Monitoring Monochloramine, Total Ammonia, and Free Ammonia in the Chlorination of Treated Wastewater

This paper describes the utility of continuous monitoring of Monochloramine, Total Ammonia, and Free Ammonia in the chlorination of municipal wastewater. The analyzer used for monitoring will be described followed by an outline of the chemistry involved in the analyzer. Next, typical performance of this analyzer on clean water samples will be covered as will response over the breakpoint curve using a flow system to feed various ratios of ammonia and hypochlorite. Various potential interferences will be discussed in comparison to other typical means of monitoring treated municipal wastewater. Finally, data from monitoring treated wastewater will be presented.The addition of chlorine to a wastewater stream will form a variety of chloramines. The predominant species is monochloramine. This is true at slightly basic pH when the ratio of chlorine to ammonia is at or below 5:1. As the pH drops or the ratio of chlorine increases, dichloramine and trichloramine can form. In addition, organic nitrogen compounds will also react with chlorine to form organic chloramines. Effective disinfection of wastewater requires the appropriate dosing of chlorine to form sufficient levels of monochloramine. The presence of organic chloramines needs to be considered since they react with chlorine to form stable compounds which are not effective disinfectants. Dichloramines and trichloramines also need to be considered since they produce odors and they are often a sign of the overfeed of chlorine. This overfeed adds cost in chemical consumption for both chlorination and dechlorination.Monitoring the chlorination of wastewater for disinfection typically involves maintaining an effective level of disinfectant via a process analyzer. Total Chlorine and ORP currently are two different technologies used to estimate the disinfectant levels. These methods will be discussed in comparison to monitoring Monochloramine, Total Ammonia, and Free Ammonia. The relative advantages and disadvantages of each will also be covered.Monitoring Monochloramine, Total Ammonia, and Free Ammonia is accomplished using a Hach APA6000TM process analyzer. The monochloramine value gives a specific indication of the level of disinfectant present over both Total Chlorine and ORP. In addition, the Total Ammonia gives the incoming ammonia levels to further aid in control. The analyzer covers a dynamic range from 0.10 to 10.00 mg/L as Cl2. Accuracy is 0.10 mg/L as Cl2 or 5% of point, whichever is greater. Precision is 0.05 mg/L as Cl2 or 3% of point, whichever is greater. The cycle time is approximately 5 minutes and the unit is capable of sample sequencing two different streams. The analyzer uses a patented technology, Carrierless Sequential Injection Analysis (CSIA) (patent # 5,849,592), for fluid handling. The performance of this analyzer on standards will be discussed. Accuracy and precision data will be presented as will interferences. Operation in the field will be reviewed.