THE VORTEX DROP STRUCTURE IMPLEMENTATIONFOR ODOR AND CORROSOIN CONTROL

William P. Moeller, Jr., P.E., Eugene Natarius, Ph.D.Metropolitan Council Environmental Services

3565 Kennebec DriveEagan, Minnesota 55122-1036

ABSTRACT

Hydrogen sulfide gas emissions cause extensive corrosion and odor problems in wastewaterconveyance and treatment systems. A significant source of these emissions is from drop struc-tures such as interceptor drop maintenance holes, forcemain discharges, joint structures, and wetwell drops in sewer pumping stations. The Vortex Drop Structure (VDS), invented by EugeneNatarius, is an effective energy dissipater and aerator, which considerably improves the dropstructure. It dissipates the flow energy and aerates the wastewater, preventing emission of odor-ous gases, oxidizing the hydrogen sulfides, and protecting the drop structure from corrosion andabrasive wear.

Metropolitan Council Environmental Services, a regional public agency serving the Minneapolis/St. Paul area, successfully implements the VDS. Four installed VDS have been working on maininterceptors with no odor complains from the public. The cost of chemical feed for odor controlhas been significantly reduced, and the structures are virtually maintenance-free. Additional VDSinstallations are in progress at a pumping station wet well, and on a forcemain discharge.

Testing to measure VDS effectiveness was performed at two installations. The analysis of mul-tiple wastewater samples taken simultaneously upstream and downstream of the Vortex DropStructures shows a significant decrease of dissolved hydrogen sulfides and a sharp rise in thedissolved oxygen concentrations downstream of the structures. The results of air qualitymonitoring around VDS installations indicate a strong effectiveness. Odor complaints fromadjacent homeowners have virtually disappeared.

KEYWORDS

Sewer drop structure, gas emission, Vortex drop structure, odor control, corrosion control, wastewateraeration, abrasive wear control, solution, implementation.

INTRODUCTION

Typical sewer drops create turbulent flow, which releases Hydrogen sulfide (H2S) gas. This

emission from the drop structures can lead to rapid, extensive damage to concrete and metalsewer piping and mechanical equipment, and is a main source for odor problems.

There are beneficial aspects of vertical drops in situations where sewage is still fresh andcontains a relatively low amount of dissolved sulfides. Intensive flow turbulence and re-aerationat these drops boost the level of dissolved oxygen in such sewers. The O

2 boost prevents exhaus-

tion of the dissolved oxygen for a considerable length of pipe.

But in most practical cases, sewage contains a significant amount of potentially volatile dis-solved molecular H

2S. This H

2S gas is released in drop structures and causes corrosion and odor

problems. Substantial physical evidence and test data show that most hydrogen sulfide emissionoccurs at the drops, rather than in the pipes. This is further confirmed by public odor complaintsmost often coming from areas neighboring the sewage drops.

Due to its elevation, flow upstream of the drop possesses great potential energy. This energy mustbe dissipated to solve the problem. A known method is to create a wall-hugging spiral flow in thevertical drop structure to dissipate the energy by friction [1]. This vortex flow is formed by acircular, or volute-shaped chamber situated concentrically on top of the vertical drop.

Applying this method to a typical drop of interceptor sewage flow is complicated by two factors.First, the upstream flow velocities are usually not enough to create a stable tangential flow on thevertical wall of a standard maintenance hole (MH). Second, quite often the MH is used forlateral connections at elevations lower than the main influent pipe.

The method can be improved considerably if the influent flow is accelerated and directed into ashaft of a predetermined diameter installed in the drop structure (Vortex Shaft) See Fig.1 (U.S.Patent pending).

The vortex form with a vortex channel of decreasing radius creates accelerating spinning flow.This accelerating flow is directed through a special top cut into a vortex shaft, with a muchsmaller diameter, installed inside of a sewer drop structure. The flow continues spiraling down-ward in a combined field of gravity and centrifugal forces. Due to the sharp reduction in diameterand significant increase of centrifugal forces, the flow maintains intimate contact with the vortexshaft wall creating a stable air core without loosing its integrity. In the vortex shaft, the flowdrags air down creating a slightly negative air pressure above the vortex. This effect prevents gasfrom escaping up above the vortex form. The air is entrained and mixed with the flow. The mostintensive processes of vigorous mixing and aeration occur in the submerged part of the vortexshaft. The flow exits the vortex shaft at the bottom into an energy-dissipating pool. Theremaining flow energy is dissipated through mixing and internal friction. A tranquil flow satu-rated with air exits the energy dissipating pool to the effluent line.

