From Master Planning to Implementation Odor Mitigation Experience from the East Honolulu WWTP

The East Honolulu Wastewater Treatment Plant is owned and operated by Hawaii American Water (HAW). This paper focuses on a chemical evaluation and optimization project conducted in 2007, as a follow-up of a 2006 odor master plan. The project consisted of a series of bench scale and pilot tests using various chemicals at several locations – influent pump station, primary clarifiers and sludge drying beds, with the intent to reduce odors.Ferric chloride, sodium hypochlorite, and chlorine were tested at the primary clarifiers because it was observed that large portion of dissolved sulfide is generated within the clarifiers. Although tests indicated that lower doses of sodium hypochlorite were required to achieve similar sulfide reduction to ferric chloride, a cost comparison favored retaining the existing ferric chloride system upstream of the plant. When the dissolved sulfide level at the pump station was low, ferric chloride flowed downstream and reacted with sulfides generated in the primary clarifiers. Ferric addition upstream of the plant also offered the benefit of odor and corrosion control along the conveyance system.Alum, ferric chloride, and potassium permanganate were tested at the sludge drying beds. No significant odor reduction was noticed. The pilot test showed that alum best reduced ammonia emissions; however, high doses tended to increase hydrogen sulfide and total odor emissions, likely due to the increased pH by alum. It was also observed that when no chemical was added, the sludge had lower odors than those measured during the master plan sampling. This odor reduction is likely due to the digester cleaning that occurred between the two sampling events.Based upon the master plan and pilot testing, the plant implemented several process and operation improvements to reduce odors. The existing pump station chemical feed system was upgraded with flow pacing capability. The sludge blanket in the primary clarifiers has been monitored and controlled so that the blanket is maintained at the minimum thickness, reducing sulfide generation. Other odor sources, such as WAS storage tank and RAS channel, were eliminated or modified. Finally, both digesters were cleaned so that sludge is better stabilized and fewer odors are emitted.