Date of Award
Master of Science (MS)
Charles B. Bott
All wastewater treatment plants that operate multiple hearth furnaces (MHF) and are required to nitrify must manage the inhibitory effects of free cyanide (HCN, CN-) in the scrubber return flows due to inhibitory impacts on nitrifying bacteria.
HRSD Boat Harbor Treatment Plant (BHTP) a 25 MGD facility consisting of primary and secondary treatment, employs an anoxic selector process for nitrification and partial denitrification and operates a MHF. There is a desire to improve TN removal performance at BHTP due to an annual mass-based bubble permit limit on a combined discharge from seven HRSD plants, and there are no discharge limitations for ammonia or TKN at BHTP.
Due to a limited footprint, management made the decision of dedicating one aeration tank for sidestream treatment of incinerator scrubber water (SW) for biological oxidation of cyanide, an approach which has been used effectively in several plants around the US and HRSD (Daigger et al., 1998). However when this aeration tank, used as a mainstream biological cyanide treatment process (MBCNTP), was put into service for first time, nitrification was not achieved.
Three 22 L sequencing batch reactors (SBR’s) with different configurations were used to investigate the feasibility of sending SW to the head of the plant, dosing with potassium cyanide (KCN) to find the maximum cyanide concentration before inhibition of nitrifying bacteria, determining the dosage rate of ferrous sulfate to form soluble Fe-CN complexes and/or insoluble Fe-CN precipitates, and to investigate if it is feasible to use one aeration tank from the BNR process as a MBCNTP.
After approximately 8 months of operation using SBRs and after performing several jar tests, it was determined that cyanide in the SW was the primary inhibitor, additionally, concentrations above 0.08 mg/L at 20 °C and concentrations above 0.26 mg/L at 28 °C were observed to have a negative impact on nitrification, when operating at 15 days total SRT, 10 days aerobic SRT.
Chemical precipitation of cyanide using ferrous sulfate could be an alternative, however trying to maintain the ideal conditions can be expensive since enough ferrous sulfate must be added to maintain the right Fe-CN ratio and enough sodium hydroxide to increase the pH to optimal conditions.
Additionally, temperatures in the MBCNTP system should be maintained below or at 40 °C to successfully degrade cyanide. Nonetheless, this parameter could be difficult to control with the new MACT 129 regulation, which basically changed the way the incinerators are operated.
Salazar-Benites, Germano M., "Quantifying Cyanide Inhibition of Nitrification and Developing Cost-Effective Treatment Processes" (2017). Civil & Environmental Engineering Theses & Dissertations. 15.