Odor and Corrosion Control Pre-fabricated Vortex Insert Assembly

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					    Vortex Insert Assembly Controls Odor and Corrosion
                      in Sewer Drops
                             Eugene M. Natarius, Ph.D., P.E.
                                    Vortex Flow, Inc.
                                  2039 Norfolk Avenue
                             St. Paul, Minnesota 55116-2652


The Vortex Insert Assembly1 is a new realization of the simple and economical Vortex
Drop method1 developed for eliminating odor and reducing corrosion in sewer drops,
forcemain discharges, pumping station wet wells, and interceptor joint structures. The
pre-fabricated assembly installs quickly, and transforms problem drop structures into

The Vortex Drop Structure (VDS) is a sewer drop with the Vortex Insert Assembly (VIA)
installed. It is an effective and proven solution for the problems associated with sewer

The VIA is a simple, pre-fabricated insert for existing or new drop structures. It redirects
influent sewage flow to create a slight vacuum, preventing emission of odorous gasses
and entraining air into wastewater. It is well suited for odor and corrosion control in all
types of sewer drops.


Sewer drop structures, gas emission, odor control, wastewater aeration, abrasive wear


In sewer drops a free-falling stream of wastewater flow releases hydrogen sulfide (H2S)
and other odorous gasses. These emissions are the cause of public odor complaints
coming from areas surrounding sewer drops. The H2S emission from the drop structures
initiates chemical processes that can result in rapid, extensive damage to concrete and
metal sewer piping and mechanical equipment. The substantial kinetic energy of the
falling wastewater can also damage the structure walls by abrasive wear.

    U.S. Patent pending.
With the combination of chemical corrosion and mechanical wear, sewer drops are very
vulnerable points of conveyance systems. Unfortunately, present technologies for liquid
and gas phase treatment require large capital investment and expensive continuous

A new and more versatile realization of the Vortex Drop method is the Vortex Insert
Assembly (VIA) [1]. The VIA, pictured in Figure 1, is a simple, pre-fabricated insert for
existing or new drop structures. Made from relatively inexpensive materials, the VIA
installs quickly and converts a typical sewer drop into an energy dissipater and aerator. It
creates a slight vacuum, preventing emission of odorous gasses. Air is drawn in and
vigorously mixed with the wastewater, oxidizing the hydrogen sulfides. The VIA also
solves the problem of abrasive wear on the structure by removing direct contact between
the flow and structure walls.

                        Fig. 1. The Vortex Insert Assembly.

Figure 2 shows a diagram of the VIA installed in a drop structure. The VIA can be made
from non-corrosive materials such as PVC, High Density Polyethylene (HDPE), and
fiberglass. The Vortex Form at the top has a channel of decreasing radius and a
supercritical slope, which creates accelerating spinning flow. No entrance flume or other
complex components are needed to create the necessary acceleration. The VIA’s Vortex
Form is contoured to maximize acceleration over a relatively short distance, and
eliminates all means for gas emission.
Figure 2. The Vortex Drop Structure
The flow is directed through a special top cut into a vortex shaft with a much smaller
diameter. The top cut serves also as a vortex regulator. It is shaped to mirror the flow
velocity profile, ensuring proper vortex in the entire flow range regardless of flow

The flow continues spiraling downward in a combined field of gravity and centrifugal
forces. Due to acceleration, the sharp reduction in diameter, and significant increase of
centrifugal forces, the flow maintains intimate contact with the vortex shaft wall. This
creates a stable air core. In the vortex shaft, the flow drags air down creating a slightly
negative air pressure above the vortex, preventing gas from escaping up and out of the
structure. The air is therefore entrained and mixed in with the wastewater.

The most intensive processes of vigorous mixing and aeration occur in the submerged
part of the vortex shaft. The wastewater is saturated with oxygen at this point. The
dissolved H2S concentration reduces dramatically.

The flow exits the vortex shaft at the bottom into an energy-dissipating pool. The
remaining flow energy is dissipated through mixing and internal friction. A calm flow
saturated with air exits the energy dissipating pool to the effluent line.

As a result, in the time it takes for the wastewater to pass through the structure, the
wastewater is dramatically changed. Its oxygen level is boosted, dissolved H2S
concentration is dropped to immeasurable levels, and the flow is made tranquil. The
effect of aeration changes the sewer flow characteristics and reduces potential for
corrosion on the long distance of sewer pipe downstream.

The Vortex Shaft top cut extends the hydraulic limits of efficient operation and provides
a stable spinning flow with air core at the minimum flow that in 10-12 times less than the
VIA hydraulic capacity. This is important for the systems with relatively low initial
flows, systems having inflow/infiltration problems, and the combined systems where the
flow range can be extreme.

Many existing drop structures have the outflow pipe at the base of the structure and do
not have pool. For those drop structures the VIA is fabricated with a submerged flow
exit. It has an energy dissipating pool as part of the Assembly. The existing structure
bottom is to be flat and clean for proper installation.

As part of the VIA is a vent that balances the air pressures in the upper and lower pipes
and provides air re-circulation to the Vortex Form and into the air core.

The VIA is effective in the flow drop height range from 4 ft to 100-110 ft.

Each VIA is optimized for its installation using three parameters: inner drop shaft
diameter, drop height, and design flow rate. From this, the best Vortex Form and Vortex
Shaft dimensions are derived.
The VIA is specifically designed for fast installation. It is pre-fabricated for its
destination structure to minimize the on-site labor and installation time. It is
recommended that the drop structure walls have a corrosion protective coating applied
prior to installation.

The process of installation consists of simply lowering the VIA into the open drop
structure, and sealing around the influent connection. Should access be needed to the
lower lines during the structure’s lifetime, the VIA can be designed for easy removal and
re-installation. For cases where flow bypassing is especially difficult, the VIA can be
designed for insertion directly into falling flow.


