Spin Away Odor & Decay in Sewage Drop Structures
The corrosion and odor problems of hydrogen sulfide emissions can be lessened by using this vortex drop structure design
Hydrogen sulfide (H2S) emission often causes corrosion and odor problems in wastewater collection and treatment systems. It can lead to rapid, extensive damage to concrete and metal sewer pipe and mechanical equipment. A significant source of H2S gas emission in sewers is drop structures such as interceptor drop maintenance holes, joint structures, forcemain discharges and wet well drops in sewer pumping stations. All of these sources create turbulent flow that releases the gas.
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 boost prevents exhaustion of the dissolved oxygen for a considerable length of pipe. However, in most practical cases, sewage contains a significant amount of potentially volatile dissolved molecular H2S. This H2S gas releases 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. Along with that, odor complaints from the public most often come from areas neighboring the sewage drops.
The Vortex Solution
Due to its elevation, flow upstream of the drop possesses great potential energy. This energy must be dissipated to solve the problem of gas release. One known method is to create a wall-hugging spiral flow in the vertical drop pipe to dissipate the energy by friction.1 This vortex flow is formed by a circular or volute-shaped chamber situated concentric on top of the vertical pipe.
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 dramatically by installing a special vortex drop shaft into a standard MH. The installation should include the following:
- Mounting a vortex-forming volute chamber on the top of the vortex shaft. At the chamber, use a bottom elevation lower than the influent pipe invert to create supercritical flow in a short-length entrance flume.
- Connecting the volute chamber to an entrance flume. The chamber bottom has a cylindrical element concentric with the vortex shaft and with the same inside diameter as the shaft.
- Placing a collar on top of the vortex shaft to provide a connection to the volute chamber.
- Stabilizing the vortex shaft in a vertical position by connecting it to the MH walls and bottom with non-corrosive frames.
- Using cut-out holes submerged in the flow near the shaft bottom.
Why It Works
Sewage flow coming to a drop structure is directed by an entrance flume. This flume is tangential to the volute form installed at the top of the vortex shaft. A supercritical slope of the short-entrance flume provides rising flow velocities with a partial potential energy transition to a kinetic energy of vortex. Although the flume cross section is getting narrow, the water level does not rise. The volute vortex form directs spinning flow inside the vertical vortex shaft, increasing flow velocities and friction losses. A top of the vortex shaft cut by a curve reflects a flow profile velocity distribution in vortex form. Without losing its integrity, the gravity flow is transformed into a flow with combined gravity and centrifugal forces. Flow acceleration creates a stable air core in the vortex shaft. This causes a slight negative pressure that prevents escape of odorous emission gases.
Through submerged holes at the shaft's bottom, the flow exits into an energy-dissipating pool and then into the effluent pipe. The area and shape of the exit holes control both water level and the vertical length of wall-hugging spiral flow inside the vortex shaft. The "shaft-in-shaft" design allows lower-level lateral connections to the MH structure without disrupting the main flow. It prevents H2S gas release at the drop structure and subsequent corrosion of upstream pipe and helps limit corrosion of the downstream pipe. The H2S gas released inside the vortex shaft partially oxidizes in the air core of the shaft and partially dissolves back into the energy-dissipating pool. Aqueous and dissolved sulfides are balanced in the effluent pipe after leaving the drop structure.
The Metropolitan Council Environmental Services (MCES) have used the vortex method to control odor and improve collection system flow drop structures situated on main interceptors. The MCES is a regional 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 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 daily flow (ADF) of 3.3 millions gallons a day (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 5,000-gallon chemical underground storage tank was installed on the FM at a flow metering station approximately two miles upstream of the discharge point. From 60 to 80 gallons of Bioxide solution were injected daily to oxidize dissolved H2S hydrogen sulfide and control odor at the drop. Expenses on chemicals averaged up to $5,700 a month.
In November 1997, the drop structure was improved by installing a vortex form and shaft into the existing MH. Improvements included
- Removing and replacing 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.
- Bolting a corrosion-protected metal base plate to the MH bottom and covering it with two-inch 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 seven 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 included two phases. One included chemical injection and the second 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 p;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 similar 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 the damage was repaired by a PVC lining, but 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 a day. This averaged $3,300 in chemical cost per month. Considerations taken into account were that 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 July 1998, the structure was improved by the installation of an inner 24-inch high density polyethylene HDPE vortex shaft. Installation included the following:
- Constructing the vortex form and entrance flume in advance to minimize bypassing time. PVC sheets 3¦8-inch thick were used for this purpose. 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 sand bags in the effluent pipe.
- Removing sand bags and installing the vortex shaft into the frame, fixing it to the top of the MH wall by three aluminum anchors.
- Installing the concrete slab with its hole centered above the vortex shaft.
- Mounting the vortex-forming PVC fabrication on the concrete slab. 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 and grouting the PVC fabrication.
- Installing the top slab, an adjacent ring and cover on the top.
The new vortex flow with its stable air core was created above the vortex shaft. The structure improvement was completed in one day.
Preliminary observations show no odorous emission from the improved structure. Because there was no chemical feed from an upstream location at the time of observation, the decrease in odor may be attributed to the new vortex design. Area residents have not complained about sewer odors since the improvement was installed.
MCES also recently 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 1,800 feet of interceptor tunnel. Corrosion related to H2S emission from the drops had severely damaged the 6 ¥ 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 an 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 described vortex drop design and improvement technique, along with the preliminary results of decreased emissions, show that the design can be beneficial and effective in situations where H2S emission creates corrosion and odor problems. These situations could include main interceptors, forcemain discharges, sewer pumping stations and locations where the sewer flow drops more than four feet. If corrosion and odor appear in these places, consider the vortex drop design as a viable solution.
- D.K.B. Thistlethwyte, The Control of Sulfides in Sewerage Systems, Ann Arbor Science Publishers Inc., Ann Arbor, Michigan, 1972.
- Tom Wahlberg, Continuous Hydrogen Sulfide Monitoring at Select Sewer Locations, MCES Report Number H980611A, St. Paul, Minnesota, 1998.