MINOR LOADING AND CONFIGURATION DIFFERENCES SIGNIFICANTLY AFFECT CLARIFIER PERFORMANCE AND CAPACITY AT THREE WWTPS IN PHILADELPHIA

The Philadelphia Water Department (PWD) has initiated a program to minimize the Combined Sewer Overflows (CSOs) to local receiving waters. One of the strategies to increase treatment capacity and reduce the CSO volume is to maximize flow to be treated by the existing WPCPs during periods of wet weather. To implement this strategy effectively, PWD needs to have a clear understanding of the factors that affect the hydraulic capacity of the three WPCPs in their jurisdiction.The objective of the project was to determine the reliable maximum treatment capacity of the existing facilities and identify low capital cost methods of increasing the ability of the facilities to treat peak hydraulic flows associated with wet weather conditions. This paper presents the results of twelve stress tests performed on the primary and secondary clarifiers at the three facilities. The major physical characteristics of the clarifiers are very similar. However, the measured performance between clarifiers was significantly different. By comparing the results from the three facilities, the study documented the effect of variations in influent characteristics, operational procedures, and minor physical modifications on clarifier performance. The allowable peak overflow rates for the primary clarifiers ranged from 80 to 140 m3/m2/d. The allowable peak overflow rates for the secondary clarifiers ranged from 68 to 85 m3/m2/d. The capacity of the secondary clarifiers was limited at solids loading rates greater than 165 kg/m2/d.The stress test results defined the peak hydraulic capacity of the clarifiers. Data from the stress tests was also used to predict clarifier effluent quality as a function of hydraulic loading at flow rates less than peak capacity. This information was used to predict mass loading and percent removal achieved to ensure all NPDES permit requirements can be met. A regression analysis of the stress test results was used to determine the maximum flow rate that could be maintained for an infinite period of time (sustainable flow) that allows the facility to meet the daily, weekly, and monthly average mass loading and percent removal requirements. The confidence limits from the regression analysis can be adjusted to reflect the level of risk associated with the predicted result.