The vortex flow acceleration combined with its direction into a much smaller diameter innershaft provides effective energy dissipation with intensive airflow mixing and aeration before theemission occurs. This method yields the following benefits:

• Elimination of odorous gas emission• Protection of the structure and pipes from intensive corrosion• Aeration of wastewater flow• Effective energy dissipation eliminating abrasive wear of the structure• Creation of conditions for boosting dissolved oxygen and oxidation of dissolved sulfides• Reliable function independent of flow fluctuations• Elimination of the need for air treatment• Significant improvement of existing drop structure rehabilitation methods.

INTERCEPTOR DROP IMPROVEMENTS

The Metropolitan Council Environmental Services (MCES) has used the new vortex method tocontrol odor and improve collection system flow drop structures situated on main interceptors.The MCES is a regional public agency that provides wastewater collection treatment to a seven-county metropolitan area surrounding Minneapolis and St. Paul, Minnesota.

The first vortex drop structure constructed by MCES is located in Minneapolis on HumboldtAvenue South. The structure has a 15-foot flow drop and is located at a MH downstream of thedischarge point of a 27,000-foot long forcemain (FM). This FM has an average flow (ADF) of3.3 million gallons daily (MGD). For years, the drop structure was the main cause of aneighborhood odor problem. The sewer odor was a constant nuisance in this otherwise elegantarea of century-old homes and trees.

Along with the odor problem, the upstream 30-inch concrete pipe was deteriorated by H2S

corrosion. It had been repaired twice; first in 1989 by sliplining a length of about 300 feet, andagain in 1996 with cured-in-place pipe. In July 1997, a chemical injection system containingchemical pumps and a 5000-gallon chemical underground storage tank was installed on the FMat a flow metering station approximately two miles upstream of the discharge point. Between 60and 80 gallons of Bioxide solution were injected daily to oxidize dissolved hydrogen sulfideand control odor at the drop. Expenses for chemicals averaged up to $5,700 a month.

The drop structure was improved by installing a vortex form and shaft into the existing MH (Fig.2,a). Improvements included the following:

• Removal and replacement of the existing structure, using a standard 48-inch concrete MHon a new base.

• Constructing a box-like concrete entrance flume and connecting it to the vortex shaft top(Fig. 2, b).

• Bolting a corrosion-protected metal base plate to the MH bottom and covering it with 2-inch thick concrete reinforcement.

• Installing the 24-inch outside diameter shaft on the metal base with free flow exit be-tween vertical channels. The shaft is fusion-welded, high-density polyethylene (HDPE).

• Sealing the space between the vortex shaft and the new base.

The drop structure is connected to a 54-inch brick interceptor by 7 feet of 30-inch reinforcedconcrete pipe.

Continuous 35-day gas monitoring in main and lateral sewers around the new vortex drop struc-ture has shown very low concentrations of H

2S. The monitors were installed into MHs upstream

and downstream of the vortex drop, into the drop structure above the vortex, and in two locallines connected to the drop structure. The H

2S monitoring program consisted of two phases. One

included chemical injection and the other did not.

The average upstream and downstream H2S gas concentrations in the phase without chemicals

were even slightly less than the corresponding data with the chemical injection: 0.38 ppm and0.32 ppm vs. 0.59 ppm and 0.41 ppm.

The average concentrations in laterals were slightly higher in the phase without chemicals: 2.51 ppm and0.5 ppm vs. 1.34 ppm and 0.38 ppm [2].

Continuous differential pressure monitoring across the vortex structure cover was very near zero in bothphases. The readings during the majority of the testing appear to be in the range of instrument drift:+0.03 to -0.03 inch of W.C. [2]. The results show practically no gas emission after the vortex dropshaft installation.

Another indication of success is that most neighborhood residents are pleased with the odor abatementeffort. To date, no new odor complaints have been received.

MCES used a modified approach with a drop structure improvement technique in the Minneapolissuburb of Golden Valley. H

2S corrosion had significantly damaged a 5-foot diameter MH, 19-foot

drop, and upstream 30-inch diameter reinforced concrete pipe on Natchez Avenue. In 1990 MCESrepaired the damaged pipe and drop MH with a PVC liner. In 5-6 years the falling wastewater dam-aged the liner again. A severe odor problem continued to cause complaints from area residents.

