Radiotracer Investigation Of An Aerobic Tank Of A Wastewater Treatment Plant

ABSTRACT Water is used in almost all human activities and large amounts of wastewater are produced every time. Wastewater need to be treated before it is discharged into the environment because it contains possible harmful substances which can pollute the environment and threaten life. Waste water treatment methods vary from industry to industry according to the process utilized. In most cases, the contaminants are not fully removed from waste water before discharge, due to certain malfunctions such as stagnant regions and short circuiting (bypassing or channeling) in the various processing tanks or units of treatment plants. Radiotracer residence time distribution methodology can be effectively used to pinpoint malfunctions or anomalies in process vessels and to consequently optimize performance. In this study, radiotracer residence time distribution methodology was used to determine the hydrodynamic parameters of the aerobic digester of an effluent treatment plant. 700 mCi of Technicium-99 m was introduced as a Dirac signal into the inlet of the digester of dimensions 11.50 m long, 2 m wide and 1.65 m deep. The volumetric flow rate into the digester was 1 m3 /h. The concentration of radiotracer in the exit stream of the digester was determined using on-line tracer monitoring method. Sodium iodide NaI(TI) scintillation detectors were installed at the inlet and exit streams of the digester to monitor tracer concentration directly. The experimental residence time distribution (RTD) data measured at the outlet of the digester was corrected for background, decay and normalized to obtain the required RTD function (E(t) curve). The method of moments was then used to determine the MRT and evaluate the RTD parameters. The experimental mean residence time (MRT) was far less than the theoretical MRT calculated with respect to the volume and flow rate. In this case, the percentage effective volume was estimated to 11.4 % translating into approximately 88.6 % dead volume in the digester. Using an RTD Software to model the flow structure in the digester, it was observed that the perfect mixers in series with exchange model best described the flow structure.