Abstract—A quadratic model (p＜0.0001) was developed by using a central composite design of 50 experimental runs (42 non-center + 8 center points) to assess the efficiency of back- ground chlorine residuals in combating accidental microbial episodes in a scaled-up distribution network (DN) (rig). A known amount of background chlorine residuals were maintained in the DN and a required number of bacteria, Escherichia coli K-12 strain, was introduced by an injection port in the pipe-loop system. Samples were taken at various time intervals at different pipe lengths. A spread-plate count was performed to count the bacterial number. Microbial concentration and time (p ＜0.0001), pipe length (p ＜0.022), background chlorine residuals (p ＜0.07) and time 2 (p ＜0.09) were observed as significant factors. The model that was developed was significant. The ramp function of variables shows that, at the microbial count of 10^6, at 0.76 L/min, with a pipe length of 133 meters, a back ground residual chlorine of 0.16mg/L was enough for the complete inactivation of a microbial episode in approximately 18 minutes. Index Terms—Central composite design (CCD), distribution network, Escherichia coli, residual chlorine. I. INTRODUCTION The purpose of a water supply-distribution system is to deliver safe, potable water which is adequate in quantity and acceptable in terms of taste, odor and appearance [1]. Surface run-off, cross-connection and the leakage of sewage disposal systems as well as septic tanks‟ wrecked or leaking pipes, back siphonage from a plumbing fixture or cross-connection into a water supply line and intermittent water supply are some other reasons behind the bacterial nemesis of the drinking-water industry, especially in developing countries [2].These external contamination events can act as a source of inoculum, introducing nutrients and resulting in the decrease of residual disinfectant concentrations within the distribution system, causing degradation of water quality. A poorly maintained distribution system can act as a vehicle for Manuscript received March 15, 2014; revised June 10, 2014. This work was supported by International Research Support Initiative Programme of Higher Education Commission, Pakistan (IRSIP-HEC, Pak). S. Rasheed and I. Hashmi are with the Institute of Environmental Sciences and Engineering, School of Civil and Environmental Engineering, National University of Sciences and Technology, Islamabad, Pakistan (e-mail: sajidarasheed@iese.nust.edu.pk, imran.hashmi@iese.nust.edu.pk). Q. Zhou, G. K. Kim, and L. Campos are with University College London, London, UK (e-mail: qizhi.zhou@ucl.ac.uk, j.k.kim@ucl.ac.uk, l.campos@ucl.ac.uk). pathogens transmission and may even contribute significantly to gastrointestinal diseases in the community [3]. Nowadays, preserving the water quality throughout the water distribution system is therefore the most challenging technological issue. An important mitigation measure that can be employed to protect against intentional or accidental microbial intrusion/contamination is the maintenance of suitable chlorine residuals throughout the distribution network, often regarded as “residual maintenance strategy” [4]. Chlorine provides the residual barrier the distribution system worldwide. The recommended chlorine residual for water, that is centrally treated, at the point of delivery should fall within 0.2–0.5 mg/l [5]. This residual chlorine concentration reduces the risk of general contamination by the accidental entry of microorganisms due to either back siphonage or to a re-growth of the microorganism within the distribution network as breakage from the biofilm [6]. Thus, controlling this concentration in drinking water is a very important aspect, since the decrease in its level below that recommended may cause a secondary development of microorganisms [7]. Changes in chlorine residuals can be used as indicators of microbial contamination [8]. Experience has shown that the maintenance of the chlorine residual cannot be relied upon to totally prevent the occurrence of bacteria. As water flows from the treatment plant to the consumer‟s tap, the water quality deteriorates because of a decreasing residual chlorine concentration, especially for „long residence‟ times. It was observed that, as the chlorine residual decreased from 4.6 ppm at the plant to 0.2 ppm at the household, there was a statistically significant increase in total and thermo tolerant coliforms [4]. Although chlorine residuals greatly contribute to the inactivation and re-growth of indicator bacteria, i.e. faecal coliforms in the pipeline, the question awaiting an answer, is the level of inactivation at the recommended levels of chlorine residual by the World Health Organization (W.H.O) and the effect of environmental factors upon the efficiency of background chlorination and the required time to combat the microbial attack [9]. Furthermore, the ability of disinfectant residuals to inactivate microorganisms between the time that they enter the distribution system and the time they reach the consumer is still to be analyzed. So the present study was designed to quantitatively assess a distribution system‟s vulnerability against microbial intrusion and to evaluate the efficiency of background residual chlorine in combating any accidental microbial event from occurring in the system and the factors that contribute towards its failure in connection with the microbial episode. Combating Accidental Microbial Episodes by Back-Ground Chlorine Residuals in a Scaled-up Distribution Network (Rig) Using a Central Composite Design (CCD) S. Rasheed, I. Hashmi, Q. Zhou, J. K. Kim, and L. C. Campos 122 DOI: 10.7763/IJESD.2015.V6.573 International Journal of Environmental Science and Development, Vol. 6, No. 2, February 2015