Application of a calcium hydroxide slurry with metabolic modifier for control of hydrogen sulfide odors at the siesta key wastewater treatment plant

In April 2016, Sarasota County Utilities investigated odor complaints from the Siesta Key WWTP. It was determined that the complaints were due to hydrogen sulfide odors from the effluent of the plant’s existing odor control scrubber. This scrubber processes foul air from the headworks and equalization tank. The Siesta Key WWTP has been in operation since the late 1960s, and is slated to be decommissioned within a year and a half and converted to a master pump station. However, the aging wet chemical scrubber has been struggling to handle increased hydrogen sulfide loadings, resulting in several complaints from the surrounding neighborhoods. A solution was necessary to mitigate these odors.Due to the short life span of the facility, improvements to the vapor-phase system were deemed cost-prohibitive. Therefore, a liquid phase treatment method was necessary for this application. Due to the nature of the upstream collection system, it was preferred to feed chemical directly into the force main discharge at the headworks. This feed configuration resulted in a detention time on the order of seconds, meaning any viable technology must be fast acting with rapid dissolution.Upon evaluating the conditions of the application, it was determined that a pH shift technology using a calcium hydroxide based slurry with a metabolic modifier would be excellent for controlling odors at the headworks and EQ tank, reducing the loadings on the chemical scrubber. Furthermore, this alkaline calcium hydroxide slurry rapidly dissolves in wastewater, providing an almost instantaneous effect at the application point. As a result, the calcium hydroxide slurry could be fed directly into the force main discharge at the headworks and have a significant impact on hydrogen sulfide concentrations at the plant.Hydrogen sulfide (H2S) is a partially soluble, volatile gas notorious for causing offensive odors and corrosion in collection systems and treatment plants. While in an aqueous solution, H2S exists in equilibrium with two ionic forms: hydrosulfide (HS-) and sulfide (S=). These ionic compounds are completely non-volatile, and are unable to escape into the vapor phase or result in any offensive odors or corrosion issues. (WEF, 2001)At a neutral pH of 7.0, approximately 50% of the sulfide in solution will be in the H2S form, and can escape to the atmosphere causing odor and corrosion problems. By shifting the pH of the solution to nearly 8.3, more than 95% of sulfide will be trapped in solution as non-volatile ionic sulfides (EPA, 1985).Prior to chemical treatment, H2S loadings at the inlet of the scrubber averaged 75 ppm, while the outlet of the scrubber averaged 3.4 ppm with peaks up to 24 ppm. Based on the plant’s current flow rate of 4.9 ML/d (1.3 MGD) and an untreated pH of 7.4, approximately 757 L/d (200 GPD) of the calcium hydroxide slurry was fed into the headworks force main discharge. The solution was applied using an advanced dosing controller to match dose-to-demand and maintain a relatively constant wastewater pH. Once optimized, an average pH of 8.2 was achieved. H2S loadings to the inlet of the scrubber were reduced to 5.8 ppm, with 0 ppm from the scrubber outlet, effectively eliminating odors in the surrounding area.This paper describes the investigation methods used to quantify the problem, the implementation and optimization of the resultant solution, and presents the results of the optimized process.