Durational Biosolids Odor Control Using Peroxide Regenerated Iron Technology (PRI-TECH®) at Metropolitan St. Louis Sewer District's Lower Meramec Treatment Plant

Summary:
The Metropolitan St. Louis Sewer District (the District) was concerned with odors from solids dewatering, cake hauling, and landfilling activities. Odors from sludge disposal at the landfill were threatening to close this outlet for District sludges, severely limiting options for solids management. To meet this need, the District conducted a comprehensive evaluation of liquid treatment for sludges at their facilities, beginning with bench scale testing for all three plants and culminating in a full-scale evaluation of the PRI-TECH® process at the Lower Meramec WWTP. USP Technologies' PRI-TECH process (iron salt dosing followed by downstream hydrogen peroxide dosing) was selected for full-scale evaluation based on the bench scale testing. The District has used ferrous chloride dosing into the sludge thickeners for hydrogen sulfide (H2S) control, providing a significant amount of total and soluble iron prior to dewatering. The PRI-TECH process was implemented with the addition of hydrogen peroxide into the conveyance lines just before the belt filter press. Odor control performance was measured using H2S meters, gas detection tubes, and the recorded descriptions and observations of the District's solids handling staff. Process Selection: Initial chemical control methods were qualified using USP's shake test methodology on samples of thickened solids treated with ferrous. Samples were dosed with oxidants, mixed, the headspaces were purged, and then the samples were shaken to volatize measurable H2S, methyl mercaptan, and ammonia. The next step of the bench scale testing assessed the durational odor control for dewatered sludge. This bench scale evaluation involved pretreating thickened solids with oxidants, dewatering with a Crown Press Belt Press Simulator, incubating samples, and then measuring purgeable odorants from the dewatered cake. Calcium nitrate addition was also evaluated before or after dewatering. While calcium nitrate was effective, the addition of iron upstream of the dewatering operations showed that the dosing of H2O2 at dewatering was both effective and offered a lower cost. Full-Scale Evaluation: Dosing of hydrogen peroxide and data collection at both belt filter presses began on 1/15/2019. The sludge prior to dewatering and filtrate was analyzed for iron, sulfide, pH, and phosphate. H2S at the belt filter presses was monitored. Dewatered cake samples were obtained from the press and analyzed for purgeable odorants at 0, 24, and 48 hours after dewatering. Ferrous chloride dosing rates were held constant throughout the evaluation. The goal of this phase of the evaluation was to determine the minimum dosage required to eliminate odorants present at the dewatering process. Since odorants continue to be generated in the dewatered cake, the dosage that eliminates odorants at this step may be considered the lowest dosage that will offer durational control. Dosages ranging from 125 500 mg/L of H2O2 eliminate ~90% or more of H2S at the belt filter press and from within cake immediately upon dewatering.. H2S Removal Dosage Rate Finding: A sample of sludge taken at the sample port at the active sludge pump was analyzed. It contained 11 mg/L of aqueous total sulfide, 300 mg/L of total iron, and 200 mg/L of soluble iron. The pH was 6.2. At the lowest H2O2 dosage tested, 15 mg/L, H2S under the belt was lowered from 32 to 17 ppmv, a 47% reduction. At dosages of 125 mg/L and higher H2S was reduced by at least 90%. Filtrate Analysis: With increasing dosages of hydrogen peroxide the total and soluble iron levels within the filtrate were reduced. This is likely due to PRI-TECH reactions whereby ferrous (Fe2+) state iron is oxidized to the ferric (Fe3+) state. A drop in phosphate concentrations was observed, with baseline filtrate containing 40 mg/L and the peroxide treated samples containing 20 mg/L. Cake Solids Improvement: Cake solids percentages displayed an apparent linear response to hydrogen peroxide dosing, increasing approximately 6% points from baseline to the 500 mg/L dosage. One mechanism likely contributing to this improvement is sulfide oxidation with hydrogen peroxide, allowing more of the iron present to bind with phosphorus bearing compounds rather than sulfides. Durational Odor Control Results: In dewatered cake samples ammonia and amines were not detected. It is likely that the low pH (6.2) of the feed sludge inhibits volatilization of nitrogenous compounds. H2S appeared to be generated on a linear basis within the 48 hour time frame, with lower overall H2S levels seen with increasing H2O2 dosages. Mercaptans were not detected at 24 hours but at all but the 500 mg/L dosage were detected at 48 hours. District staff trained in odor monitoring techniques recorded observations on odor characteristics, intensity, and prevalence throughout the solids handling and transport process. This feedback was considered to determine the hydrogen peroxide dosing rates ultimately necessary to meet durational odor control needs.

Conclusion:
The District decided to continue with the PRI-TECH application and awarded USP a five year full-service supply contract through a competitive RFP process. Through the full-scale evaluation period the H2O2 dosing rate range of 200 - 250 mg/L was selected as the most cost-effective treatment level that met odor control objectives.