Systemwide Evaluation of Iron-Based Sulfide Control Across Wastewater Collection and Treatment Systems

BACKGROUND The Los Angeles County Sanitation Districts (Districts) operate a regional wastewater system serving approximately 850 square miles, including more than 1,400 miles of sewers and 11 treatment plants. Upstream sewers converge into three major Joint Outfall Systems-JOA, JOB, and JOD-that convey wastewater to the A.K. Warren Water Resource Facility (WF), the Districts' largest facility, treating approximately 250 mgd (Figure 1a). To control sulfide-related corrosion and odors, the Districts operate five sulfide control facilities (SCFs) that dose ferrous chloride (FeCl₂) within the wastewater collection system (WCS) (Figure 1a). FeCl₂ is also added at the WF anaerobic digesters to control hydrogen sulfide (H₂S) in digester gas (Figure 1b). Over the past decade, FeCl₂ unit costs have increased approximately 2.5-fold, with 2025 procurement alone totaling about $40 million. Iron dosed upstream directly affects iron and sulfide loading to the WF, with reductions in WCS doses increasing iron demand at the digesters (Figure 2), highlighting the need for systemwide optimization rather than facility-specific optimization. The objectives of this study were to evaluate the Districts' iron-based sulfide control practices across the WCS and WF, quantify the systemwide fate of iron and sulfur, and assess competing iron reaction pathways. The results of this work serve as a foundation for future studies to identify and evaluate opportunities for coordinated systemwide optimization. METHODOLOGY Historical flow, sulfate, sulfide, and iron usage data were analyzed to quantify iron loading and sulfide control practices across the WCS and WF. These data were also used to develop a systemwide mass balance. GIS hotspot analysis of H₂S monitoring data identified spatial patterns and recurring sulfide problem areas. Targeted upstream and downstream sampling at selected SCFs was used to calibrate a chemical equilibrium model (CEM) and then simulate near-field and downstream iron reactions and solids formation. RESULTS AND DISCUSSION Analysis of historical data for the WF influent sewers shows that JOB had the highest sulfate concentrations (Figure 3), largely attributable to wastewater from battery recycling and other industrial sources. Sulfate can be reduced to sulfide under anaerobic sewer conditions. Consequently, JOB was dosed with the most iron, resulting in the highest iron loads (Table 1) and the highest concentrations of total iron and insoluble sulfide (Figure 4), a proxy for sulfide precipitates such as ferrous sulfide (FeS). Calculated and observed iron concentrations showed reasonable agreement for all three sewers, with doses in JOA and JOB frequently exceeding 20 mg-Fe/L and even 30 mg-Fe/L in some years -above the typical sulfide-control range of 5–20 mg-Fe/L (WEF, 2017; Gutierrez et al., 2010). These results suggest upstream iron dosing in JOA and JOB may not be fully optimized. A GIS hotspot analysis of H₂S monitoring data (Figure 5) identified statistically significant sulfide clusters in three areas that are consistent with operator observations. These clusters were confirmed to have elevated sulfide levels, based on 90th-percentile H₂S concentrations aggregated by manhole (Figure 6) for representative low (2019) and high (2022) sulfide years, as measured at the WF. Near-field sampling downstream of the South Gate JOA SCF shows close agreement between measured and CEM-predicted total and dissolved iron concentrations (Figure 7a), confirming model calibration. Modeling indicates approximately 30% of dosed iron reacts rapidly with carbonate and orthophosphate to form siderite and vivianite, while FeS formation is limited immediately after dosing due to low dissolved sulfide availability (Figure 7b). Reductions in orthophosphate and alkalinity across multiple sampling events further support rapid near-field iron scavenging (Figure 8). Serial CEM simulations (Figures 9–11) show iron solids form progressively as sewage flows downstream in JOA, JOB, and JOD, with shifting contributions of FeS, vivianite, and siderite driven by evolving sulfide, phosphate, carbonate, and pH conditions. Aggregated systemwide, nearly all iron dosed upstream reaches the WF as solid-phase minerals (Figure 12). The systemwide iron mass balance (Figure 13) indicates that most iron within the WF influent is retained in solids and exits the WF in biosolids. The iron alone (ignoring any co-precipitated anions) comprises an estimated 10% of the biosolids mass, highlighting the strong linkage between upstream sulfide control and downstream solids handling and cost. Model results were compared to a sewer deposit sample collected from a location downstream of the Beach Avenue JOD SCF. Characterization of the deposit was broadly consistent with the model-predicted precipitation of FeS and siderite: the elemental composition was dominated by iron, sulfur, carbon, and oxygen, and sulfur was co-located with iron. (Figure 14). SUMMARY AND FUTURE WORK Results show iron dosing for sulfide control in the collection system is often conservative, with substantial iron consumed by carbonate and phosphate reactions and transported to the WF as solids. Findings support the need for coordinated, systemwide sulfide control. Future work will evaluate alternative control strategies, sulfate source control from industrial dischargers, and optimized iron dosing using integrated BioWin modeling.