THE VORTEX DROP STRUCTURE IMPLEMENTATION FOR ODOR AND CORROSOIN CONTROL William P. Moeller, Jr., P.E., Eugene Natarius, Ph.D. Metropolitan Council Environmental Services 3565 Kennebec Drive Eagan, Minnesota 55122-1036 ABSTRACT Hydrogen sulfide gas emissions cause extensive corrosion and odor problems in wastewater conveyance and treatment systems. A significant source of these emissions is from drop structures such as interceptor drop maintenance holes, forcemain discharges, joint structures, and wet well drops in sewer pumping stations. The Vortex Drop Structure (VDS), invented by Eugene Natarius, is an effective energy dissipater and aerator, which considerably improves the drop structure. It dissipates the flow energy and aerates the wastewater, preventing emission of odorous gases, oxidizing the hydrogen sulfides, and protecting the drop structure from corrosion and abrasive 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 main interceptors with no odor complains from the public. The cost of chemical feed for odor control has been significantly reduced, and the structures are virtually maintenance-free. Additional VDS installations 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 multiple wastewater samples taken simultaneously upstream and downstream of the Vortex Drop Structures shows a significant decrease of dissolved hydrogen sulfides and a sharp rise in the dissolved oxygen concentrations downstream of the structures. The results of air quality monitoring around VDS installations indicate a strong effectiveness. Odor complaints from adjacent homeowners have virtually disappeared. KEYWORDS Sewer drop structure, gas emission, Vortex drop structure, odor control, corrosion control, wastewater aeration, 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 metal sewer 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 and contains a relatively low amount of dissolved sulfides. Intensive flow turbulence and re-aeration at these drops boost the level of dissolved oxygen in such sewers. The O2 boost prevents exhaustion of the dissolved oxygen for a considerable length of pipe.
But in most practical cases, sewage contains a significant amount of potentially volatile dissolved molecular H2S. This H2S gas is released in drop structures and causes corrosion and odor problems. Substantial physical evidence and test data show that most hydrogen sulfide emission occurs at the drops, rather than in the pipes. This is further confirmed by public odor complaints most often coming from areas neighboring the sewage drops. Due to its elevation, flow upstream of the drop possesses great potential energy. This energy must be dissipated to solve the problem. A known method is to create a wall-hugging spiral flow in the vertical drop structure to dissipate the energy by friction [1]. This vortex flow is formed by a circular, 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 the vertical wall of a standard maintenance hole (MH). Second, quite often the MH is used for lateral connections at elevations lower than the main influent pipe. The method can be improved considerably if the influent flow is accelerated and directed into a shaft 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 much smaller diameter, installed inside of a sewer drop structure. The flow continues spiraling downward in a combined field of gravity and centrifugal forces. Due to the sharp reduction in diameter and significant increase of centrifugal forces, the flow maintains intimate contact with the vortex shaft wall creating a stable air core without loosing its integrity. In the vortex shaft, the flow drags air down creating a slightly negative air pressure above the vortex. This effect prevents gas from escaping up above the vortex form. The air is entrained and mixed with the flow. The most intensive processes of vigorous mixing and aeration occur in the submerged part of the vortex shaft. 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 tranquil flow saturated with air exits the energy dissipating pool to the effluent line. The vortex flow acceleration combined with its direction into a much smaller diameter inner shaft provides effective energy dissipation with intensive airflow mixing and aeration before the emission 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 to control 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 sevencounty metropolitan area surrounding Minneapolis and St. Paul, Minnesota. The first vortex drop structure constructed by MCES is located in Minneapolis on Humboldt Avenue South. The structure has a 15-foot flow drop and is located at a MH downstream of the discharge point of a 27,000-foot long forcemain (FM). This FM has an average flow (ADF) of 3.3 million gallons daily (MGD). For years, the drop structure was the main cause of a neighborhood odor problem. The sewer odor was a constant nuisance in this otherwise elegant area 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, and again in 1996 with cured-in-place pipe. In July 1997, a chemical injection system containing chemical pumps and a 5000-gallon chemical underground storage tank was installed on the FM at a flow metering station approximately two miles upstream of the discharge point. Between 60 and 80 gallons of Bioxide solution were injected daily to oxidize dissolved hydrogen sulfide and 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 MH on 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 2inch thick concrete reinforcement. Installing the 24-inch outside diameter shaft on the metal base with free flow exit between 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 reinforced concrete pipe. Continuous 35-day gas monitoring in main and lateral sewers around the new vortex drop structure has shown very low concentrations of H2S. The monitors were installed into MHs upstream and downstream of the vortex drop, into the drop structure above the vortex, and in two local lines connected to the drop structure. The H2S 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 and 0.32 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 and 0.5 ppm vs. 1.34 ppm and 0.38 ppm [2]. Continuous differential pressure monitoring across the vortex structure cover was very near zero in both phases. 