Operating Chemically Enhanced Clarification for Optimum Disinfection Performance

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ABSTRACT
The Milwaukee Metropolitan Sewerage District conducted demonstration tests in 2005 to evaluate chemically enhanced clarification (CEC) of wet-weather flows at the District’s two wastewater treatment plants (WWTPs). CEC offers the potential to treat intermittent high wetweather flows, produce high-quality effluent, and fit into the limited space at the plant sites. Two CEC processes were tested at the South Shore WWTP: DensaDeg®, manufactured by Infilco Degremont, and ACTIFLO®, manufactured by Krüger. Primary clarifier influent was the wastewater source, and it was split between the two CEC processes. Testing was conducted during both dry- and wet-weather conditions.

The CEC demonstration testing program had the following specific objectives: evaluate the performance of the selected CEC technologies, evaluate the feasibility of UV disinfection for CEC effluent, compare CEC treatment to existing secondary treatment, and determine key design criteria for full-scale application of CEC for wet-weather flows at Jones Island and South Shore WWTPs.

The CEC demonstration testing at the South Shore WWTP provided a wealth of data that assisted the District in conceptual design evaluations of wet-weather treatment alternatives. The CEC processes provided high levels of treatment at very high overflow rates, minimizing their footprint for inclusion at either the Jones Island or South Shore WWTPs.

INTRODUCTION
The Milwaukee Metropolitan Sewerage District operates two wastewater treatments plants (WWTPs) with a combined peak hour capacity of 630 mgd (2,385,000 m3/d). Following major storms in the service area, wastewater flow into the system can exceed 1 billion gallons per day (3,785,000 m3/d). Significant storage capacity within the system offsets the need to provide treatment capacity equal to the peak daily flow. However, the District’s 2020 Facilities Plan is evaluating alternatives for increasing treatment capacity during major storms.

METHODOLOGY
The District and its consultant, CH2M HILL, conducted demonstration tests in 2005 to evaluate chemically enhanced clarification (CEC) of wet-weather flows at either of its two wastewater treatment plants. CEC offers the potential to treat intermittent high wet-weather flows, produce high-quality effluent, and fit into the limited space available at the Jones Island and South Shore plants. Testing was conducted during both dry-weather and wet-weather conditions. Disinfection effectiveness on CEC effluent was measured during those tests in terms of removal of E. coli, fecal coliform and male-specific coliphage (as a surrogate for human viruses). During wet-weather events, additional analysis was performed for inactivation of viruses, Cryptosporidium, and Giardia. The CEC processes provided high levels of treatment at very high surface overflow rates, minimizing their footprint to allow inclusion at either Jones Island or South Shore.

Two CEC processes were recommended for onsite demonstration testing: DensaDeg®, manufactured by Infilco Degremont (Exhibit 1), and ACTIFLO®, manufactured by Krüger (Exhibit 2). The demonstration testing collected additional operating information to allow the project team to complete the technology evaluation and develop conceptual designs. Since CEC effluent requires disinfection, the demonstration test program also collected information to evaluate the effectiveness of ultraviolet (UV) light on CEC effluent. Demonstration testing was conducted at South Shore from April 28 until July 6, 2005.

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