OMSAP Public MeetingSeptember 1999

Why the Outfall?

OMSAP Public MeetingSeptember 1999

Protecting the Harbor/Bays Ecosystem

1. Cleaner effluent through source reduction andsecondary treatment

2. Dilution

3. Monitoring and Contingency Planning

OMSAP Public MeetingSeptember 1999

Outfall Siting

Siting process beganin 1986. Minimumrequirements wereset by EPA:

n Initial dilution of 50n More than one tidal

cycle away fromshoreline

n Avoid sensitive andunique resources

n Harbor site ruled out

OMSAP Public MeetingSeptember 1999

Outfall Siting Process: Siting Criteria

Siting criteria developed by Facilities PlanningCitizens Advisory Committee:

n Attain compliance with state and federal waterquality criteria

n Protection of commercial on-the-water activitiesn Maintenance and enhancement of aestheticsn Avoidance of areas of important habitat

OMSAP Public MeetingSeptember 1999

Outfall Siting Process: Technical Analysis

7 potential sites evaluated independently by EPAand MWRA: Broad Sound to a location 10 milesoffshore

n Siting studies in 1987 and 1988n Engineering and modelingn 2 years of oceanographic sampling including biological,

physical and chemical studies

OMSAP Public MeetingSeptember 1999

Outfall Siting Process: Site Selection

After extensive regulatoryreview and publiccomment, 9.5-mile siteselected. FEISdetermined that:

n Water depth andcurrent patternspromote effectivedilution

n Least likely to affectsensitive resources

n Feasible to construct

OMSAP Public MeetingSeptember 1999

Outfall Design: Tunnel and Risers

n 380 feet drop shaft from plantn 9.5 mile tunnel, bored through bedrockn Diffuser system (55 risers each with 8 ports) along final

1.25 miles provide maximum dilution

OMSAP Public MeetingSeptember 1999

Outfall design

OMSAP Public MeetingSeptember 1999

Outfall Design: Scale Model

n Roberts scalemodel of outfalland diffusersystem used totest different portconfigurations.

n 8 ports gave mosteffective, rapiddispersion.

OMSAP Public MeetingSeptember 1999

Outfall Design: Diffuser Head

Diffuser head ready for installation

OMSAP Public MeetingSeptember 1999

OMSAP Public MeetingSeptember 1999

OMSAP Public MeetingSeptember 1999

Dilution: Harbor Location

n Average depth about 30 feetn Available dilution is about 14 to 1.n With onshore winds, effluent can reach the

shoreline.n Effluent plumes reach the surface and are visible.n Effluent is flushed to Massachusetts Bay in a

surface plume.

OMSAP Public MeetingSeptember 1999

Dilution: Model Harbor Outfalls

USGS-Hydroqualmodel

n 77 square miles<200-fold

n Lower dilutioncontours extendalong shorelinesouth of Boston

n Parts of CCBshoreline 600-1000.

OMSAP Public MeetingSeptember 1999

Dilution: Bay Outfall

n Average depth about 100 feetn 1.25 mile diffuser system provides effective

dispersion.n Available dilution is about 150 to 1.n Effluent is more than one tidal cycle away from

shoreline.n Circulation is greater and more variable than in

Harbor, providing better mixing.

OMSAP Public MeetingSeptember 1999

Dilution: Model Bay Outfall

USGS-Hydroqualmodel

n 3 square miles<200

n Harbor and SouthShore 400-600.

n Most of MB andall of CCB >1000

OMSAP Public MeetingSeptember 1999

Bay Outfall: Mixing in Winter

n Water column is well-mixed.

n Freshwater effluentplume rises rapidly,and mixes to samedensity as seawaterwithin a few tens ofmeters.

OMSAP Public MeetingSeptember 1999

Bay Outfall:Mixing and Stratification in Summer

n Water columnstratification trapseffluent below pycnocline(about 15m below thesurface).

n Nutrients less availableto phytoplankton atsurface, where there isthe most light.

OMSAP Public MeetingSeptember 1999

Outfall Benefits: Less Chlorine Use

n Chlorine disinfection necessary to protect public health; minimizingchlorine use is environmentally beneficial.

n Disinfection effectiveness, K, depends on concentration of chlorine, C,and duration of exposure, t :

K = Ct.n Length of outfall increases t, allowing less chlorine use.

Present Outfall

Contact time ~45 minutes 4-15 hours(10 hours average)

Approx. targetchlorine residual 1.0-3.0 mg/l 0.5-1.0 mg/l

Annual chlorine use 4.6 MG 1.38 MG

OMSAP Public MeetingSeptember 1999

CSO/System Master Plan andWet Weather Hydraulic Flow Capacity

n Size of treatment plant, interceptors,pumping, CSO facilities integrated inMWRA’s CSO/System Master Plan.

n Maximum plant pumping (and new outfall)capacity = 1270 MGD.

n New outfall is necessary to achieve levels ofCSO control in Plan.

OMSAP Public MeetingSeptember 1999

CSO/System Master Plan andWet Weather Hydraulic Flow Capacity

n Two existing major Harbor outfalls maximumcapacity = 400 MGD. (MWRA average = 370MGD)

n Flows >400 MGD activate two shorelineemergency outfalls in Winthrop Bay; total capacity~1000 MGD.

n At flows >1000 MGD headworks must restrictflow, increasing combined sewer overflows.

n New outfall will provide hydraulic capacity of 1270MGD.

OMSAP Public MeetingSeptember 1999

Protecting Resources in the Harbor/BaysEcosystem

n Outfall-diffuser systemdesigned to maximizedilution and mixing.

n Benefits Harbor andBay ecosystem.

n Monitoring andContingency Planningto detect outfall-relatedimpacts and linkfindings to action plan.