Combined Sewer System Long Term Control Plan - DC Water

Selection of Final LTCP
Rate) treatment facility at BPWWTP. A dedicated force main would be constructed from the
proposed tunnel dewatering pumping station at Poplar Point to the high rate facility. The tunnel
contents would be dewatered through the high rate facility, disinfected, dechlorinated and discharged.
During wet weather events, it was envisioned that the new high rate facility would be used in place of
the existing excess flow treatment system at BPWWTP, and would treat up to 350 mgd of excess
flow during rain events. This would provide a higher quality effluent than the existing excess flow
treatment system. Figure 11-6 presents the major components of the expanded plan.
11.3.6 Alternatives Based on Zero Overflows in the Period 1988-1990
An alternative was developed to provide zero overflows in each of the three analysis years. Due to
the large CSO storage volumes required, it was not feasible to simply increase the diameter of the
tunnels proposed in the Draft LTCP. Instead, it was necessary to extend the tunnels as follows:
Anacostia – extend the Anacostia tunnel from Poplar Point to BPWWTP
Potomac – Extend the Potomac tunnel from the Potomac Pumping Station to connect with the
Anacostia Tunnel near Main and O Street Pumping Stations
Piney Branch- Delete the Piney Branch storage tunnel proposed in the Draft LTCP. Instead,
extend the Anacostia tunnel along Florida Avenue up to Piney Branch to capture this
overflow.
Blue Plains – construct a 350 mgd high rate treatment facility and a central tunnel dewatering
pump station at the plant. Dewater the tunnels through the high rate facility.
11.3.7 Alternatives Sized for 5-Year 24-Hour Storm
This alternative was sized for the 5-year 24-hour storm. The tunnels were configured as above for
the zero overflow in three years option. Due to the large storage volume required, it was not feasible
to capture all of the CSO in the tunnel. Instead, the first 773 mg is captured in the tunnel and the
remainder is conveyed by the tunnel to Blue Plains where it treated by a 1,725 mgd sedimentation/
disinfection facility.
Figure 11-7 presents the major components of plans based on zero overflows in 1988-1990 and based
on the 5-year 24-hour storm. A summary of the alternatives is included in Table 11-12.
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