Assessment Of The Hydrodynamics Process Of Anaerobic Ponds Of Dandora Wastewater Treatment Plant: Radiotracer Application

Abstract

Wastewater is a serious environmental concern when released to the environment untreated. Dandora Waste Water Treatment Plant treats about 80,000 m3(80%) of wastewater generated within Nairobi and its environs per day before disposal into Nairobi river. Therefore, this study has assessed the operations of Dandora Waste Water Treatment Pond for effectiveness, specifically, the anaerobic ponds by using radiotracer technology. Radiotracers are widely used in industries to optimize processes by investigating reactor mixing processes and troubleshooting. In waste water treatment plants processes, radiotracers are very sensitive and mostly applied in online diagnosis of various operations. Approximately 5000 MBq of Iodine- 131 radiotracer was injected 40 m away from the inlet of anaerobic pond and its passage through the channel and the entire pond was monitored by seven (7) NaI detectors to determine the Residence Time Distribution (RTD). Volumetric flow rate of the inlet channel at the time of injection was estimated to be 0.059 m3s-1 using radiotracer methods which provided the theoretical mean residence time (MRT) to be 3.5 days. The measured MRT was then compared with the theoretical MRT and dead volume was estimated, from which it was found that about 93% of the geometric volume within the pond was dead. The measured RTD was modeled using various RTD models and eventually a suitable model was identified to describe and visualize the flow pattern of wastewater within the pond. RTD obtained using the online detection system shows a typical bypass of 3.0 - 3.8% followed by mass flow homogenization of 96.2 - 97 %. It was evident that the retention time of anaerobic pond of interest was less than the theoretical retention time thus indicating a reduction in active volume of the pond due to sludge build-up and stagnant volumes. Based on the measured MRT, percentage of dead volume and identified flow patterns, it was concluded that the hydraulic performance of the pond was catastrophically poor and no longer suited for the purpose it was designed for. It is recommended that the pond be de-sludged and follow-up experiment conducted. The results of the wastewater effluents for weekly averages for COD, BOD, TSS and TDS exceed the design and Kenya standards limits.