Disinfection control strategies

At a typical water treatment plant, the dominant chloramine species will be monochloramines. Chloramine generating reactions are 99% complete within a few minutes. Chloramines are a weak disinfectant that are less effective against virases or protozoa than free chlorine but produce fewer disinfection by-products. The use of chloramines as a DBF control strategy is well estabhshed in the United States. Chloramines ate generated onsite at the treatment plant. Anhydrous ammonia and ammonia sulfate ate examples of armnonia containing chemicals used by water systems to form chloramines. In most situations in the Urrited States, chloramines are used as a secondary disinfectant to maintain a residual in the distribution system. 

In 1978, analyses of THMs showed peak concentrations above 300 pg/L with an annual average of 140 pg/L. In light of these results, the Philadelphia Water Department began to reevaluate its disinfection strategies. Factors that affected the selection of an alternative included THM, bacterial, taste, odor, algae, and corrosion control residual duration and economics. 

P. S. Singer, THM control using alternate oxidant and disinfectant strategies. Proceedings 1986 AWWA Conference, American Water Works Association, Denver, CO, pp. 999-1017. 

Smface water source (Ottawa River) with low alkalinity (25 mg/L) and total hardness (35 mg/L) but high organic content (DOC = 6.5 mg/L) and colour (30-35 TCU). Secondary disinfection is achieved with the use of monochloramine. Historically, the City of Ottawa has used CaO quicklime to raise the pH of treated water to 8.5 pH units but this practice resulted in increased turbidity and poor pH control. A new strategy was selected whereby sodium hydroxide and carbon dioxide to achieve a treated water pH of 9.2 and alkalinity >35 mg/L (as CaCOa) were to be phased in from 2002-2003. The initial phase of changing to sodium hydroxide coincided with an increase in lead levels. 

The primary use of anhydrous ammonia (ammonia gas) in water treatment is to combine with chlorine to form chloramines. Chloramines are used both as primary and secondary disinfectants. Use as a secondary disinfectant (residual in the distribution system) is more common. A typical treatment strategy is to use free chlorine to satisfy the USE PA regulatory CT requirements as a primary disinfectant. Ammonia is then added to combine with the free chlorine residual to form chloramines for use as the secondary distribution system disinfectant. The ammonia added is carefully controlled to ensure that all the free chlorine is combined and little free ammonia remains. This control is necessary because the presence of free chlorine can form regulated by-products. Free ammonia can increase the growth of nitrifying bacteria, thus causing residual demand that could lead to conditions that could violate the Total Coliform Rule. 

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