Reducing Sludge Blanket Depth in Circular Scraper Secondary Clarifiers Using a Sludge Collection Ring

The proper functioning of secondary settling tanks (SSTs) is critical to the effectiveness of the activated sludge process. Excessive inventories of settled solids and/or deep sludge blankets in SSTs can result in poor clarification performance.The goal of this research was to evaluate the effectiveness of a sludge collection ring versus a conventional sludge hopper in reducing sludge blanket depths and solids inventories in circular scraper type SSTs.Two identical eighty foot diameter circular scraper type SSTs were studied. One was fitted with a sludge collection ring; the other included a standard hopper. Several seven hour tests were run at identical influent and recycle flow rates, with the rake mechanisms both on and off, and measurements were made of sludge blanket depths. Samples of MLSS and RAS, as well as a clarifier composite, were also analyzed for total suspended solids (TSS). Calculations were performed to estimate total sludge inventories and short-circuiting flows within the basins.Testing showed that the SST with the sludge collection ring consistently produced a lower sludge blanket and lower solids inventory than the hopper SST in all but one test scenario. Sludge blanket depths averaged approximately one foot lower in the SST with the sludge collection ring. Only at a low RAS flow were the two sludge blankets at essentially equal average levels.It was hypothesized that this improvement may be due to less dilution of the RAS flow. However, calculations showed that the average short-circuiting flow in the SST with the sludge collection ring was generally greater than or equal to that in the hopper SST. Further research is required to identify the controlling parameters.