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 drop shaft installation. Another indication of success is that most neighborhood residents are pleased with the odor abatement effort. To date, no new odor complaints have been received. MCES used a modified approach with a drop structure improvement technique in the Minneapolis suburb of Golden Valley. H2S 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 MCES repaired the damaged pipe and drop MH with a PVC liner. In 5-6 years the falling wastewater damaged 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 gravity feeding system administered a constant dose of 55 gallons per day. This averaged $3300 in chemical cost 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 feet lower 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. 3/8inch thick PVC sheets were used for this (Fig. 4, c). Also constructed in advance were the concrete base slab, concrete box, and top slab above the vortex. Bolting an aluminum frame to the MH bottom during main interceptor and lateral flow bypassing, 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 MH wall 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 chemical feed from an upstream location since the time of improvement, the decrease in odor is attributed to the new vortex design. Area residents have not complained about sewer odors since the improvement was installed. A special sampling program was done to investigate a change in wastewater parameters when it goes through the vortex drop structure. The wastewater samples were taken simultaneously upstream and downstream of the structure every 30 minutes over a 12-hour period. Measurements of dissolved H2S,
wastewater temperature, pH, and dissolved oxygen were made. The dissolved oxygen reading was taken using a DO meter, followed by the pH and temperature readings. The samples were prepared and 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 dissolved oxygen concentrations upstream and downstream of the structure is shown on Fig. 5. Evidently there is a significant decrease in dissolved hydrogen sulfides and a sharp rise (by 6-8 times) in dissolved oxygen concentrations downstream of the vortex drop structure. The air entrained in the VDS provides oxygen into solution that oxidizes dissolved sulfides. The vortex drop structure therefore works as an effective flow aerator. MCES also used the vortex structure design for two 52-foot drop structures on an interceptor near the Mississippi River in Minneapolis. This work was part of an emergency repair that included replacement of 1800 feet of interceptor tunnel. Corrosion related to H2S 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 drop structure to the north accommodates flow from a lateral gravity line with ADF of 1.0 MGD. Both vortex 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 drop shaft was made from 18-inch HDPE pipe (SDR 26). Both vortex drop structures have a maximum vortex form inside diameter of five feet. After flow was diverted to the new vortex drop structures and a new tunnel, a number of measurements showed zero H2S 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 900 feet downstream of the main vortex drop structure. The results are shown in Table. 2, [3]. The comparison of dissolved sulfides and dissolved oxygen concentrations upstream and downstream of the structure is shown in Fig. 6. Downstream of the structure most of the dissolved sulfide concentrations are less than 0.2 mg/l which is the level of test sensitivity. The dissolved oxygen concentrations are up by 10-30 times - almost to the level of saturation. Only during late afternoon and evening hours was the rise downstream only about 67 times higher. The results of both sampling programs prove that the vortex drop structure works as an effective aerator boosting level of dissolved oxygen in wastewater. We assume that the oxygen delivered into wastewater from the air entrained in the vortex drop structure oxidizes dissolved sulfides and decreases the overall wastewater BOD.
Fig. 5 - Dissolved Hydrogen Sulphide (DH2S) and Dissolved Oxigen (DO) Upstream and Dow nstream of the Vortex Drop Structure on Natchez Ave., Golden Valley, MN (12/20/99)
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Fig. 6 - Dissolved Hydrogen Sulphide (DH2S) and Dissolved Oxigen (DO) Upstream and Dow nstream of the Vortex Drop Structure on Hiaw atha Ave./52nd St., Minneapolis (5/20/99)
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Installed upstream of a wastewater treatment plant, the vortex drop structure could provide substantial savings 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 station is equipped with four submersible pumps. It pumps against 15700 foot long 20-inch and 30-inch diameter 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 pipe invert is 8.25 feet above wet well bottom. An average flow drop height is about 4.5 – 5.0 feet. The vortex form - vortex shaft assembly is installed as a pre-fabricated unit suspended on one-beam support so that it provides flow exit from the vortex shaft to a wet well (Fig. 7). The lower end of the shaft is always submerged. ½-inch PVC sheets were used for making the vortex form. The form was welded to 24-inch schedule 40 PVC pipe that is used for the vortex shaft. A steel belt and anchors firmly keep the assembly in the wet well corner. The flow comes from a channel with a 12-inch Parshall flume meter and enters the vortex form. The vortex assembly dissipates the flow energy, aerates the flow, 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 diameter forcemain made from PCCP. The pumping station in Minneapolis suburb of Wayzata has three constant speed pumps with a one pump flow of 3900 gpm and station firm capacity of about 4700 gpm. The pumping cycle is 15-20 min. In 1995 there was a forcemain break at the discharge point due to severe H2S 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 drop continued release of hydrogen sulfide gas and deteriorated a downstream 36-inch concrete line. About 1000 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 project includes 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 plastic collar 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 two feet deep. Vigorous mixing and flow aeration should occur in the pool. The airflow mix will flow over the plate to the effluent line.
CONCLUSIONS The new vortex method described herein and its implementation, along with the results of wastewater and air quality investigations, show that the method is beneficial and effective in situations where H2S emission creates corrosion and odor problems. These situations could include sewer collection and treatment system locations where the sewer flow drops more than 4 feet. If corrosion and odor appear in 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. H2S 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
