Importance of Laminar Flow Design in Septic Tanks

A survey of septic tank design shows that short-circuiting is minimized and effluent quality is improved when quiet, laminar, fluid-flow is encouraged, such as in long, narrow tanks with little hydraulic ‘dead space’. Longitudinal partitions provide better solids retention by providing longer travel distance, but transverse partitions are advantageous only when the opening between compartments is large enough to permit quiet flow. Field testing using synthetic solids shows that a partitioned standard tank with two 150mm openings provides better relative retention of incoming sewage than a short, singlecompartment standard tank of the same depth, even when dosed at the same relative volumes. Dosing rates of 10% of tank volume result in more than double the solids short-circuiting than 5% volume dosing. Proprietary tanks (WaterTubeTM) designed with no airspace and a long, narrow, shallow aspect to induce closed-conduit, laminar flow provided the best solids retention, with negligible flow-through even during sustained, high-rate pumping. Test results using sewage at the Massachusetts septic system test facility at Buzzards Bay showed that the proprietary flooded tank had no scum and very little sludge halfway along the pathway, with scum only in the inlet riser, confirming the relation between air space and formation of hard, moldering scum.