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Disinfection is an integral part of water treatment because it inactivates pathogens that are not physically removed by filtration. The degree of inactivation required is dependent on CT. CT refers to the product of the

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residual disinfectant concentration in mg/L, “C,” and the disinfectant contact time in minutes, “T.” “T” is the effective disinfection contact time, which is actually “T10” – time for 10% of the water to pass through the point where “C” is measured. There is a relationship between CT values and inactivation rates (or log inactivation) for a given disinfectant. Since the determination of log inactivation of a microbiological contaminant is more technically demanding than the calculation of CT, CT is used as a surrogate for log inactivation for a given disinfectant under specific water quality conditions (e.g., temperature, pH).

D

T

Chlorine

Chlorine has several forms and is the most widely used disinfectant in public water supplies. Hypochlorites are available in solid (tablet or granule), liquid (solution pump-fed) or gaseous forms. The use of gaseous chlorination at small water supplies may not be among the best disinfection options due to the hazardous nature of the material. Use of gaseous chlorine places greater demand on the need for isolated plant space, trained and attentive operating staff and protection from any hazards. Use of hypochlorite solutions also warrants some precautions. With time, the disinfectant strength of the solution decreases and toxic chlorate levels in solution can increase. Awareness regarding the potential for producing elevated levels of halogenated disinfection by-products (e.g., trihalomethanes, inorganic byproducts, and others) is also essential.

Chloramines

Chloramines, while possessing certain advantages over other disinfectants (e.g., long residual

effects and low production of disinfection by-products), are not widely used due to high costs and the complexity in operation. Compared to free chlorine and ozone, chloramines possess less potency as a germicidal agent, and would therefore require longer CTs. Chloramine disinfection requires careful monitoring of the ratio of added chlorine to ammonia. Failure to do so can result in odor and taste problems or biological instability of water in the distribution system. Excess ammonia (i.e., low chlorine:ammonia) can promote growth of nitrifying bacteria, which convert ammonia to nitrites and nitrates. The dose of ammonia should be tempered by any natural ammonia occurring in the source water.

Chlorine Dioxide

Chlorine dioxide, although a powerful oxidant, may be more difficult to handle than other forms of chlorine. Chlorine dioxide requires trained staff to manage its use and is so reactive that it may not provide a residual disinfectant in the distribution system.

Chlorine dioxide may be used for either taste and odour control or as a pre-disinfectant. Total residual oxidants (including chlorine dioxide and chlorite, but excluding chlorate) may not exceed 0.30 mg/L during normal operation or 0.50 mg/L (including chlorine dioxide, chlorite and chlorate) during periods of extreme variations in the source water supply.

Chlorine dioxide provides good Giardia and virus protection but its use is limited by the restriction on the maximum residual of 0.5 mg/L ClO2/chlorite/chlorate allowed in finished water.

Where chlorine dioxide is approved for use as an oxidant, the preferred method of generation is to entrain chlorine gas into a packed reaction chamber with a 25% aqueous solution of sodium chlorite (NaClO2).

Dry sodium chlorite is explosive and can cause fires in feed equipment if leaking solutions or spills are allowed to dry out.

Ozone

Ozone is a powerful oxidant with a high disinfectant capacity. Ozone is very effective in inactivating cysts, bacteria and viruses. Inactivation of 4-log to 6-log reduction can be achieved within very short contact periods. Design of ozone as a primary treatment should be based on simple criteria including ozone contact concentrations, competing ozone demands, and a minimum contact time to meet the required cyst and viral inactivation

requirements.

Ozonation technology requires careful monitoring for ozone leaks which pose a hazard. Use of

ozonation may also increase biodegradable organics in water which may affect distributed water quality. Additional treatment, such as granulated activated carbon

filtration, may be used as necessary to mitigate the problem. Also, where bromides are present in source water there is an increased potential for the formation of disinfection by-products (i.e., brominated organics and bromate) which should be minimized.

Secondary disinfection with chlorine or chloramines may help in this regard by balancing treatment needs with the need for also protecting distributed water quality.

UV Radiation

Ultraviolet (UV) radiation is an effective disinfectant in treating relatively clean source waters. UV is a useful disinfection technology option given its simplicity of installation, ease of operation and maintenance, and low costs relative to chemical disinfection.

UV radiation as a germicidal agent is effectively applied at a wavelength of 253.7 nanometers through the application of low or medium pressure mercury lamps. UV dose is expressed in units of millijoule per square centimeter (mJ/cm2), the product of the intensity (I) of the UV lamp (mW/cm2) and time (T) of exposure (sec). UV treatment of water is therefore comparable to the CT as described above for chemical disinfection, since UV dose is expressed in terms of the IT values. At a germicidal fluence of 40 mJ/cm2, UV has been effective in inactivating Giardia to achieve 3-log reduction, Cryptosporidium and Bacillus subtilis at 4.5-log reduction, and MS2 coliphage at about 2-log reduction.

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hardness, suspended solids, and other factors can reduce UV

transmission and cause lamp fouling, thus decreasing the effectiveness of disinfection. In addition to pre- treatment and automatic cleaning systems to remove dissolved and/or suspended materials, which foul the lamps and impede UV performance, a secondary disinfectant is necessary to provide residual protection in distribution systems. UV intensity sensor readings, flow through the reactors, and temperature and lamp status should be monitored

continuously to determine the daily average and minimum UV dose per reactor. Remote alarms, automatic cleaning of UV components, and annual UV sensor maintenance are also important design components to prevent deposition or scaling and to minimize on-site operator attention.

A full-scale equipment validation is paramount. It is extremely

important for all UV units to have undergone manufacturer

performance testing to verify the unit’s capability with respect to inactivation of the target organisms. The validation protocol should be in accordance with one of:

• German Association on Gas and Water protocol

(DVGW Technical Standard W294, UV Systems for Disinfection in Drinking Water Supplies –

Requirements and Testing)

• Austrian protocol (ONORM M 5873-1);

• NWRI/AWWA document titled Ultraviolet

Disinfection: Guidelines for Drinking Water and Water Reuse

• Proposed USEPA UV Guidance Manual for Drinking Water.

The effectiveness of the various alternative disinfection technologies on various pathogens is shown in Table 7.4. The effectiveness may vary according to water temperature and the concentration of disinfectant.

Table 7.4 Effectiveness of Alternative Disinfectants on Different Pathogens

Micro organism E-coli and Inactivation Ability Disinfectant

E. Coli Giardia Cryptosporidium Viruses

Chlorine Very effective Effective Not effective Very effective

Ozone Very effective Very effective Very effective Very effective

Chloramines Effective Not effective Not effective Not effective

Chlorine dioxide Very effective Very effective Effective Very effective Ultraviolet radiation Very effective Very effective Very effective Effective

7.7

Water Distribution System