Environmental Science & Engineering - www.esemag.com - September 2003
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Vortex flow inserts – for odour and corrosion control
Richard St-Aubin, P.Eng., IPEX Inc.
The Vortex Flow Insert (VFI) is a specially engineered
drop structure that uses the kinetic energy of
falling wastewater to eliminate odour discharges
from municipal manholes and pumping stations.
While vortexes have been used for many years to suppress
turbulence and minimize air injection in large manhole
drop structures, a recent innovation by noted
hydraulics expert Dr. Eugene Natarius not only allows the
VFI to suppress turbulence, but to actually oxidize hydrogen
sulphide (H2S) and other odorous substances in wastewater.
This has the effect of actually improving wastewater
quality by dramatically increasing the concentration of dissolved
oxygen in the flow. As a result, the VFI has been
patented, and has been awarded a special Technical
Innovation Award from the American Public Works
Association.
Vortex Flow Inserts are successfully operating in over
20 locations in the United States, and have also been
installed in Australia. The most recent installation in
Canada was in Sarnia, Ontario, where the VFI drastically
reduced H2S generation in a drop manhole near the Plank
Road Pumping Station.
How the VFI works
Incoming sewage is accelerated to supercritical flow
conditions in the upper vortex form that has an increased
slope. As the flow is accelerated, it flows around a swirl of
decreasing radius, thus generating a centrifugal force.
The sewage flow proceeds to a much smaller diameter
drop pipe, where it continues to spin and drop at the same
time. This creates a whirlpool effect, forming an air core
down the centre of the drop pipe.
This whirlpool or vortex effect creates a slightly lower
pressure in the air core, creating a down draft that draws
odorous gases down to the bottom of the manhole. The
H2S and other gases are entrained back into the sewage in
the specially designed energy dissipating pool.
Because of the vigorous aeration and mixing that occurs
in the energy dissipating pool, the dissolved oxygen content
of the sewage is drastically increased, thus oxidizing
the H2S gas.
In order for the VFI to be effective, a drop depth of at
least one metre is required; however, a drop of three metres
or more is considered optimal. The VFI will perform in
drops of well over 30 metres, and can be specially engineered
to handle greater distances. They have been successfully
installed both in drop manholes and in pumping
station wet wells. While the main applications for VFIs are
for control of odour and gas generation, they have also been
used as energy dissipaters for such applications as turbine
outlets.
The advantage of this approach for controlling odours at
sewer drops is that it requires no active maintenance.
These vortex units are designed to be self-cleansing and
they will work effectively at flows
between 10% and 110% of their rated
capacity. This allows a vortex unit to handle
the widely varying flows that are characteristic
of many sanitary sewers.
Current liquid phase odour control
methods include injection of various
chemicals such as ferrous sulphate or
hydrogen peroxide into the sewage flow.
While the VFI typically has a lower capital
cost than most chemical injection systems,
the real savings come when considering
the annual chemical costs. Based
on a flow of 1 million (US) gallons per
day (44 L/s), with a concentration of 5 ppm dissolved sulphide,
annual chemical costs could range between $30,000
and $40,000 annually. In addition, some commonly used
chemicals (such as iron salts) often require double containment
tanks and piping systems.
The vortex flow will perform well in virtually any area
where falling wastewater creates an odour or corrosion concern.
When installing a vortex unit in a wet well, it is
important to recognize that the vortex unit forces air into
the flow, and, therefore, should be located a reasonable distance
from pumps in order to prevent air bubbles from
being injected into the pipeline.
In areas where the total drop may be less than adequate
for complete odour attenuation, the VFI can work in tandem
with traditional chemical injection to reduce chemical
costs.
The performance of one of the first Vortex Flow Inserts,
which was installed in the Minneapolis/St-Paul area in
1998, was documented at WEFTEC in 2000. This paper
described the installation of a VFI at a manhole that had
been a persistent odour problem for many years in a
Minneapolis neighborhood. In addition to the odour problem,
the upstream 750mm concrete pipe required rehabilitation
both in 1989 by sliplining, and in 1996 with cured in
place pipe. The manhole was located downstream of a discharge
point of an 8.1 km sewage forcemain, with an average
daily flow of 12,500 m3/day, and a drop of 4.5 metres.
A chemical injection system was installed in 1997 to mitigate
the odour and corrosion concerns, consisting of chemical
pumps as well as an underground 19,000 L chemical
storage tank. While the system was effective at controlling
hydrogen sulfide induced odour and corrosion, chemical
costs averaged up to $5,700 (US) per month.
A VFI was installed in November 1997, and concentrations
of dissolved H2S in the wastewater were measured in
two phases – one with only the chemical injection system
operating, and the other with only the VFI operating. In all
cases the VFI performed at least as well as chemical injection,
and in some cases was even more effective at reducing
H2S concentrations.
Perhaps the most important statistic is that, to date, no
new odour complaints have been received from area residents.
Scientific papers describing the VFI in greater detail are
available from the author, Richard St-Aubin, e-mail: ricsta
@ipexinc.com.
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