To cure the problem, MCES fed Bioxide into the gravity line at a point 2.5 miles upstream. A gravityfeeding system administered a constant dose of 55 gallons per day. This averaged $3300 in chemicalcost per month. Considerations were as follows: the ADF at the structure is approximately 3.1 MGD;the drop structure has one lateral connection with ADF about 1.4 MGD; the connection enters 12 feetlower than the main influent pipe.

In 1998, the structure was improved by installation of an inner 24-inch high-density polyethylene(HDPE) vortex shaft, designed as shown on Figure 3. Installation included the following:

• Constructing the vortex form and entrance flume in advance to minimize bypassing time. 3/8-inch thick PVC sheets were used for this (Fig. 4, c). Also constructed in advance were theconcrete base slab, concrete box, and top slab above the vortex.

• Bolting an aluminum frame to the MH bottom during main interceptor and lateral flow bypass-ing, under protection of sandbags in the effluent pipe.

• Removing sandbags and installing the vortex shaft into the frame, fixing it to the top of the MHwall by aluminum anchors (Fig. 4, a).

• Installing the concrete slab with its hole centered above the vortex shaft (Fig 4, b).• Mounting the vortex-forming PVC fabrication on the concrete slab (Fig. 4, c). The cylindrical

part under the bottom was attached to the vortex shaft collar caulked in advance.• Positioning and fixing the fabrication to the base slab, mounting the concrete box (Fig. 4, d ),

and grouting the PVC fabrication.• Installing the top slab (Fig. 4, Photo e), an adjacent ring, and cover on the top.

Photo in Figure 4, f shows the new vortex flow with its stable air core created above the vortex shaft.The structure improvement was completed in one day.

The observations show no odorous emission from the improved structure. Because there is no chemicalfeed from an upstream location since the time of improvement, the decrease in odor is attributed to thenew vortex design. Area residents have not complained about sewer odors since the improvement wasinstalled.

A special sampling program was done to investigate a change in wastewater parameters when it goesthrough the vortex drop structure. The wastewater samples were taken simultaneously upstream anddownstream of the structure every 30 minutes over a 12-hour period. Measurements of dissolved H

2S,

wastewater temperature, pH, and dissolved oxygen were made. The dissolved oxygen reading wastaken using a DO meter, followed by the pH and temperature readings. The samples were preparedand delivered to the lab for dissolved sulfide analysis.

The results of the sampling are shown in Table 1, [3]. The comparison of dissolved sulfides and dis-solved oxygen concentrations upstream and downstream of the structure is shown on Fig. 5. Evidentlythere is a significant decrease in dissolved hydrogen sulfides and a sharp rise (by 6-8 times) in dissolvedoxygen concentrations downstream of the vortex drop structure. The air entrained in the VDS providesoxygen into solution that oxidizes dissolved sulfides. The vortex drop structure therefore works as aneffective flow aerator.

MCES also used the vortex structure design for two 52-foot drop structures on an interceptor near theMississippi River in Minneapolis. This work was part of an emergency repair that included replacementof 1800 feet of interceptor tunnel. Corrosion related to H

2S emission from the drops had severely

damaged the 6 by 3.5-foot concrete tunnel. The main drop structure has ADF of 8.8 MGD. The dropstructure to the north accommodates flow from a lateral gravity line with ADF of 1.0 MGD. Bothvortex shafts were built near 12-foot diameter access shafts connected to a new 54-inch PVC tunnel.The main vortex shaft was made from fusion welded 32-inch HDPE pipe (SDR 26), and the north dropshaft was made from 18-inch HDPE pipe (SDR 26). Both vortex drop structures have a maximumvortex form inside diameter of five feet.

After flow was diverted to the new vortex drop structures and a new tunnel, a number of measurementsshowed zero H

2S air concentrations at the tunnel and energy dissipating pool. All people who inspected

the structures confirmed there was no odor down at the tunnel.

A 14-hour long sampling program similar to one described above was performed upstream and 900feet downstream of the main vortex drop structure. The results are shown in Table. 2, [3]. Thecomparison of dissolved sulfides and dissolved oxygen concentrations upstream and downstream of thestructure is shown in Fig. 6.

Downstream of the structure most of the dissolved sulfide concentrations are less than 0.2 mg/l which isthe level of test sensitivity. The dissolved oxygen concentrations are up by 10-30 times - almost to thelevel of saturation. Only during late afternoon and evening hours was the rise downstream only about 6-7 times higher.

