The efficiency of gravity distribution devices for on-site wastewater treatment systems

A detailed analysis of different types of gravity distribution devices, designed to split on-site wastewater effluent equally between percolation trenches, has been carried out both in the laboratory and also in the field under realistic loading conditions. Five different types of distribution device have been compared: a V-notch distribution box, stilling chamber box, T-splitters with and without baffles and tipping bucket device. The trials carried out in the laboratory with clean water showed that flow distribution for all devices was sensitive to both the off-level installation angles and variable flow rates, with the most stable performance achieved using the T-splitters with baffles and tipping bucket devices. In parallel to this, the on-site flow regime experienced at two sites was continuously monitored using a tipping bucket and data-logger over eighteen month periods, finding that the most common flow rates at the distribution unit were in the range of 0.1–2.5 L/min. The on-site performance of these devices receiving both septic tank and secondary treated effluent showed that significant solid deposition and biofilm development had severely affected the equal distribution between the trenches, hence highlighting the need for regular maintenance to ensure efficient performance over time after installation.