Characterizing shallow secondary clarifier performance where conventional flux theory over-estimates allowable solids loading rate

Abstract

The performance characteristics of relatively shallow (3.3 and 3.7 m sidewater depth in 30.5 m diameter) activated sludge secondary clarifiers were extensively evaluated during a 2-year testing program at the City of Akron Water Reclamation Facility (WRF), Ohio, USA. Testing included hydraulic and solids loading stress tests, and measurement of sludge characteristics (zone settling velocity (ZSV), dispersed and flocculated total suspended solids), and the results were used to calibrate computational fluid dynamic (CFD) models of the various clarifiers tested. The results demonstrated that good performance could be sustained at surface overflow rates in excess of 3 m/h, as long as the clarifier influent mixed liquor suspended solids (MLSS) concentration was controlled to below critical values. The limiting solids loading rate (SLR) was significantly lower than the value predicted by conventional solids flux analysis based on the measured ZSV/MLSS relationship. CFD analysis suggested that this resulted because mixed liquor entering the clarifier was being directed into the settled sludge blanket, diluting it and also creating a 'thin' concentration sludge blanket that overlays the thicker concentration sludge blanket typically expected. These results indicate the need to determine the allowable SLR for shallow clarifiers using approaches other than traditional solids flux analysis. A combination of actual testing and CFD analyses are demonstrated here to be effective in doing so.

Marrying Step Feed with Secondary Clarifier Improvements to Significantly Increase Peak Wet Weather Treatment Capacity: An Integrated Methodology

“…These results represent the culmination of a process that began in 2010 with completion of the initial assessment described above (CH2M HILL, 2010) based on modeling the biological treatment process using an International Water Association (IWA) Activated Sludge Model (ASM) and CFD analysis of proposed secondary clarifier modifications (the condition of the existing secondary clarifiers necessitated some level of modification). The potential increase in peak wet weather secondary treatment capacity identified in that evaluation, while quite surprising at the time, has been verified by subsequent field testing as described here and elsewhere (CH2M, 2010;Daigger et al, 2016;Siczka et al, 2014Siczka et al, , 2015Siczka et al, , 2016. The following lessons learned through application of the process followed for the City of Akron can be applied when this approach is used at other locations:…”

“…Secondary clarifier stress tests were conducted in the relatively standard manner by sequentially altering the loading conditions on the clarifiers while collecting effluent TSS samples sequentially, and then analyzing the data to develop performance versus loading curves (Daigger and Buttz, 1998;Ontario Ministry of the Environment and Energy, 1996;Wahlberg, 2001). The results of these investigations have been reported elsewhere (CH2M, 2015;Daigger et al, 2016;Siczka et al, 2014Siczka et al, ., 2015 and were used to initially indicate potential increases in peak wet weather secondary treatment capacity and to guide further testing. They indicated that SORs approaching 3 m/h could be successfully sustained in the modified clarifiers.…”

“…Test critical assumptions and develop contingency plans. Modification of Unit 6 to implement step feed and clarifier improvements and modification of Unit 5 clarifiers, followed by the testing program documented here and elsewhere (CH2M, 2010); Daigger et al, 2016;Siczka et al, 2014Siczka et al, , 2015Siczka et al, , 2016) allowed assessment of the actual peak wet weather secondary treatment achievable. Compliance with other effluent parameter limits (ammonia, phosphorus) was also verified.…”

“…The model includes laminar and turbulent flow with Newtonian flow in the inlet and clarified zone and non-Newtonian flow in the sludge blanket. The model was calibrated and validated using operational and stress test data, as described elsewhere (CH2M, 2015;Daigger et al, 2016;Siczka et al, 2014Siczka et al, , 2015Siczka et al, , 2016. The stress tests were accompanied by Vesilind settling tests as well as DSS and FSS tests.…”

“…The ease with which these capabilities could be retrofitted to Unit 6 for testing purposes made it the obvious focus for initial investigations. The objective of the work presented here and elsewhere (CH2M, 2015;Daigger et al, 2016;Siczka et al, 2014Siczka et al, , 2015Siczka et al, , 2016) was to determine the increased peak wet weather secondary treatment capacity achievable, with the objective of reducing the size of supplemental peak wet weather secondary treatment facilities needed. Full-scale testing was conducted using a portion (one of six trains) of the existing City of Akron WRF, supplemented with CFD analysis of the modified clarifiers.…”