Sustainably Eliminating Collection System Odors with Oxygen
Application: Collection System Odor and Corrosion Control
Owner: City of Houston
Engineer: BlueInGreen, LLC
- Reduce pumping bypass odor and corrosion by minimizing H2S production
- Demonstrate oxygen injection technology in pumping bypass/force main applications
- Determine design criteria for full-scale implementation, including OUR and sulfide requirements
In late November of 2012, The City of Houston (COH), IPR, and BlueInGreen came together to pilot the SDOX-CS system to show the effects of Dissolved Oxygen on Hydrogen Sulfide reduction within a bypass system operated by IPR. The study focused on a bypass system that was located in the 8000 block of Woodway drive in Houston, TX.
Hydrogen Sulfide, H2S, is a toxic substance that acts as a respiratory depressant in both humans and wildlife, and is probably the most difficult compound plaguing wastewater collection systems today. Hydrogen Sulfide gas forms within a collection system as a result of bacterial action on organic matter under anaerobic conditions. The formation of Hydrogen Sulfide raises two primary concerns for those charged with maintaining a city’s or industry’s collection system: Odor and Corrosion to steel and concrete structures. One concern for collection systems containing Hydrogen Sulfide is the distinct “rotten egg” odor associated with the gas and the harmful effects on workers in direct contact with H2S. The second concern is the corrosive properties of Hydrogen Sulfide, where millions of dollars are spent annually to correct the damage done to structures within a collection system.
The study operated under the assumption that if the SDOX-CS delivered enough Dissolved Oxygen to satisfy the oxygen demand of the water plus enough to react with the Hydrogen Sulfide present, then all the Hydrogen Sulfide would be converted to sulfates. As the Hydrogen Sulfide is removed through the oxidation reaction, the equilibrium of the water shifts to form more Hydrogen Sulfide from the remaining dissolved sulfides in the water. So, depending on the amount of Dissolved Oxygen in the collection system and the amount of contact time, the oxidation reaction can continue until all Hydrogen Sulfide is removed. The two key design parameters for these types of systems are oxygen uptake rate (OUR) and oxygen requirements for sulfide oxidation. Outlined by the EPA and confirmed in this study, typical values for OUR range from 10 to 15 milligrams of dissolved oxygen per liter per hour of retention time. Furthermore, the requirements for sulfide oxidation range from 2 to 5 pounds of dissolved oxygen per pound of dissolved sulfide, depending on wastewater characteristics.
The following figures provide a few snapshots of the data collected during our demonstration. Figure 1 shows background data at a manhole about 300 feet above the injection site, with hydrogen sulfide levels averaging >5-mg/L and maximum levels of ~30- mg/L occurring during low flow conditions. Figure 2 is a chart representing the distance the DO traveled in the bypass pump-around, with the first bar being the DO level upstream (<1 mg/L) of the injection site and 3000 feet from injection site. We were able to keep DO (>7 mg/L) in the system up to 300 yards beyond the bypass outfall. Finally, Figure 3 shows dissolved oxygen levels upstream and downstream of SDOX-CS injection during unit operation. It is seen here, the SDOX-CS technology can effectively increase DO concentrations within the collection system to levels that mitigate the formation of odorous H2S.
In addition to the above, the analytical data also shows an average BOD reduction of ~20% with a corresponding ~8% decrease in COD. Therefore, unlike competing technologies which address odor problems with chemical solutions that typically increase solids loading at wastewater treatment facilities, oxygen injection has the potential to reduce loadings at treatment facilities, effectively offsetting some operational costs while simultaneously increasing the expected lifespan of those facilities.