The quality of the effluent from activated sludge wastewater treatment plants is highly dependent on the performance of the final clarifier. The final clarifier performs solids/liquids separation through sedimentation. The standard design criteria (overflow rate and detention time) must be selected such that the effluent suspended solids requirements are met.
Numerous research programs have been conducted to evaluate the factors affecting the sedimentation efficiency of activated sludge mixed liquor. Flocculation has been documented as a significant factor. The flocculation process is an agglomeration of small primary particles of the surface of larger particles to achieve higher settling velocities. Increased settling velocities promote enhanced sedimentation efficiencies.
This research reports on some design aspects of clarifier feed wells and the associated impact on the flocculation process. A number of experiments were conducted using a clarifier with a fixed diameter. The effluent suspended solids concentration was evaluated for different feed well diameters. In the seven experiments performed, the feed well diameter was varied from 2 to 5.5 inches with a constant clarifier diameter of 8 inches. An enlarged feed well diameter provides longer detention times in the feed well and, hence, promotes solid contact and flocculation.
The experimental results, however, indicate that use of a larger feed well diameter increases the effluent suspended solids. In the clarifier used for the experiments, the clarification zone was concentric with the feed well. Any increase in the feed well diameter produced a decrease in the available clarification zone. The reduced clarification zone resulted in a shorter detention time for clarification and an increase in overflow rate. The decreased available detention time together with the associated increase in overflow rate negated any beneficial effect of enhanced flocculation in the larger feed well.
In the range of values in the experimental program, a feed well diameter of 25% of the clarifier diameter was found to yield the best results with respect to effluent suspended solids. This value appears to be a fair trade-off between improved flocculation in an enlarged feed well and impaired suspended solids removal due to a shorter detention time and an increased overflow rate in the clarification zone. This optimum configuration appears to be consistent with current practice.
Dietz, John D.
Master of Science (M.S.)
College of Engineering
Length of Campus-only Access
Masters Thesis (Open Access)
Shahnami, Mahnaz, "The Use of Enlarged Feed Wells as Flocculator Units in Activated Sludge Secondary Clarifiers" (1986). Retrospective Theses and Dissertations. 4948.