OC San Strategies and Pilot Testing to Optimize Collection System Odor Control

Orange County Sanitation District (OC San) chemically treats the wastewater in its collection system to mitigate sulfide levels, nuisance odors, and corrosion potential. Ferrous chloride, magnesium hydroxide, and calcium nitrate are the chemicals predominantly used for odor control. However, market volatility and supply chain issues have impacted the availability and cost of chemicals over the past several years. OC San has been proactively taking measures to develop strategies to address these issues, including using a combination of chemicals, optimizing chemical dose requirements, and identifying alternate chemicals or chemical formulations. The increasing market volatility in recent months has made optimization a high priority. A chemical dosing strategy was developed that involves adding an iron-based compound, increasing the pH using an alkaline material such as magnesium hydroxide, and, in some instances, adding calcium nitrate. Increasing the wastewater pH increases the solubility of sulfide (as bisulfide HS- ) and minimizes the release of odorous hydrogen sulfide (H2S) to the headspace. Further, iron salts are more effective in precipitating dissolved sulfide compounds as ferrous sulfide at higher pH than at lower pH. Finally, since wastewater microorganisms prefer nitrate over sulfate as an electron acceptor, adding calcium nitrate will lower the microbial reduction of sulfate and the formation of sulfide. Increasing the wastewater pH improves the effectiveness of calcium nitrate as the preferred electron acceptor. Hence, at elevated pH through alkaline addition, the iron and nitrate dose requirements can be significantly reduced to achieve the same level of odor reduction compared to chemical dosing at the native pH of the wastewater in the collection system. In 2019, OC San received a United States patent (US 10,435,316 B2) for this approach to reducing chemical dosing requirements and chemical costs for odor control. This patent requires adding at least one iron-based compound. The patent allows adding an alkaline compound to increase pH and also allows adding a nitrate compound to minimize the reduction of sulfate to sulfide. OC San intends to make this chemical usage approach available to other wastewater utilities. OC San has performed several field evaluations to verify and demonstrate the effectiveness of these chemical combinations. Historically, OC San used calcium nitrate compound in these efforts. Recently, in an effort to further increase its chemical supply security, OC San identified another nitrate-based compound that can potentially be used for odor control. Calcium ammonium nitrate (marketed as CAN-17) is predominantly used in the agricultural industry as a fertilizer. Since it is manufactured in a process that is very different from that used for calcium nitrate, it can provide additional chemical supply security. Table 1 provides a comparison of key characteristics of calcium nitrate and calcium ammonium nitrate. Briefly, calcium ammonium nitrate has a higher nitrate nitrogen content (17% nitrate) than calcium nitrate (9% nitrate). However, calcium ammonium nitrate has a higher ammonia content (5.4% ammonia nitrogen) than calcium nitrate (0.58% ammonia nitrogen), and that might increase the ammonia in the wastewater treatment plant influent. Currently, the cost of calcium ammonium nitrate is approximately 25% lower than calcium nitrate for equal nitrate content. Initially, OC San performed a theoretical analysis comparing the wastewater quality and cost implications of using calcium ammonium nitrate instead of calcium nitrate. The potential cost savings based on historic chemical usage and the potential increase in the treatment plant influent ammonia levels with calcium ammonium nitrate were estimated. The preliminary evaluation indicated that OC San could save approximately $250,000 annually by using calcium ammonium nitrate. The treatment plant influent ammonia levels would increase by less than 0.25 mg/L. A pilot study in the collection system was performed to verify these findings. The testing was performed at the Baker-Main trunkline using the chemical dosing approach identified in the OC San patent. The average wastewater flow during the testing was approximately 17 MGD. Initially, magnesium hydroxide and calcium ammonium nitrate or calcium nitrate were added to the sewer line near the Main Street Pump Station (Figure 1). Subsequently, ferrous chloride was added three miles further downstream of the Main Steet Pump Station and injected about midway to the treatment plant. An average of approximately 240 gallons per day (gpd) of magnesium hydroxide and 280 gpd of ferrous chloride were dosed. The nitrate dosing was 183 gpd of (8.42% N) calcium nitrate. The pilot testing with calcium ammonium nitrate used 183 gpd or 131 gpd (11.6% N). Various parameters related to collection system odor, including headspace H2S, dissolved sulfide, sulfate, ammonia, nitrate, pH, DO, and BOD, were monitored. The testing was conducted over four to five weeks for each of the nitrate formulations. Samples were collected upstream and downstream of the ferrous chloride addition. Figure 1 shows the monitoring (sample collection) locations. The goal of the testing was to maintain a headspace H2S level below 45 ppm. The results indicated that calcium ammonium nitrate at both tested concentrations maintained the headspace H2S level well below the target concentration of 45 ppm throughout the testing period (Figure 2). The average H2S level at the higher and lower calcium ammonium nitrate doses was approximately 13 ppm and 20 ppm, respectively. However, during the calcium nitrate testing, the daily average H2S exceeded the target concentration on 8 of the 28 days of testing. The average ammonia levels during the high and low calcium ammonium nitrate doses were approximately 50 mg/L and 48 mg/L, respectively. The average ammonia level during calcium nitrate dosing was approximately 46 mg/L. This difference resulted in a difference of less than 1 mg/L in the ammonia level at the plant influent. This indicated that using calcium ammonium nitrate did not significantly increase the plant-influent ammonia levels. This presentation will provide an overview of OC San's odor control pattern and benefits, present results from initial testing using calcium nitrate and ferrous chloride with and without magnesium hydroxide, review the test results using calcium nitrate or calcium ammonium nitrate as the nitrate compound, and report data with and without ferrous chloride addition during the recent tests.