MICROBIAL DECAY FACTOR METHOD: A SIMPLE PRE-WARNING TOOL OF ONSET OF SEVERE NITRIFICATION IN CHLORAMINATED DISTRIBUTION SYSTEM Arumugam Sathasivan, Siew Teng Tan, Weixi Zhan, Afrah Al - Ithari Department of Civil Engineering and Construction Curtin University of Technology GPO Box U1987, Perth, WA 6845, Australia ABSTRACT Maintaining adequate disinfectant residual in systems with long retention times is a challenge due to microbial acceleration of chloramine decay. Various findings showed that nitrification, major cause of microbiological acceleration, can be controlled by controlling free ammonia and chloramine residual. Chloramine control to the required level will be difficult, if microorganisms in the water accelerate chloramine decay. In the past such acceleration is not quantified and hence is not utilised as a control parameter. Recently, a sensitive method - “microbial decay factor” (Fm) – was developed to measure this acceleration. It was shown that measurement of Fm can provide early warning as opposed to traditional indicators. This paper shows how Fm could be utilised as a pre-warning tool in a real distribution system using a hypothetical distribution system reservoir. INTRODUCTION Since the mid-1970s, the use of (mono)chloramine to disinfect drinking water has greatly increased. This maintains a longer- lasting disinfectant residual and reduces chlorinated disinfection by-product (CDBP) formation. When the chloramination process is well controlled, chloramine reacts with the dissolved organic matter remaining in drinking water after treatment, at a relatively slow stable rate. This, together with the auto- decomposition of chloramine, constitutes the basic “chemical decay rate”. It is generally slow enough to maintain an adequate residual for several hundreds hours. Another important factor adding to the instability of monochloramine in the distribution system is the existence of nitrifying bacteria – thus resulting in microbial decay. The main source of nutrients for these nitrifiers is free ammonia compounds released when combined forms of chloramine is decayed. Nitrite (NO2 - ) is produced by the oxidation of ammonia; then nitrate (NO3 - ) is produced by the oxidation of nitrite. This process is known as nitrification. Nitrification is very well known to cause accelerated chloramine decay in the distribution systems and severe losses of chloramine have been reported. Once severe nitrification is triggered it is usually hard to revert back to the original non-nitrifying situation without serious interference. Therefore it is very important to understand when possibly nitrification is induced. Traditional indicators such as nitrite, nitrate, ammonia, and drop in chloramine residuals are often used to assess the disinfection status of a chloraminated water supply. Traditional indicators have got three major drawbacks (Sathasivan et al., 2005): These do not measure how much micro-organisms or chemical reactions contribute to chloramine decay, thus it was difficult to know why chloramine is dropping when temperature increases in summer; Slow activity of nitrifiers in winter could be interpreted as not existing; and When traditional indicators reach levels of concern it is usually too late to implement any changes. Hence, the use of these indicators leads to reactive approach rather than preventive approach. NEW DEVELOPMENT IN NITRIFICATION MONITORING AND CONTROL Recently, a simple measurement method termed the microbial decay factor (Fm) was proposed to measure the microbiologically assisted chloramine decay, relative to chemical chloramine decay in a water sample (Sathasivan et al., 2005). In this method, the microbiological and chemical components of chloramine decay are determined by comparing the chloramine decay in an unprocessed sub-sample with the decay in a sub-sample with the microbiological contribution removed by inhibition or filtration. The authors showed that the method is capable