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   Introduction
   1. Background
   2. Supplement
   3. Conveyances
   4. RTF
   5. Schedule
   Appendices

Appendix 7: Advantages and disadvantages of alternative 2 and alternative 3

Alternative 2

330 cfs RTF plus 4.2 million gal conveyances storage plus 12.5 million gal underground storage tank (total system storage with RTF is 8.45 million gal)

System will capture 92% of total average annual wet weather flow for treatment at Metro
USEPA-developed, proven CSO technology (vortex separator)
Capable of treating back-to-back storm events
Lower post storm cleanup requirements
Designed to operate under varying flow conditions
Reduced land area requirements
High degree of settleable solids and floatables removal
Less difficult construction
Less costly per gallon of CSO abated
Captures smaller intensity storms for post-storm treatment at Metro
Least project cost ($55,240,000)
Minimizes deep excavation
Least construction impacts on neighborhood surrounding RTF and underground storage facility
Minimizes impact on MIS and Metro
Lowest 25-year present worth cost ($62,500,000).

Disadvantages

Conveys smaller volume of CSO to Metro for treatment in comparison to Alternative 3
Average 9 treated (disinfected) discharges per year
Higher power requirements
Larger aboveground facilities
Requires large influent pumps to convey flow to vortex separators
Citizens opposition claims:
-Aboveground "sewage treatment plant"
-Use of chlorine for disinfection
Higher O&M compared to Alternative 3.

Alternative 3

12 million gal underground storage tank plus 4.2 million gal conveyances storage plus additional sewer separation (total system storage is 16.2 million gal)

Advantages

Conveys a larger volume of CSO to Metro for treatment in comparison to Alternative 2
Lower power requirements
Smaller aboveground facilities
Higher degree of settleable solids, floatables, and nutrient removal
Lowest O&M cost.

Disadvantages

Does not have the ability to handle large back-to-back storm events
Higher post-storm cleanup requirements
Requires larger land area in comparison to Alternative 2
Requires larger capacity dewatering pumps to dewater the facility in the same amount of time as Alternative 2
Requires larger odor control system
More difficult construction
More costly per gallon of CSO abated
Highest project cost ($80,410,000)
Extends time to start construction (1 year minimum)
Deep excavation will require substantially more sheeting and bracing, dewatering, soil removal/disposal, and construction materials
Greater construction impacts than Alternative 2 on neighborhood surrounding the underground storage facility
Highest present worth cost ($86,200,000).