FINAL CLARIFIER FAILURE MECHANISM NOT PREDICTED BY STANDARD STRESS TESTING PROTOCOLS

Miami-Dade Water and Sewer Department (MDWSD) own and operate the South District Wastewater Treatment Plant. The treatment plant receives predominantly domestic wastewater, but also treats septage and leachate. The plant is a 112.5 mgd, high purity oxygen (HPO) conventional activated sludge process. The secondary treatment process consists of 6-stage Oxygenation Tanks, followed by circular Final Clarifiers. The return activated sludge is recycled back to the first stage of the Oxygenation Tank. Waste activated sludge is thickened and anaerobically digested. The digested sludge is dewatered by centrifugation, and the dewatering centrate is also treated. The secondary effluent receives disinfection, before deep well injection – refer to Figure 1.Miami-Dade Water and Sewer have embarked on a program to upgrade their wastewater collection system. One of the main objectives of this rehabilitation program is to cut back on the number of wet weather storm overflows from the collection system. The rehabilitated collection system has resulted in substantial peak wastewater flows entering the treatment facility under storm and hurricane conditions. A peak wet weather management plan is being developed to upgrade and retrofit the existing plant capacity. The Final Clarifiers play a pivotal role in maintaining effective treatment during wet weather conditions. A series of full scale testing under simulated wet weather conditions was conducted to establish the final clarifier capacity. Conventional and unique failure mechanisms were observed and specific recommendations to control the failures were developed.An Oxygenation Tank and associated Final Clarifier were isolated from the mainstream treatment process to allow simulated wet weather testing. The testing protocol followed the guidelines of the WEF/ASCE clarifier testing protocol. Wet weather flow and plant operating conditions were simulated during an extended testing period, lasting three days for each test. The long duration of the test (compared to the typical clarifier testing protocol) was dictated by past observations that the health of the activated sludge culture progressively deteriorates during prolonged storm events. The summer storm events can be of long duration and a short-term test was not considered appropriate.