Odor and Corrosion study in a Forcemain utilizing Oxygen
Application: Collection System Odor and Corrosion Control
- Reduce collection system odor and corrosion
- Demonstrate oxygen injection technology’s effectiveness in a forcemain application
- Determine design criteria for full-scale implementation, including OUR and sulfide requirements
In the spring and summer of 2013, a large southern city in the US, Fresco Water Services, and BlueInGreen partnered to pilot the SDOX-CS™ system to show the effects of dissolved oxygen on hydrogen sulfide (H2S) formation within a forcemain portion of the city’s sewer collection system. The city, like almost every city, is battling odor and corrosion problems within the city’s sewer system. Currently, the city spends in excess of $50M annually to replace the collection system infrastructure due to corrosion.
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. H2S gas forms within a collection system as a result of bacterial action on organic matter under anaerobic (low oxygen) conditions. The formation of H2S is evident by its distinctive “rotten egg” odor. Not only is the smell unpleasant, but it also poses a serious work hazard and can have physically harmful effects on workers that come in direct contact with the gas. On the financial side, H2S possesses aggressive corrosive properties which often lead to premature failure or shortened service lives for concrete and steel structures within a collection system.
BlueInGreen’s SDOX-CS system, as seen in Figure 1, uses patented technology to supersaturate wastewater with oxygen by taking a side stream off the water line, injecting oxygen and returning the supersaturated, oxygenated water back to the main line. The reduction of H2S is most efficiently accomplished by delivering dissolved oxygen into the collection system and promoting aerobic conditions. The SDOXCS injection point was placed on the discharge side of an existing lift station into one of three lines that fed a 30” forcemain. The 30” line is one of three additional 30” lines that feed the wastewater treatment plant (WWTP). The WWTP is approximately 1.5 miles from the oxygen injection point. The goal of the project was to oxygenate the wastewater, maintain aerobic conditions in the pipe, and impede the development of H2S in the water line all the way to the WWTP.
Initially, to determine the oxygen needed to properly oxidize the existing sulfides, a portable flow meter was installed, and it was determined that the flow rate coming from the injection point was about 200 GPM. This flow rate gave an HRT of roughly 18 hours. The amount of oxygen needed to keep the forcemain in an aerobic state was calculated to be 1,800 lb/day. Since the SDOX-CS unit was vastly undersized for this particular application, dissolved oxygen, DO, was a difficult parameter to measure. But, in these situations the Oxidation Reduction Potential, ORP, is a better indicator of the biological reaction taking place within the collection system. Normally an ORP of greater than -150 mV describes the lower boundary of the ORP range where H2S will not form, and Figure 2 shows the successful ability of the SDOX-CS system to keep ORP levels within the range where sulfides will not form. To further show the positive outcome of ORP analysis, Figure 3 shows the analytical data from the WWTP’s headworks with a daily average H2S reduction of 43%.
Competing technologies 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.