The Vortex Drop method has been in use in the Minneapolis/St/ Paul metro area since
1998 [2]. The Vortex Drop Structures work on the main interceptor drops, force main
discharge, and in the pumping station wet well. The design flow range is up to 24 MGD
and drop height is up to 52 ft. New projects incorporating Vortex Drop Method are under
way in Hennepin, Anoka, and Washington Counties in Minnesota. The design flow range
for each of two VDS in Hennepin County is 8.5 MGD - 33 MGD, and 4 MGD - 54 MGD
for each of three VDS in Washington County.

Experiments were conducted at multiple installations of Vortex Drop Structures to
measure H2S concentration in the pipes and air quality in areas surrounding the

Multiple wastewater samples were taken simultaneously upstream and downstream of
two VDS installed in Hennepin Co. [2]. Analysis showed a significant decrease of
dissolved H2S and a sharp rise in the dissolved oxygen concentrations downstream of the
structures (Table 1). The results of air quality monitoring around the Vortex Drops
indicated no H2S gas emission[2].

At previously troublesome interceptors, following implementation of the Vortex Drop
method, chemical feed for odor has been eliminated. Even more significant savings will
be seen over the life of the interceptors since no repairs or rehabilitation due to corrosion
are needed. Most importantly, odor complaints from adjacent homeowners have
             Table 1. The average H2S and Oxygen dissolved concentrations.

                       UPSTREAM OF VDS                        DOWSTREAM OF VDS

      VDS             Hydrogen            Oxygen,           Hydrogen             Oxygen,
   Installation      Sulfide, mg/l         mg/l            Sulfide, mg/l          mg/l
 Natchez Ave,
 Golden Valley            2.88              1.05                0.93               4.57
 Hiawatha Ave.,
 Minneapolis              0.38              0.41                0.27               4.28

As part of a centralization program an existing WWTP in Chaska, Minnesota, was
abandoned and an interim sewer pumping station was built in 2000. A severe odor
problem in the surrounding residential area was documented.

The existing sludge pump chamber in the headwork building was transformed into a wet
well housing four identical pumps with flow capacity 2200 gpm (3.2 MGD). The influent
flow comes in the existing channel, goes through metering flume, and drops into the new
wet well 4-7 ft.

A pre-fabricated VIA made from PVC was installed into the wet well (Figure 3, 4) to
reduce turbulence and odor in the wet well and in the headwork building.

Figure 3. The VIA installed into the Chaska Pumping Station Wet Well.
         Figure 4. The working VIA in the pumping station wet well.

The VIA effectively operates over a wide range of flow, from 1.1 MGD up to 8.0 MGD
and over a changing drop height from 3,5 ft to 5.5ft. The wastewater in wet well is
saturated with small bubbles of air at any flow. The aeration facilitates oxidation of
sulfides in wet well and in the long 16000 ft force main. The maintenance personnel
noticed reduction in odorous gas emission. There were no public odor complains from
surrounding area after the installation.

Hunter Water Corporation, the government owned organization providing water supply
and sewer services to a population of 500,000 in Newcastle, Australia, built a Maryland-
Minmi Transportation Redirection project in 1999-2000. The project included three
sewer pumping systems with long force mains. Each of the force mains has a barometric
loop at the discharge point to ensure stable operation. The third force main discharge is
located at a treatment plant inlet.

The Vortex Insert Assembly was installed in each of the barometric loops to reduce
emission of H2S gas. The barometric loop at the pumping system 2 discharge with the
VIA is shown on Figure. 5.

Hunter Water Corporation (HWC) performed hydrogen sulfide gas concentration
monitoring above two working Assemblies and carried out liquid sampling in the
pumping station wet wells immediately downstream of the Vortex Assemblies No.1 and
No.2 (Figure 6). The Vortex Assembly No. 3 effluent flow cannot be isolated from
various sources in the treatment plant inlet. Therefore no sampling or gas monitoring data
was carried out at this installation.
         Figure 5. The VIA installed into barometric loop.

Figure 6. The vortex flow in the VIA installed into barometric loop.
According to the HWC data the average hydrogen sulfide gas concentration for one week
of continuous monitoring was 1.29 ppm on discharge of the pumping system 1, and 2.27
ppm on discharge of the pumping system 2. For a long force main discharge with a flow
drop an average H2S gas concentration is typically at least ten times higher. The
dissolved hydrogen sulfide concentrations from samples were 0.34-0.70 mg/l and 4.2-4.5
mg/l downstream of the system 1 and system 2 discharges accordingly. The HWC
compared the levels of dissolved H2S and found them significantly lower than those
recorded at non-vortex barometric loops.

Compared to similar HWC force main systems with barometric loops, odorous gas
emission was significantly lower in the VIA installations. HWC concluded that the VIA
is effective in reducing H2S gas emission. VIA implementation allowed reduction of
chemical feed dosing rates. Another VIA for the Stockton to Shortland Wastewater
Transfer System in New South Wales, Australia is in fabrication now.


The VIA, a new realization of the Vortex method, is effective for odor and corrosion
control in sewer conveyance systems. It can be used in new and existing sewer drops of
any practical height and flow rates. For more information on the Vortex Insert Assembly
and the Vortex Drop method visit


1. Odor and Corrosion Control: Insert Assembly Improves Sewer Drops, E.M. Natarius,
WATER Engineering and Management, June 2001, pp.14-15, 25.

2. The Vortex Drop Structure Implementation for Odor and Corrosion Control, W.P.
Moeller, E.M. Natarius, WEFTEC 2000, Session 42, Anaheim, CA.