Impact of Activated Sludge and Clarifier Configurations on Final Clarifier Capacity

OBJECTIVE
One of the most common methods to define final clarifier solids loading rate (SLR) capacity is Solids Inventory Control State Point Analysis (SPA). However, using SPA can over-estimate final clarifier SLR capacity. (Ekama) To account for clarifier inefficiencies, an SLR 'de-rating' factor (reduction in maximum allowable SLR) is typically applied to SPA results to reduce the maximum allowable SLR. This paper presents how the SLR de-rating factor can be impacted by activated sludge operating mode, surface overflow rate, and clarifier configuration.

METHODOLOGY
Bench- and stress-testing was completed to evaluate final clarifiers capacity and performance. Clarifier evaluations were conducted in accordance with WERF/CRTC Protocols for Secondary Clarifier Performance (Wahlberg) with subsequent computation fluid dynamics modeling of the final clarifiers to maximize capacity and improve system performance. Optimized final clarifier SLR capacity was then compared to the SPA maximum allowable SLR (Wahlberg and Keinath) for the same operating conditions. SLR de-rating factors as defined by equation 1 were developed for clarifiers having the general features shown in Figure 1.

SLR 'De-rating' factor (Percent) = 100*(1 — Maximum SLR by CFD modeling/SPA maximum allowable SLR) (Equation 1)

Maximum SLR capacity by CFD modeling is based upon analysis using a calibrated two-dimensional clarifier model (McCorquodale et al) with SLR capacity defined by loading conditions in which either the predicted sludge blanket exceeds half the clarifier side water depth (SWD) or effluent suspended solids exceed a target value (ranged from 15 to 35 mg/L) during a 4 to 6 hour sustained loading condition. Results were divided into two datasets: biological nutrient removal facilities (BNR) and high-rate activated sludge systems.

FINDINGS
Figure 2 shows SLR de-rating factors for BNR facilities with sustained surface overflow rates (SORs) ranging from 600 to 1,300 gallons per square foot-day gal/sf-d) ranged from 0 to 15 percent. This range and value is less than 20 percent de-rate factor recommended by Ekama to account for clarifier inefficiencies not included in SPA. For all plants in this dataset, the sludge blanket depth was the limiting capacity factor. In addition, Figure 2 shows a slight increase in SLR de-rating factor as the SOR per foot of SWD increases.

Figure 3 presents the SLR de-rating factor for high-rate activated sludge systems operating at solids retention times of 2.5 days or less. This data set consists of conventional, TF/SC, and high-purity oxygen activated sludge facilities. Figure 3 shows the SLR de-rating factor at a typical peak SOR of 1200 gal/sf-d ranged from 25 to 30 percent, a substantial increase compared to a BNR facilities presented in Figure 2. In general, the higher SLR de-rating factor at this SOR was related to poorer bio-flocculation resulting in high effluent suspended solids. Figure 3 also shows as SOR increase above 1200 gal/sf-d the SLR de-rating factor increases to values of 40 to 45 percent. The higher SLR de-rating factors correlated to conditions in which the SOR per foot of SWD exceeded 100 gal/sf-d.

Figure 4 presents several additional BNR system de-rate factors for clarifiers with SWD less than 12 feet and an extremely large clarifier. Figure 4 shows the SLR de-rating factor for the extremely large clarifier with scraper sludge collection is roughly 40 percent. Replacing the scraper mechanism with a hydraulic suction collector to improve sludge transport reduces the de-rate factor to 35 percent while using in-board launders with hydraulic suction collectors would further reduce the de-rate factor to 25 percent. Figure 4 also shows the SLR de-rating factor with scraper removal mechanisms can be 5 to 10 percentage points higher than hydraulic suction sludge removal.

This paper will also illustrate how the duration of the peak loading condition can impact the SLR de-rating factor and present similar SLR de-rating factors for peripheral feed and 'Squircle' clarifiers.

CONCLUSIONS
When using SPA for final clarifier design or plant operations, the SLR capacity can change significantly based system operations, duration of peak flow, and clarifier configuration. This analysis assists the designer or operator in selecting an appropriate SLR de-rating factor for determining clarifier capacity and/or target operating conditions. For typical BNR systems an SLR de-rating factor or 10 to 15 percent appears reasonable whereas for high-rate activated sludge systems an SLR de-rating factor of 25 percent or more may be needed.