HomeMy Public PortalAboutDownDefender_ApprovalLetter_020723Metropolitan
St. Louis Sewer
District
2350 Market Street
St. Louis, MO 63103-2555
(314) 768-6200
February 7, 2023
Hydro International
94 Hutchins Dr.
Portland, ME 04102
RE: Downstream Defender Hydrodynamic Separator (HDS)
Highway Use Level (HUL) and Redevelopment Use Level (RUL)
Approval within the District
Dear Sir,
The Metropolitan St. Louis Sewer District (MSD) is providing HUL
and RUL approval for the use of the Hydro International Downstream
Defender HDS unit as a stand-alone water quality BMP, subject to
the following provisions:
• The Downstream Defender is only approved for use on public
highway and roadway projects, and for redevelopment sites
less than 5 acres. This approval is based upon compliance
with requirements listed in MSD's Proprietary Water Quality
Products and the MSD's Stormwater Management Program (Revised
Jan. 2009).
• The Downstream Defender must be sized such that the Maximum
Treatment Flow Rate (MTFR) for the unit exceeds the Water
Quality flow rate calculated for the unit's tributary area.
The Downstream Defender's approved MTFRs will be those shown
in Table 1 below, to match the most recently certified flow
rates provided by the New Jersey Department of Environmental
Protection (NJDEP). Procedures for calculating the Water
Quality flow rate are provided in Appendix D.10 of the
Maryland Stormwater Design Manual (2000).
Table 1: Downstream Defender Models and Approved MTFRs
Model Manhole
Diameter (ft)
MTFR (cfs)Treatment
Chamber Depth
(ft)
4-ft 4 1.12 3.33
6-ft 6 2.52 4.83
8-f t 8 4.49 6.42
10-ft 10 7.00 7.92
12-ft 12 10.08 9.42
• The Downstream Defender will be configured as an off-line
unit, downstream from a diversion manhole. In most cases,
the BMP will be installed without the need to store the Water
Quality volume upstream of the unit.
• All devices shall provide a minimum sediment storage capacity
of 10 cubic feet.
• All proprietary lids and covers should be captive components.
The minimum size access hole is 30 inches in diameter.
• Confined space entry shall not be a requirement for routine
maintenance. No special tools or attachments should be
required to provide for routine maintenance with a vacuum
pumping truck.
• Project specific design calculations and maintenance plans
furnished by the manufacturer must be included within the
project's Stormwater Management Facilities Report prepared by
the consulting engineer.
MSD reserves the ability to withdraw or modify this approval based
on subsequent information, including information indicating that
this BMP does not satisfy MSD rules, requirements, or construction
specifications.
As of the date of this letter, the Downstream Defender's
certification status with NJDEP has expired. Should Hydro
International fail to be recertified with NJDEP before January 1,
2025, the Downstream Defender will be removed from MSD's list of
approved BMPs.
Sincerely,
Curtis Zurliene, PE
Civil Engineer (BMP Committee Chairman)
Engineering / Development Review
Metropolitan St. Louis Sewer District