Odor Elimination in a Collection System utilizing Oxygen
Application: Collection System Odor and Corrosion Control
Owner: City of Houston
Engineer: BlueInGreen, LLC
- Reduce collection system odor and corrosion
- Demonstrate oxygen injection technology’s effectiveness in a gravity sewer line
- Produce a cost savings over the current chemical based odor control system
The City of Houston and BlueInGreen partnered to determine the effects of dissolved oxygen on hydrogen sulfide (H2S) within the city’s sewer collection system. The city regularly received odor complaints from a particular location and wanted to mitigate the odor problem by dissolving oxygen into the wastewater to reduce hydrogen sulfide levels. Historically the problem was addressed by adding chemicals, calcium nitrate and ferrous sulfate, directly into the sewer line. The chemical solution is very costly and only masks the odor problems. By injecting oxygen into the sewer, the build-up of H2S, the source of the problem, can be eliminated. A secondary goal was to see what effect using oxygen in the sewer might have on corrosion issues. The City of Houston spends over $50M annually to replace the collection system infrastructure. H2S is a toxic substance and is one of the most difficult compounds plaguing wastewater collection systems today. H2S gas forms within a collection system when bacteria are working in an oxygen free environment (anaerobic conditions). The formation of H2S poses two concerns for municipal and industrial collection systems: it has a foul smell which results in odor complaints, and it is caustic which causes corrosion to the infrastructure. BlueInGreen’s SDOXCS® system uses patented technology to supersaturate wastewater by taking a sidestream off the water line, injecting oxygen and returning the supersaturated water back to the line. Reduction of H2S is most efficiently accomplished by delivering dissolved oxygen into the collection system itself and promoting aerobic conditions. The SDOX-CS injection point was placed in a 48” gravity line at a manhole approximately 2,700 feet upstream from the manhole causing the odor issues. The problem manhole was at the head of an existing lift station.
Dissolved oxygen (DO) was measured at various manholes along the line, from the injection site to the lift station. One of the pilot goals was to meet the 2 to 5 mg/L of DO required to oxidize dissolved sulfides in the wastewater. Figure 1 shows the successful initial rise in DO levels, as well as, the subsequent depletion of oxygen along the line. The Oxygen Uptake Rate (OUR) was 13mg/L of DO per hour, which is right in line with EPA guidelines of 10-15mg/L of DO per hour for raw wastewater. Another parameter measured was the Oxygen Reduction Potential ORP. ORP is a snapshot taken of a wastewater sample that shows the effectiveness of the biochemical reaction taking place within the collection system. ORP ranges from +2000mV to - 2000mV, with sulfide formation taking place in the -50mV to -200mV. The goal for the pilot was to show a positive increase in ORP, which would indicate a decrease in sulfide formation. Figure 2 & 3 show the effect of DO and ORP levels on H2S at the target manhole near the lift station. Even with minimal DO levels, ORP is greatly impacted in a positive manner. The graphs, from two separate days, show how H2S levels tend to drop as ORP levels are shifted in the positive direction. The periodic dip and rise in DO and ORP levels were a direct result of turning the SDOX-CS unit off and on to show the corresponding effect on the two parameters.
Chemicals were dosed, prior to the SDOX-CS being used, at a rate of 200lb of calcium nitrate and 600lb per month of ferrous sulfate during the summer, and 100lb and 300lb per month respectively during the winter. The average cost of chemicals was over $30,000 per month. The average cost to run the SDOX-CS is $1500 per month, which produces a cost savings of $28,500 monthly.
The data illustrates the effectiveness of SDOX-CS technology at promoting aerobic conditions in collection systems for the purposes of odor and corrosion control. The aerobic state created in this section of the sewer system will facilitate the degradation of sulfides, thus allowing healthy operation of the system without the worry of short term odor complaints. Utilizing the SDOX-CS to solve the problem rather than chemicals will save the city $342,000 per year. Keeping the development of H2S at bay over long periods of time will deliver long term corrosion control and even further cost savings by protecting the existing infrastructure and extending its useful life.