock. They may be below, at ground surface or partially or totally above ground surface. The polishing filter produces a high quality effluent as it contains a reduced organic load from secondary treatment systems compared with septic tank systems, and thus the biomat is less developed.

Gravity discharge Treated wastewater from the secondary filter should flow by gravity to a distribution box, which distributes the flow evenly into the several trenches which should be 500 mm wide at 2-m spacing and designed according to the criteria given in the Code of Practice, with the exception that the maximum length of each trench should not exceed 10 m.




Pumped discharge The treated wastewater from the secondary treatment unit is pumped to a manifold and percolation pipes using typically 32 mm laterals with 4–6 mm orifices (0.6 m apart) at 0.6 m spacing between laterals facing downwards over a 200-mm layer of gravel.