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99%

Chemical coagulation, with adequate concentrations of aluminum sulfate or ferric chloride, of surface water used as a source of drinking water or of waste-water that has received biological treatment can remove 99 percent of the viruses. Hepatitis A virus, rotavirus, and poliovirus removal of 98.4 to 99.7 percent was also achieved in a pilot plant by softening during Ca2+ and Mg2+ hardness reduction.23 A high pH of 10.8 to 11.5, such as softening with excess lime, can achieve better than 99 percent virus removal, but pH adjustment is then necessary.

Filtration using sand and/or anthracite following coagulation, flocculation, and settling can remove 99 percent or more of the viruses, but some viruses penetrate the media with floc breakthrough and turbidity at low alum feed.24 Diatomaceous earth filtration can remove better than 98 percent of the viruses, particularly if the water is pretreated. Activated-carbon adsorption is not suitable for virus removal. The infectivity of hepatitis A virus is destroyed by 2.0 to 2.5 mg/l free residual chlorine. Reverse osmosis and ultra filtration, when followed by disinfection, can produce a virus-free water. However, it has been found that both enteroviruses and rotaviruses could be isolated from water that received complete treatment containing more than 0.2 mg/l free chlorine, less than one coliform bacteria per 100 ml, and turbidity of less than 1 NTU.25 The WHO states that a contaminated source water may be considered adequately treated for viruses infectious to humans if it has a turbidity of 1 NTU or less and is disinfected to provide a free residual chlorine of at least 0.5 mg/l after a contact period of at least 30 minutes at a pH below 8.0.26

A conventional municipal biological wastewater treatment plant can produce an effluent with less than 10 PFU/l. When followed by conventional water treatment incorporating filtration and chlorination, a virus-free water can be obtained.27 The product of the contact time (t) in minutes and free residual chlorine, or other approved disinfectant, in milligrams per liter (C) produces a value that is a measure of the adequacy of disinfection. The Ct value for a particular organism will vary with the water pH, temperature, degree of mixing, turbidity, and presence of interfering substances, in addition to disinfectant concentration and contact time. For example, a turbidity less than 1 to 5 NTUs and a free chlorine as HOCl (that penetrates the cell wall of microorganisms and destroys their nucleic acid) are necessary. A smaller Ct value is effective at lower pH and at higher temperature.28 The Ct value effective to inactivate 99.9 percent of the Giardia cyst will also inactivate 99.99 percent or greater of the bacteria and viruses at a given pH and temperature.

For protozoa (Giardia lamblia) a Ct value of 150 to 200 at pH 8.0 or less is required with water at 50°F (10°C). Experimental results based on animal infectivity data show that a 99.99 percent cyst inactivation can be obtained at Ct values of 113 to 263, at pH 6, temperature 33°F (0.5°C), and chlorine concentration of 0.56 to 3.93 mg/l for 39 to 300 minutes. At pH 8, temperature 33°F (0.5°C), chlorine concentration of 0.49 to 3.25 mg/l, and contact time of 132 to 593 minutes, the Ct values varied from 159 to 526. If a large enough Ct value can be maintained to ensure adequate Giardia cyst disinfection to EPA satisfaction, then filtration may not be required.29

Chlorine dioxide can achieve 99.9 percent Giardia lamblia inactivation at Ct values of 63, in water at 34°F (1°C) or less, to 11 at 77°F (25°C) or greater. Inactivation using ozone is achieved at Ct values of 2.9, in water at 34°F (1°C) or less, to 0.48 at 77°F (25°C) or greater. These Ct values also achieve greater than 99.99 percent inactivation of enteric viruses. See state regulatory agency for required Ct values to inactivate Giardia lamblia and enteric viruses using chlorine, chloramine, chlorine dioxide, and ozone.

Naegleria fowleri cyst is a pathogenic flagellated protozoan. It causes primary amebic meningoencephalitis, a rare disease generally fatal to humans. The organism is free living, nonparasitic, found in soil and water. Naegleria gruberi, a nonpathogenic strain, was used in experimental inactivation studies. At pH 5.0 the N. gruberi cyst was inactivated in 15.8 to 2.78 minutes by 0.45 to 2.64 mg/l free chlorine residual at 25°C (77°F). At pH 7.0 it was inactivated in 21.5 to 2.9 minutes by 0.64 to 3.42 mg/l; and at pH 9.0 in 11.5 to 2.36 minutes by 15.4 to 87.9 mg/l residual chlorine. Also, it was reported that Acanthamoeba sp. 4A cysts (pathogenic) were inactivated after 24 hours by an initial chlorine dose of 8.0 mg/l but ending with a chlorine residual of 6.0 mg/l. Naegleria fowleri was reported to be inactivated by 4 mg/l chlorine residual in 10 minutes at a temperature of 77°F (25°C) and a pH of 7.2 to 7.3.30 Giardia lamblia cysts are inactivated in 60 minutes by 2.0 mg/l free chlorine residual at pH 6.0 and 41°F (5°C); in 60 minutes by 2.5 mg/l free chlorine at pH 6.0, 7.0, and 8.0 and 60°F (15°C); in 10 minutes by 1.5 mg/l free chlorine at pH 6.0, 7.0, and 8.0 and 77°F (25°C); and in 30 minutes by 6.2mg/l total chlorine at pH 7.9 and 37°F (3°C).31

