Secondary disinfection

Chlorine. It is a toxic, yellow-green gas under normal pressures but becomes a liquid under high pressures. It is very effective as both a primary and secondary disinfectant. It is lethal, however, at concentrations as low as 0.1% by volume. 

The water is treated with UV radiation for primary disinfection, then chlorinated for secondary disinfection. An applied chlorine dosage of only about 1 mg/L is necessary. The entire water treatment system is housed in a 2.97 m (32 fU) building. The UV disinfection system consists of six irradiation chambers, two control cabinets with alarms, chart recorders, relays, hour-run meters, lamp and power on-lights, six thermostats, electrical door interlocks, mimic diagrams, and six UV intensity monitors measuring the total UV output. Each irradiation chamber contains one 2.5-kW mercury vapor, medium-pressure arc tube, generating UV radiation at 253.7 nm. 

Monochloramine Established All Secondary disinfectant only some by-product concerns... 

Ozone Established All Very effective and requires a secondary disinfectant... 

Ultraviolet radiation Established All Simple, no established harmful by-products and requires secondary disinfectant... 

Chlorine Effective. Widely used. Variety of possible application points. Inexpensive. Appropriate as both primary and secondary disinfectant. Harmful halogenated byproducts. Potential conflict with corrosion control pH levels, when used as a secondary disinfectant. 

Ozone Very effective. Minimal harmful by-products identified to date. Enhances slow sand and GAC filters. Provides oxidation and disinfection in the same step. Requires secondary disinfectant. Relatively high cost. More complex operations because it must be generated on-site. 

Ultraviolet radiation Very effective for viruses and bacteria. Readily available. No harmful residuals. Simple operation and maintenance. Inappropriate for water with Giardia cysts, high suspended solids, high color, high turbidity, or soluble organics. Requires a secondary disinfectant. 

Chloramines Mildly effective for bacteria. Long-lasting residual. Generally does not produce THMs. Some harmful by-products. Toxic effects for kidney dialysis patients. Only recommended as a secondary disinfectant. Ineffective against virases and cysts. 

Toward the end of the water treatement process to minimize THM formation and provide secondary disinfection Prior to the rapid mixing step in all treatment processes, except GAC and conventional treatment processes prior to fdtration for GAC post-sedimentation for conventional treatment. In addition, sufficient time for biodegradation of the oxidation products of the ozonation of organic compounds is recommended prior to secondary disinfection. Toward the end of the water treatment process to minimize presence of other contaminants that interfere with this disinfectant. 

Best applied toward the end of the process as a secondary disinfectant. 

The chlorination process applied to the water treatment system before the filtration process is prechlorination, predisinfection, or primary disinfection. When chlorination is applied to water treatment after filtration, it is termed postchlorination, postdisinfection, or secondary disinfection. The disinfectants used in primary disinfection and secondary disinfection are termed primary disinfectant and secondary disinfectant, respectively. 

Chlorine is the most common primary and secondary disinfectants used in the United States. It is available as a gas, solid, or aqueous solution. Chlorine gas is used most frequently, especially hy large utilities, because of its lower cost. Chlorine in its solid form is calcium hypochlorite, Ca(OCl)2 the liquid form is available as sodium hypochlorite solution, NaOCl. 

Free available chlorine concentration. The concentration must be high enough to always be detectable at the farthest points in the distribution system to effect both primary and secondary disinfection. 

Water treatment plants employ both primary and secondary disinfection (a) Primary disinfection achieves the desired level of microorganism kill or inactivation and (b) secondary disinfection ensures a stable residual concentration of disinfectant in the finished water to prevent microbial growth in the distribution system (4,7,14,17,18). 

Major primary disinfectants are chlorine, ozone, chlorine dioxide, and ultraviolet (UV) radiation. Major secondary disinfectants are chlorine and monochloramine. Some disinfectants can be used for both processes. 

Groundwater systems that apply disinfection to comply with regulations may have to add filtration if they contain iron and manganese. Insoluble oxides form when chlorine, chlorine dioxide, or ozone are added to these systems thus, filters would be needed for their removal. In addition, both ozonation and chlorination may cause flocculation of dissolved organics, thus increasing turbidity and necessitating filtration. The presence of such insoluble substances will require the use of secondary disinfection after filtration as well (4,14). 

Secondary disinfectants provide an essential residual that prevents regrowth in the distribution system. Although chlorine is the most widely used secondary disinfectant, chlorine dioxide and monochloramine are appropriate as well. As secondary disinfectants, chlorine and chlorine dioxide are handled in the same manner as for primary disinfectants. The use of monochloramine as a secondary disinfectant is discussed in detail in this section. 

Chloramination process can be applied to both water treatment and wastewater treatment (1,29). In the field of potable water treatment, chloramine is recommended as a secondary disinfectant because it is ineffective as a virucide, and is only marginally effective against Giardia cysts. It is formed from the combination of ammonia and chlorine (hypochlorite or hypochlorous acid). The chemical is generated on site, usually by injecting ammonia gas or adding an ammonium sulfate solution to chlorinated water. 

Because ozone is employed only for primary disinfection, a chlorinated compound (chlorine or chloramine) must be added for secondary disinfection following ozonation, i.e., to provide a residual for the distribution system. Consequently, the secondary byproducts, those formed by the reaction of chlorine or chloramine with the primary byproducts of ozonation, become a concern to water treatment specialists. Although some studies have examined by-products produced by two-step oxidation sequences of this type, no compounds have yet been reported that are not produced by one of the two oxidation processes acting alone. 

Chlorine dioxide is effective as a primary and secondary disinfectant, hut some chlorite ion is produced. The use of chlorine dioxide has been associated with hematological effects in laboratory animals, which may result from the production of chlorite and chlorate ions. Neurological effects have also been identified. 

The remaining alternative, monochloramine, is a weak disinfectant. The very high CT values required to inactivate 99.9% Giardia and 99.99% enteric viruses make monochloramine impractical for use as a primary disinfectant. Therefore, monochloramine should only be considered as a secondary disinfectant. 

Because UV radiation disinfection provides no disinfecting residual, a secondary disinfectant is needed. Very little oxidation of organic materials occurs with typical UV radiation systems used for drinking water disinfection consequently, if oxidation is required (for iron, manganese, sulfide, nitrate, etc.), a strong oxidizing agent may be... 

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. 

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. 

Chlorine is the most common disinfectant used in water treatment. This versatile chemical is not only used as a primary and secondary disinfectant but also as an oxidant of organic and inorganic substances. In addition, chlorine used before filtration can significantly improve particulate removal. 

Chlorine, when combined with ammonia, forms chloramines. This form of chlorine is a common secondary disinfectant used in... 

Ops Tip Primary disinfection is used to satisfy requirements for inactivation of bacteria and viruses. Secondary disinfection is aiso known as residual disinfection. A residual is maintained within the distribution system to ensure the safety of water delivered to customers. 

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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