Solids Loading Limitations of Rectangular Secondary Clarifiers

Basin configuration and equipment design govern whether rectangular secondary clarifiers will experience problems of inadequate sludge transport capacity. The operating factors to be considered, other than peak flows which may be severe, are the potential for sludge bulking and the higher mixed liquor suspended solids concentrations and solids retention times employed for biological nutrient removal processes. Rectangular clarifiers longer than 20?m and loaded at more than 3.5?kg/m2?day often have sludge transport/shortcircuiting problems. Shortcircuiting of mixed liquor into the return sludge is a common situation that can be avoided in new designs and easily corrected in existing facilities. A step-by-step design approach is presented as a series of process calculations with graphs. Results from the unmodified and the improved rectangular clarifiers at Phoenix 91st Avenue wastewater treatment plant, Ariz., are presented.