Ms. Osborne
November 18, 2016
Page 2of5
and overflowing the influent sewers. The overflow flooded the paved parking area on the north side of
the wet weather pump station, significantly eroded the adjacent banking, and flowed into the adjacent
wetlands. A large electrical transformer was undermined and taken off line.
Wright-Pierce met with the Plant Operations Staff to discuss their observations of the rain event:
@
The drop shaft vortex manhole on the 48-inch Salmon Brock Interceptor (connected to the
72-inch North Merrimack Interceptor) was overflowing; it flooded the adjacent parking area,
and eroded the adjacent bank. The manhole rim is at about elevation 33.0.
The main influent pump station was receiving up to 56 mgd at one point, and was unable to
keep up with the peak flow.
The influent pump station wet well flooded the lower level to about elevation 15.0, within
approximately 3.6 feet of the intermediate floor level.
In response to the wet well flooding, the 72-inch influent gate to the main influent pump
station was closed from about 24 inches open to about 10 inches open. This further restricted
the flow to the pump station.
According to the level trending information in SCADA, the water surface elevation upstream
of the wet weather pump station influent gate and mechanical screens was 22.0.
Two wei weather influent pumps were operated (each pump rated at 21 mgd and 51-foot
TDH), and lowered the wet weather pump station wet well to below normal operating levels
(elevation of about -5.0). However, the recorded total peak flow rate of the wet weather
pump siation was measured at only 25 mgd. The total rated flow capacity of this pump
station is 60 mgd with 3 pumps operating. This indicates a possible hydraulic bottle neck
associated with either the influent gate or mechanical screens.
PROJECT OBLECTIVES
Based on our initial investigation of the rain event and existing conditions, we have identified seven
project objectives:
1.
Identify the issues that resulted in the overflow of untreated influent wastewater, and the
reasons why the wet weather pump station was only able to receive/pump about 25 mgd;
Evaluate modifications and control strategies that will minimize the potential for future
untreated wastewater overflcws;
Identify the necessary modifications required to facilitate the desired flow handling capacity;
Evaluate possible operational issues or limitations with the wet weather flow treatment
facility (Actiflo System) to effectively treat pumped wet weather flows up to 60 mgd;
Recommend modifications and operational changes to the wet weather flow treatment
system;
