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Figure 3: Critically Loaded – Thickening Figure 4: Overloaded – Thickening Figure 5: Critically Loaded – Clarification Application of State Point Analysis The SPA approach can be used to assess the behavior of clarifiers that are not limited by hydraulic inefficiencies. The clarification and thickening performance of a clarifier can be assessed by examining the SPA curves. Clarification Function The location of the State Point in relation to the solids flux curve would indicate whether clarification failure is happening or likely to happen. Clarification failure means that the solids are not adequately separated from the main flow. This can result in an effluent high in TSS. When the State Point lies on the flux curve, the settling velocity equals the OFR (also called the upflow velocity). If the State Point is outside the flux curve, the OFR would be greater than the settling velocity. As a result the feed solids would be prevented from settling and would be carried over the effluent weir resulting in clarification failure. Thickening Function The location of the UFR line in relation to the descending arm of the solids flux curve would indicate whether the clarifier is underloaded, critically loaded, or overloaded with respect to thickening. These conditions are defined below: Underloaded: The solids entering the clarifier are conveyed to the bottom and removed with the RAS. No significant solids accumulation occurs within the tank. Sludge thickening is at a minimum and is just adequate to transition from the MLSS concentration entering the tank to the RAS solids concentration exiting the tank. No appreciable sludge blanket. Critically loaded: Solids entering the tank are slightly in excess of the solids leaving the tank. As a result a sludge blanket is formed. This may be acceptable operational strategy to cope with diurnal peak solids load. Continued growth of the sludge blanket should be avoided. If not, the clarifier could easily become overloaded. Overloaded: Net transfer of solids from aeration basin to the clarifier leading to significant solids accumulation and deep sludge blankets. If left unchecked, the sludge blanket could propagate to the surface resulting in loss of solids in the effluent (clarification failure). It should be noted that when the State Pont is above the solids flux curve (clarification failure), the input solids flux is greater than the downward flux for the input solids concentration. This means that some of the applied solids cannot settle to the sludge blanket and will be lost with the effluent. Hence, a clarifier in clarification overload is automatically in thickening overload as well. A few examples of how SPA can be used to improve clarifier performance are summarized in Table 2. The potential solutions, denoted (a) and (b), are shown in dashed line in the respective figures. * Potential solutions (a) and (b) are shown in dashed lines in the respective figures Increasing the RAS flow is a quick way to transfer solids from the clarifier to the aeration basin to relieve thickening failure. However, this will also increase the solids loading rate to the clarifiers and may not be an effective long term strategy. Table 2 – Interpretation of the State Point Analysis State Point UFR Line Clarification Thickening Potential Corrective Action* Within the flux curve (Fig. 2) Below the descending limb of the flux curve Normal Underloaded None. Within the flux curve (Fig.3) Within the flux curve (Fig.4) On the flux curve (Fig.5) Tangential to the descending limb of the flux curve Intersects the descending limb of the flux curve Below the descending limb of the flux curve Normal Normal Critically loaded Critically loaded Overloaded Underloaded 3(a) Increase RAS Rate. 3(b) Reduce feed solids by converting to step-feed or lowering MLSS. 4(a) Control filaments to improve sludge settleability. 4(b) Reduce feed solids by converting to step-feed or lowering MLSS. 5(a) Increase number of clarifiers to reduce rise rate. Outside the flux curve (Fig. 6) Intersects the descending limb of the flux curve Overloaded Overloaded 6(a) Increase number of clarifiers to reduce rise rate. 52 MWEA MATTERS: FALL 2005
Figure 6: Overloaded Clarification & Thickening Figure 7: Settling Columns CONCLUSION This article outlines the concept of SPA and how it can be used by designers for capacity analysis and by operators for solids inventory control. The SPA approach uses site specific settleability data to predict clarifier performance fairly accurately, provided there are no hydraulic inefficiencies. If such constraints are present, it is likely that clarifier failure would occur sooner than predicted by the SPA. The engineer’s goal should be to use good design practices in the design of final clarifiers. However, poor sludge settleability can curtail the operating range of even the best of clarifiers. Operators should strive to attain the best settleable sludge practically possible under their operating conditions. Doing so will ensure stable clarifier operation over a wide range of operating scenarios. Finally, other methods similar to SPA (such as the Daigger Operating Chart and Ekama Approach) are also available for clarifier analysis. MWEA MATTERS: FALL 2005 53
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