A Comparison Between Distribution Devices used to Split On-site Wastewater Effluent Between Percolation Trenches

Laurence Gill, Titiksh Patel, Niall O’Luanaigh

Department of Civil, Structural & Environmental Engineering

University of Dublin, Trinity College


A rigorous site assessment is now required in Ireland in order to design an on-site wastewater

treatment system. A critical component of an effective gravity-operated on-site system is the distribution box, which must be able to promote an equal split of effluent between percolation trenches. Comparisons were made both in the laboratory using clean water and also on-site using a V-notch distribution box on two different types of wastewater effluent (septic tank and secondary treated). The trials carried out on distribution devices in the laboratory showed that flow distribution was sensitive to both the off level installation angles and variable flow rates.

The flow regime experienced at distribution devices has been continuously monitored using tipping bucket instrumentation over six month periods which established that the most common flow rates at the distribution unit were in the range of 0.1 to 2 L min-1. Laboratory tests were also carried out on three other distribution devices (a stilling chamber and two in-line flow splitter devices) which revealed that a flow splitter with upstream baffle plates achieved the best performance under varying installation angles and flow rates.  


Poor distribution of on-site wastewater effluent between subsoil percolation trenches is often the cause of septic tank / soil percolation system failures. Poor distribution results in hydraulic and biological overloading of one or more of the percolation trenches. Even if the soil hydraulic conductivity is adequate for transmitting the liquid away from the trenches, the formation of a biomat may restrict wastewater percolation from the trench (Van Cuyk et al., 2001). Hence, equal distribution of the effluent is essential to prevent the failure of the system whereby all of the effluent passes into one distribution line to the exclusion of the others. Equal distribution to soil absorption components can be particularly difficult to achieve when relying on gravity to promote such a split.


The laboratory trials using clean water showed that the Flow Splitter device with baffles showed the best flow splitting efficiency and was relatively insensitive to a range of off-level installation angles. In contrast the stilling chamber design proved very sensitive to being off level and also to different flow rates.

11th Individual and Small Community Sewage Systems Conference Proceedings

20-24 October 2007 (Warwick, Rhode Island, USA)

Publication Date: 20 October 2007

ASABE Publication Number 701P1107