Lessons Learned from Startup of the Largest Tertiary Ballasted Settling System in the U.S.


In February 2005, Onondaga County (County) began operating a 492,000 m3/day (130 mgd) peak flow ballasted settling system at the Metropolitan Syracuse Wastewater Treatment Plant (Metro WWTP) for tertiary phosphorus removal.

The system was designed to reduce the total phosphorus concentration in the BAF effluent from 0.75 to 0.12 milligrams per liter (mg/L). This performance was to be achieved at a ferric chloride dosage not to exceed 25 mg/L (as FeC13) and a polymer dosage not to exceed 0.6 mg/L (active polymer).

Shortly following startup, it became apparent that the system was falling slightly short of expectations. Daily effluent total phosphorus concentrations were generally ranging from approximately 0.15 to 0.20 mg/L. To improve the performance of the system, the County began evaluating a series of changes in operation and maintenance of the system with assistance from the system vendor, Kruger Inc., and the system design engineer, Environmental Engineering Associates, LLP (EEA). These changes included modification of recommended procedures for cleaning of the lamella tube settling modules in each train, varying ferric chloride and polymer dosages, and experimentation with alternative coagulants.

Cleaning of the lamella tube settling modules was discovered to be of critical importance to overall process performance shortly following startup of the system. Accumulation of chemical solids within and on top of the lamella tube settling modules resulted in the carry-over of fine solids to the plant effluent. To address the accumulation of solids within and on the lamella tube settling modules, the County increased the frequency of tube cleaning to once per week.
In consultation with Kruger and EEA, the County also began efforts to optimize performance by varying the dosages of ferric chloride and polymer solution added.
Based on these efforts, concern was raised about whether each ACTIFLO process flow train was receiving a consistent dosage of ferric chloride solution due to apparent differences in wastewater flow distribution to each train. Despite efforts to improve performance by optimizing the dosages of ferric chloride and polymer solution, and performing frequent cleaning of the tube settling modules, reduction of effluent total phosphorus concentrations to less than 0.12 mg/L still was not possible. During the fall of 2005, tube clogging problems continued to occur culminating in the failure and displacement of numerous modules from the system. In consultation with Kruger, the decision was made to replace the lamella tube settling modules with new modules having larger openings.

In addition, it was also decided to relocate the point of ferric chloride addition to address concerns about differences in ferric chloride dosages applied to each process flow train due to possible differences in flow distribution. A diffuser was installed for the addition of ferric chloride solution to the common influent channel to provide a more consistent and uniform dosage of ferric chloride to each ACTIFLO process flow train.

Replacement of the tube settling modules and relocation of the ferric chloride feed point were completed in January 2006. Since April 2006, the ACTIFLO system at Metro has demonstrated consistent performance with effluent phosphorus concentrations averaging 0.10 mg/L or less. In addition, the installation of lamella tube settling modules with larger openings has significantly improved the effectiveness of tube cleaning by the operators. As a result, the frequency of tube cleaning has been reduced from once per week to once every two weeks. In conclusion, several important lessons were learned in connection with startup of the ACTIFLO system at Metro. These lessons included the importance of lamella tube size and cleaning procedures as well as consistent and uniform ferric chloride dosage to each process train.

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