Odor and Corrosion Control in Wastewater Collection Systems

Post-Treatment Analysis

As discussed in Parts 2 and 3, hydrogen sulfide (H2S) levels were elevated. The average H2S indicated was 1,070 parts per million (ppm). Even if the extreme spike above 3,000 ppm were ignored, the average concentration was still about 700 ppm. After ozone/oxygen treatment, concentrations as low as 1 ppm were achieved. However, for calculation purposes, a more realistic average was 7 ppm of H2S, based on OdaLog values following the steep drop, which was considered a treatment induced value. This equates to a 99 percent reduction in H2S levels in a matter of days, with real results starting in about a day (see Figure 1). In Figure 1, the startup of the system is indicated as well as the calculated detention time range, approximately 25 hours. Notice how the H2S concentration began a rapid descent almost exactly at the 25-hour mark (14a). Additionally, the system shutoff was included, and about 25 hours later, the H2S levels began to peak again (14b). When the treatment company disconnected the system, it discovered an impingement on the gas feed line, and the treatment delivery rate had significantly decreased. This could be one reason why it took slightly less than 25 hours for concentration increase to resume following deactivation. Another important fact is that, although the H2S concentration began to increase following system deactivation, as expected, the peaks were only in the 25 to 80 ppm range and remained in that range for at least four days, which was all the data that was available. This is significant because 10 days prior, the untreated H2S concentrations spiked, at times, in excess of 1,500 ppm. This suggests that the treated environment was directly and positively affected by the oxygen and ozone. Aside from H2S vapor levels, total and dissolved sulfides and dissolved oxygen (DO) levels (liquid phase) should be examined. For this, only the third-party grab samples were used. Since both data sets were, for the most part, in agreement, this limits any sampling bias and simplifies the analysis. This data set (see Table 1) is complete and precise and supports the Figure 1 data. During all sampling, the pH remained relatively constant at slightly above 7, averaging 7.11. The temperature was in the range of 27.1 C to 35.2 C, averaging 32.4 C. These are normal values for the location and time of year, indicating that temperature and pH did not significantly affect the reductions achieved and can be eliminated as alternate explanations for the reduced H2S. What changed was the sulfide and DO concentrations, especially in the range highlighted. On Aug. 19, significant changes occurred, and the data in Table 1 further illustrate the point. The reduction in total and dissolved sulfides between the last sample on Aug. 18 and the first on Aug. 19 was 50 percent and 70 percent, respectively. While these are great reductions, even better was the 100 percent reduction to both that occurred when the first sample was collected on Aug. 20 and continued through the sampling period. A rise in DO was also apparent, increasing from 1.1 ppm to 11.5 ppm. As for the suspected feed line impingement, it was interesting to note the decrease in DO levels that took place on Aug. 23, although the levels were still elevated well beyond what is considered aerobic.

Financial Benefits

Traditional odor control methods employed by utilities and municipalities can create technical and financial compromises, which tend to produce undesirable side effects, including infrastructure corrosion, clogging and the facilitation of the sulfide producing anaerobic slime layer. The use of chemicals also requires ongoing and escalating year-over-year costs. Often, these technical and financial challenges impact performance and compromise results. Using the ozone/oxygen system can quickly and completely control H2S production and increase DO levels. The operational theory is supported by pilot tests. This treatment causes minimal to zero negative side effects. Consider a municipality that uses, on average, 210 gallons of iron salts per day in a force main. For nitrates, the approximate stoichiometric equivalent is about 33 percent less usage than iron, or about 139 gallons per day (gpd). The costs of these methods can be compared to that of an appropriately sized ozone/oxygen system, which would be expected to provide a higher level of overall performance so that an accurate comparison can be made between them, as shown in Figure 2. The assumptions used to create Figure 1 were:

• Iron—210 gpd average usage at $0.75 per gallon
• Nitrate—139 gpd average usage at $2.25 per gallon
• Ozone/oxygen treatment—Capital equipment cost and operating costs of 16 kW at $0.08 per kilowatt hour, 24 hours, 365 days and miscellaneous consumables

After year one, the ozone/oxygen treatment system costs fall below those associated with chemicals (see Figure 2). Figure 3 indicates that an ozone/oxygen treatment system will be less expensive to operate over 5 years than traditional treatments (iron or nitrate), two ends of the chemical pricing spectrum. Additionally, unseen chemical treatment costs—such as the potential for damage to infrastructure or increased usage issues that were discussed in Part 1 (Pumps & Systems, June 2012)—were not included in the data. Alternatively, ozone treatment systems will improve the collection system environment over time.

Conclusion

The application of ozone and oxygen into the wastewater environment brings with it a much more technologically and cost-effective way of treatment. It is also more effective from a sustainability standpoint.

Series References:

1. American Society of Civil Engineers, “ASCE’s Infrastructure Report Card,” ASCE, 2009, http://www.infrastructurereportcard.org/.
2. A. Matthews et al, “Control of Hydrogen Sulfide Buildup in Forcemains using Ozone and Oxygen,” Proceedings of the Water Environment Federatin, WEFTEC 2010: Session 101 through Session 112, pp. 7591-7611(21), 2010.
3. U.S. Environmental Protection Agency, “Odor and Corrosion Control in Sanitary Sewerage Systems and Treatment Plants,” Design Manual, EPA/625/1-85/018, Cincinnati, OH, 1985.
4. U.S. Peroxide, “Iron Salts – Ferric and Ferrous,” 2011, www.h2o2.com
5. Beltran, F.J., Ozone Reaction Kinetics for Water and Wastewater Systems, Lewis Publishers, 2004.
6. Lenntech, “Water Disinfection Application Standards (for EU),” 1998, www.lenntech.com.
7. Drago, J.A. et al, “Municipal Wastewater Ozonation Practice in the United States: Past, Present and Future,” Ozone: Science & Engineering, Volume 32, Issue 1, pp. 43-55, 2010.
8. Plasma Technics, Inc, “Plasma Block Product Line, Product Detail,” 2011, www.plasmatechnics.com.
9. Terry, P.A., “Application of Ozone and Oxygen to Reduce Chemical Oxygen Demand and Hydrogen Sulfide from a Recovered Paper Processing Plant,” International Journal of Chemical Engineering, Volume 2010, Article ID 250235, 2010.