The results of both sampling programs prove that the vortex drop structure works as an effectiveaerator boosting level of dissolved oxygen in wastewater. We assume that the oxygen delivered intowastewater from the air entrained in the vortex drop structure oxidizes dissolved sulfides and decreasesthe overall wastewater BOD.

Fig. 5 - Dissolved Hydrogen Sulphide (DH2S) and Dissolved Oxigen (DO) Upstream and Downstream of the Vortex Drop Structure on Natchez Ave., Golden Valley, MN (12/20/99)

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Structure on Hiawatha Ave./52nd St., Minneapolis (5/20/99)

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Installed upstream of a wastewater treatment plant, the vortex drop structure could provide substantialsavings on aeration and chemical treatment costs for the plant.

SEWER PUMPING STATION IMPROVEMENT

MCES included a Vortex Drop Structure in a new City of Chaska pumping station design. This stationis equipped with four submersible pumps. It pumps against 15700 foot long 20-inch and 30-inch diam-eter parallel barrels that cross the Minnesota River. Each pump has 2200 gpm capacity at 146 ft W.C.of total dynamic head. The firm capacity is about 6400 gpm with both barrels open. The influent pipeinvert is 8.25 feet above wet well bottom. An average flow drop height is about4.5 – 5.0 feet.

The vortex form - vortex shaft assembly is installed as a pre-fabricated unit suspended on one-beamsupport so that it provides flow exit from the vortex shaft to a wet well (Fig. 7). The lower end of theshaft is always submerged. ½-inch PVC sheets were used for making the vortex form. The form waswelded to 24-inch schedule 40 PVC pipe that is used for the vortex shaft. A steel belt and anchorsfirmly keep the assembly in the wet well corner. The flow comes from a channel with a 12-inch Parshallflume meter and enters the vortex form. The vortex assembly dissipates the flow energy, aerates theflow, and directs it under the wet well water level.

FORCEMAIN DISCHARGE IMPROVEMENT

MCES uses the vortex drop structure to improve a discharge of a12800 foot long, 24-inch diameterforcemain made from PCCP. The pumping station in Minneapolis suburb of Wayzata has three constantspeed pumps with a one pump flow of 3900 gpm and station firm capacity of about 4700 gpm. Thepumping cycle is 15-20 min. In 1995 there was a forcemain break at the discharge point due to severeH

2S related corrosion. The discharge has 15.5 feet of drop. It was rebuilt with a 24-inch riser made

from ductile iron pipe and connected to a discharge maintenance hole at the bottom. The dropcontinued release of hydrogen sulfide gas and deteriorated a downstream 36-inch concrete line. About1000 feet of the line was improved using CIPP technology.

The last 1200 feet of the forcemain are now replaced as part of a bridge rebuilding project. The projectincludes improvement of the drop with the vortex method using the existing riser as a vortex shaft (Fig.8). The vortex form made from ½-inch PVC sheets is installed above the existing riser using a plasticcollar matching the existing riser diameter. The pre-fabricated vortex form is installed in a concrete box,fixed to the bottom, and grouted all around with concrete. It serves as a reliable PVC liner (Fig. 9).

A vertical steel plate in the center of the discharge MH will create the energy dissipating pool about twofeet deep. Vigorous mixing and flow aeration should occur in the pool. The airflow mix will flow overthe plate to the effluent line.

CONCLUSIONS

The new vortex method described herein and its implementation, along with the results of wastewaterand air quality investigations, show that the method is beneficial and effective in situations where H

2S

emission creates corrosion and odor problems. These situations could include sewer collection andtreatment system locations where the sewer flow drops more than 4 feet. If corrosion and odor appearin these places, consider the new vortex drop design as a viable solution.

REFERENCES

1. The Control of Sulfides in Sewerage Systems. Edited by D.K.B. Thistlethwyte. Ann Arbor Science Publishers Inc., Ann Arbor, Michigan, 1972.

2. Continuous Hydrogen Sulfide Monitoring at Select Sewer Locations. MCES Report Number H980611A by Tom Wahlberg. St. Paul, Minnesota, 1998.

3. MCES, Industrial Waste and Pollution Prevention Section. H

2S Sampling Project Report: 1-GV-641 Golden Valley, Natchez Ave., 12/20/1999

H2S Sampling Project Report: 1-MN-340 Minneapolis, Hiawatha Ave., 5/20/1999