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 ABSTRACT 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 aerators. 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 drops. 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. KEYWORDS Sewer drop structures, gas emission, odor control, wastewater aeration, abrasive wear control. INTRODUCTION 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. 1 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 maintenance. A new and more versatile realization of the Vortex Drop method is the Vortex Insert Assembly (VIA) . 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. PRINCIPLE OF OPERATION 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 fluctuations. 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. EXPERIENCE The Vortex Drop method has been in use in the Minneapolis/St/ Paul metro area since 1998 . 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 installations. Multiple wastewater samples were taken simultaneously upstream and downstream of two VDS installed in Hennepin Co. . 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. 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 disappeared. 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 NEW INSTALLATIONS 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. CONCLUSIONS 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 www.vortexflow.com. REFERENCES 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.