Entamoeba histolytica cysts are inactivated by 2 mg/l free chlorine in 15 minutes at a temperature of 68°F (20°C) and pH 7.0,32 by 2.5 mg/l free chlorine in 10 minutes at a temperature of 86°F (30°C) and pH 7.0, by 5.0mg/l free chlorine in 15 minutes at a temperature of 50°F (10°C) and pH 7.0, and by 7.0 mg/l free chlorine in 10 to 15 minutes at a temperature of 86°F (30°C) and pH 9.0.33

The removal of nematodes requires prechlorination to produce 0.4 to 0.5 mg/l residual after a 6-hour retention period followed by settling. The pathogenic fungus Histoplasma capsulatum can be expected in surface-water supplies, treated water stored in open reservoirs, and improperly protected well-water supplies. Fungicidal action is obtained at a pH of 7.4 and at a water temperature of 78.8°F (26°C) with 0.35 mg/l free chlorine after 4 hour contact and with 1.8 mg/l free chlorine after 35 minutes contact. Complete rapid sand filter treatment completely removed all viable spores even before chlorination.34

Cysts of E. histolytica and Giardia lamblia (also worms and their eggs) are removed by conventional water treatment, including coagulation, floccula-tion, sedimentation, and filtration (2-6 gpm/ft2). Direct and high-rate filtration, diatomaceous earth filtration with good precoat (1 kg/m2), and special cartridge filters (<7 x 8-^m pore size) can also be effective. The slow sand filter is also considered effective. Pressure sand filtration is not reliable. The inactivation of Giardia lamblia by free chlorine is similar to that for E. histolytica .35

Coliform bacteria can be continually found in a chlorinated surface-water supply (turbidity 3.8 to 84 units, iron particles, and microscopic counts up to 2,000 units) containing between 0.1 and 0.5 mg/l of free residual chlorine and between 0.7 and 1.0 mg/l total residual chlorine after more than 30 minutes contact time.36

It is evident from available information that the coliform index may give a false sense of security when applied to waters subject to intermittent doses of pollution. The effectiveness of proper disinfection, including inactivation of viruses, other conditions being the same, is largely dependent on the freedom from suspended material and organic matter in the water being treated. Treated water having a turbidity of less than 5 NTU (ideally less than 0.1), a pH less than 8, and an HOCl residual of 1mg/l after 30 minutes contact provides an acceptable level of protection.37

Free residual chlorination is the addition of sufficient chlorine to yield a free chlorine residual in the water supply in an amount equal to more than 85 percent of the total chlorine present. When the ratio of chlorine to ammonia is 5:1 (by weight), the chlorine residual is all monochloramine; when the ratio reaches 10:1, dichloramine is also formed; when the ratio reaches 15:1 or 20:1, nitrogen trichloride is formed, reaching a maximum at pH less than 4.5 and at a higher pH in polluted waters. Nitrogen trichloride as low as 0.05 mg/l causes an offensive and acrid odor that can be removed by carbon, aeration (natural or forced draft), exposure to sunlight, or forced ventilation indoors.38 It can titrate partly as free chlorine and is also highly explosive. The reaction of chlorine in water is shown in Figure 2.1.

The minimum free chlorine residual at distant points in the distribution system should be 0.2 to 0.5 mg/l. Combined chlorine residual, if use is approved, should be 1.0 to 2.0 mg/l at distant points in the distribution system.39

In the presence of ammonia, organic matter, and other chlorine-consuming materials, the required chlorine dosage to produce a free residual will be high. The water is then said to have a high chlorine demand. With free residual chlorination, water is bleached, and iron, manganese, and organic matter are oxidized by chlorine and precipitated, particularly when the water is stored in a reservoir or basin for at least 2 hours. Most taste- and odor-producing compounds are destroyed; the reduction of sulfates to taste- and odor-producing sulfides is prevented; and objectionable growths and organisms in the mains are controlled or eliminated, provided a free chlorine residual is maintained in the water. An indication of accidental pollution of water in the mains is also obtained if the free chlorine residual is lost, provided chlorination is not interrupted.

1. Destruction of chlorine by reducing compounds, no disinfection.

2. Chloro-organic compounds formed, little disinfection.

3. Ammonia plus chlorine-producing chloramines, mostly monochloramine. i. Chloramines and chloro-organic compounds destroyed.

1. Destruction of chlorine by reducing compounds, no disinfection.

2. Chloro-organic compounds formed, little disinfection.

3. Ammonia plus chlorine-producing chloramines, mostly monochloramine. i. Chloramines and chloro-organic compounds destroyed.

Chlorine dosage, mg/l

FIGURE 2.1 Reaction of chlorine in water. (Adapted from Manual of Instruction for Water Plant Chlorinator Operators, New York State Department of Health, Albany.)

Chlorine dosage, mg/l

FIGURE 2.1 Reaction of chlorine in water. (Adapted from Manual of Instruction for Water Plant Chlorinator Operators, New York State Department of Health, Albany.)

The formation of trihalomethanes and other chloro-organics, their prevention, control, and removal, and the use of other disinfectants are discussed later in this chapter.

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