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Central Marin Sanitation Agency Wastewater Treatment Plant Evaluation

As part of wet weather upgrades, ferric chloride and polymer systems were installed to enable Central Marin Sanitation Agency (CMSA) to perform chemically enhanced primary treatment (CEPT). While the ferric system was used regularly, the polymer system was observed to have negative effects on clarifier performance. Hazen and Sawyer was selected to assist CMSA with evaluating and optimizing the CEPT system.

The CMSA Wastewater Treatment Plant (WWTP) located in San Rafael, CA is a 6 mgd ADWF plant with peak wet weather flows of 130 mgd.

This effort involved the following key elements:

Jar Testing: Hazen utilized jar testing to determine the ideal flocculation time and doses of ferric chloride and polymer for CEPT. Jar testing showed that while CMSA’s primary clarifiers performed well without CEPT, increased TSS and COD removal rates could be achieved with ferric chloride and polymer addition.

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Jar testing various doses of ferric and polymer.

Field Testing: Hazen conducted full scale testing to observe primary clarifier performance under normal and stressed conditions. Through careful observation and small changes, ferric and polymer dosages were optimized and an improvement in clarifier performance was observed. During testing it was also found that the aeration in the primary clarifier influent channels, where polymer was dosed, caused sludge to float to the surface in the primary clarifiers. By turning off the air in the primary clarifier influent channels, clarifier performance improved.

Under stressed conditions it was found that flocculation time in the primary clarifier influent channels was significantly reduced. During the exceptionally high peak flows that the plant regularly sees, flocculation time decreased below the minimum flocculation time of 2.5 minutes. At these high flows, the plant influent is typically very dilute, decreasing the effectiveness of CEPT to enhance primary clarifier removals.

CMSA working on site 2
Hazen conducted full scale testing to observe primary clarifier performance under normal and stressed conditions.

Hazen’s recommendations included dosing strategies for dry weather, elimination of channel air in primary clarifier influent channels and further wet weather testing to determine potential CEPT effectiveness during wet weather. Recommendations for the optimization of the CEPT system included improved control systems and strategies and alternatives to increase flocculation time.

CMSA primary clarifier
Optimization of CEPT as primary clarifier influent channel air is reduced.

Project Outcomes and Benefits

  • Reviewed five years of historical data to better understand the plant’s primary clarifier performance.
  • Utilized jar testing and full scale CEPT testing to understand the potential enhanced removal rates as well as the limitations of the full-scale system.
  • Determined optimal doses of ferric chloride and polymer, allowing plant staff to make operational improvements.
  • Full-scale demonstration of Hazen’s recommended conditions showed that increasing flocculation time and removing air from the primary clarifier influent channel improved clarifier performance.