An Application of Tube Settlers in The Secondary Clarification of Domestic Wastewater

Abstract

<p>The North-End Water Pollution Control Centre (NEWPCC) in Winnipeg, Manitoba, treats an average annual flow of 60 MGD. During peak periods, spring run-off and storms, flows to the plant exceeds 110 MGD. The capacity of the ten secondary clarifiers limit the capacity of the plant to 65 MGD.</p> <p>This report discusses the results of an attempt to upgrade the capacity of the existing clarifier at Winnipeg with the use of tube settlers. This experimental programme monitored variables which govern both the biological conditioning within the aeration tank (DO, SRT, F/M, etc.) and the hydraulic effects on the clarifier (Q and QR). A 5½ inch ID, 5 feet high transparent plexiglass column was used to monitor the zone settling velocities of various mixed liquor concentrations, from which flux plots were drawn and the limiting hydraulic and solids loadings were determined.</p> <p>A discontinuous or pulse input of Rhodamine WT dye was used to trace the hydraulic behaviour within the clarifier. The overflow was monitored continuously and the underflow or return was monitored by frequent batch sampling.</p> <p>The results indicate that under good biological conditioning (clarification limiting), the clarifier with tube settlers treated flows up to 11.5 MGD or 2000 GPD/sq.ft. of clarifier surface area (or 167 GPD /sq.ft. of tube settler area) at solids loadings of 50 to 60 lbs/day/sq.ft. This is 80% greater than the flows handled by a similar clarifier with no tubes. Under upset biological conditions (thickening limiting), the tube settlers could not handle flows in excess of 6.5 MGD or 1100 GPD/sq.ft. of clarifier surface area (or 93.2 GPD/sq.ft. of tube settling area) at solids loadings of 45 lbs/d/sq.ft.</p> <p>Although no significant short circuiting was found in the overflow, the underflow experienced serious short circuiting problems. As the underflow was increased beyond 2.5 MGD or 430 GPD/sq.ft. of clarifier surface area, short circuiting occurred deteriorating the effluent quality. Since the tracer studies were conducted during periods of thickening limiting, it is not known if the short circuiting in the underflow was singularly responsible for the deterioration of effluent quality, or if it was an interaction between thickening limitation and the short circuiting in the underflow. This is contradictory to the established design criteria of secondary clarifiers which state that an increase in underflow would increase the ability of the clarifier to handle a greater solids loading.</p> <p>Now that the ground work has been laid in correlating tube settler performance to thickener design, future studies should pursue the possibility of establishing predictive capability of tube settler performance in secondary clarification of domestic wastewater.</p>