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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.