Secondary Settling Tank Inlet Design: Full Scale Test Results Lead To Optimization

Many of the activated sludge and trickling filter plants in operation today are capacity limited by the performance of their secondary settling tanks (SST). A large percentage of these in turn are circular or square, center-feed units. Adding a tank to increase capacity at a large plant could easily cost 1-2 million or more. Such an additional unit could also require changes in yard piping and the RAS pump station, as well as take up valuable footprint area. Recent full-scale research at Hyperion showed that a relatively inexpensive modification to the clarifier inlet design resulted in doubling the tank's capacity. This paper presents a review of inlet designs, gives a hypothesis as to why some inlets are so much more effective than others, and shows data compiled from several full-scale test projects. The results suggest that designs emphasizing impingement for energy dissipation and flocculation are superior to others.