SS.
i BF
‘
A Hey gar ieee ED, en Wp gO Weer
Vv = i ir t m tr i i: hee Ee ee WATER ~ WASTEWATER ~ STORMWATER ~ INFRASTRUCTURE
Dm a
Engineering a Better Environment
November 18, 2016
W-P Project No. T11295
Nis. Noelle Osborne
City of Nashua NWTF
2 Sawmill Road
Nasnua, NH 03060
Subject: Letter Proposal for Influent Hydraulic Analysis & Evaluation of the Wet Weather
Treatment System
Dear Ms. Osborne
We are pleased to respond to your request for assistance with analysis of the Wastewater Treatment
Facility influent hydraulics, and evaluation of the wet weather treatment system. The following
paragraphs cutline relevant project background, our current understanding of the issues, proposed
scope of services, schedule and the estimatec fee.
PROJECT BACKGROUND
The Nashua Wastewater Treatment Facility includes a wet weather treatment system that was put in
service in 2009. The Wet Weather Fiow Treatment Facility project included modifications of the
treatment facility’s influent piping system, a below grade diversion structure, a wet weather pump
station, a wet weather sedimentation facility, and modifications to the City’s existing hypochlorite
contact chamber. More specifically, these improvements included:
e Site Piping Modifications — Installation of a drop shaft vortex manhole and new 48” piping to
connect the 48” Salmon Brook Interceptor to the 72” Merrimack Interceptor just upstream
of the Main influent Pump Station.
® Wet Weather Pump Station — Hydraulically actuated influent gates, two mechanical bar
screens, four 21 mgd submersible pumps, and a magnetic flow meter on the 42-inch force
main to the wet weather sedimentation facility.
Wet Weather Sedimentation Facility - A Kruger Actiflo System that includes chemical and
ballasted floc mixing, coagulation and maturing tanks, prior to lamella clarifiers. The system
discharges to a 48-inch gravity line to the chlorine contact tank.
@
® Hypochlorite Contact Chamber — The modification to the chlorine contact tank included new
influent piping with isolation sluice gates, new rotating scum collection pipes and fixed FRP
weirs.
On October 21°, Nashua experienced a rain event of nearly 3 inches over a time of 3 hours. During one
hour they received 1.8 inches. The plant influent hydraulics were overwhelmed by this flow, surcharging
Recognized for Engineering Excellence ~ Selected for Service & Value 230 Commerce Way, Suite 302
Portsmouth, NH 03801
www.wright-pierce.com
Phone: 603.430.3728 | Fax: 603.430.4083
