No More Odors Here on the Streets of Bakersfield

Faced with increasing costs for controlling hydrogen sulfide in its wastewater collection system, the City of Bakersfield, California commissioned an evaluation of its program in 2019. Among the consultant recommendations was to consider using a more efficient treatment (ferrous chloride) in place of certain of the existing Calcium Nitrate dosing stations where costs were constrained. Of priority were stations those along the higher flow interceptor segment where chemical demands and community sensitivities were highest. An expected benefit of iron dosing is its durational control capability, which should translate into fewer dosing stations. Consequently, field testing was suggested to assess the cost-benefit. As siting dosing stations for ferrous chloride solutions can be limited (due to the corrosivity of the product), a low-hazard form of ferrous chloride, SulFeLox®, was used. The study was conducted from April to June 2021 and consisted of 12 days monitoring of the baseline Calcium Nitrate scenario, followed by 60 days of SulFeLox dosing at different feed rates, and concluding with 14 days of no chemical dosing (i.e., the true baseline scenario). The interceptor segment selected for the test was a 10-mile long (5-6 hour retention) trunkline leading to the City's Plant No. 3 (Figure 1). Wastewater flows increase from approximately 0.3 mgd at the Romero PS input (S1) to 12 mgd at the McCutcheon PS (S3). In this study, the SulFeLox dosing station replaced two Calcium Nitrate stations: 1) at the Romero site (S1), as well at the Buena Vista site (S2). Comparing the Calcium Nitrate and SulFeLox results shows 66-83% lower vapor-H2S peaks at the Buena Vista (S2) and treatment plant (S4) sites, with less benefit at the McCutcheon site (S3) where vapors are impacted by H2S by other flows into the station. (Figure 2a,2b,2c). Site 2 (Buena Vista PS) is in a particularly sensitive area, and prior tests suggested upstream Calcium Nitrate feed rates would need to be 500 gpd (or more) to maintain vapor-H2S levels below 25 ppm. Figure 3 shows the vapor-H2S results in this test, where SulFeLox feed rates of 120-210 gpd achieved the compliance targets and provided H2S reductions of 84-95% (relative to 146 gpd Calcium Nitrate). On a performance basis, the field results confirmed the consultant recommendation that (for system-wide H2S control) iron dosing from one site could provide improved results relative to Calcium Nitrate dosing from two sites. SulFeLox feed rates were determined that provide interceptor-wide H2S control to 25 ppm and 10 ppm. However, a direct cost comparison between the two treatment chemistries is complicated given that system-wide H2S control could not be demonstrated with Calcium Nitrate (per the 500 gpd test in a prior study). This present study did show, however, that the cost to control to target H2S levels using SulFeLox was $600-$1000 per day, where the recurring costs for Calcium Nitrate was $850-950 per day (to control to 100-140 ppm H2S). Subsequent to this field test, the City continued to feed SulFeLox at S1 (Romero PS) and has since replaced seven other Calcium Nitrate sites with three SulFeLox sites, reducing the total number of chemical feed sites from nine to four. As a result of this work, the City is now spending approximately $663k/yr for SulFeLox to meet its system-wide performance targets, where it was previously spending $990k/yr (for Calcium Nitrate). Further, with now a year's experience with dosing iron into the collection system, the treatment plant has observed operational benefits including the elimination of the ferrous chloride feed to their digesters for sulfide control. Early in 2022, the City entered into a five-year supply agreement for providing SulFeLox iron product and related services and equipment. Future work is planned to evaluate iron regeneration ahead of the treatment plant (at McCutcheon PS, S3), recognizing that this trunkline comprises 70% of total plant flow but 100% of influent iron. Oxidizing the spent iron (as FeS) to hydrous ferric oxide (using hydrogen peroxide) is expected to provide additional benefit to treatment plant operations.