1 SENSITIVITY ANALYSIS OF WASTEWATER FLOW QUALITY IN A CHANNEL UNDER THE INFLUENCE OF HYDRAULIC RETENTION TIME VARIATIONS Amir Gholipour 1* , Abolghasem Akbari 2 1 Khavaran Institute of Higher Education, Mashhad, Iran 2 Faculty of Civil Engineering and Earth Resources, Universiti Malaysia Pahang, 26300 Gambang, Kuantan, Malaysia __________________________________________________________________________________ ____________________________ ______________________________________________________________________________ ______________________ ______________________________________________________________________ 1.0 Introduction Treatment of sewage is very important in that it addresses both the issues of reusability of the effluent as well as the protection of the environment. To this end, numerous wastewater treatment plants (WWTPs) were constructed all over the world. However, untreated sewage may be released directly into natural water bodies due to a variety of reasons, among them electricity cuts to the WWTPs (Goel, 2006). In such cases, the self-purification capacity of the stream or river is depended on to assimilate and render the contaminants in the wastewater effluent harmless. One of the most important factor in refining and purifying contaminations in natural waters is the hydraulic retention time (HRT). Together with adequate nutrients and other determinants, sufficient HRT will enable the microorganisms in the water to reduce and eventually eliminate the contamination through natural biological processes. One of the main components of dairy wastewaters is cheese whey (Sultana et al., 2015). Although different technologies have been used extensively in the past for cheese whey treatment, constructed wetlands (CWs) applications are limited. Furthermore, the effect of crucial operational parameters (e.g. temperature, pollutant loading rate) have not been thoroughly studied. Having this in mind, two horizontal subsurface flow pilot-scale CW units (one planted and one unplanted) were used to treat secondary cheese whey, in order to examine the effect of different chemical oxygen demand (COD) influent concentrations (1200 to 7200 mg /L), hydraulic Keywords: Hydraulic retention time Flow quality QUAL2K BOD NO 3 International Journal of Civil Engineering and Geo-Environment Journal homepage:http://ijceg.ump.edu.my ISSN:21802742 A B S T R A C T ARTICLEINFO Urgent water supply issues coupled with environmental factors have resulted in a greater focus on treating waste water effluent. As technological treatment is costly, the use of naturally-purifying systems is significant. In this study, the sensitive analysis of flow quality under the influence of Hydraulic Retention Time (HRT) variations has been carried out using the numerical model of Q2K. The study is conducted in the proposed 10 km concrete open channel for transmission of sewage from Mashhad city to Parkand-Abad wastewater treatment plant (WWTP), Iran. Results show that after 12 hr from entering flow to the channel, Dissolved Oxygen (DO) concentrations increase with rising HRT along the transmit channel. However, BOD, Organic Nitrogen (NO), Total Coliform (Tcoli), Organic Phosphorous (PO), Particle Organic Matter (POM), Total Suspended Solids (TSS) and Total Organic Carbon (TOC) was reduced by 70%, 35%, 35%, 35%, 80%, 85%, 15%, 10% and 80% respectively. Likewise, when HRT becomes greater; NO 3 , PH and NH 4 increased which means greater HRTs in aerobic processes have undesirable influence on NO 3 and NH 4 indexes concentration. As well, the numerical outcomes are in conformity to experimental data.