Water chlorination is carried out by using both free and combined residuals. The latter involves chlorine application to produce chloramine with natural or added ammonia. Anhydrous ammonia is used if insufficient natural ammonia is present in the wastewater. Although the combined residual is less effective than free chlorine as a disinfectant, its most common application is as a post-treatment following free residual chlorination to provide initial disinfection. Free residual chlorination establishes a free residual through the destruction of naturally present ammonia. High dosages of chlorine applied during treatment may result in residuals that are esthetically objectionable or undesirable for industrial water uses. Dechlorination is sometimes performed to reduce the chlorine residual by adding a reducing agent (called a dechlor). Sulfur dioxide is often used as the dechlor in municipal plants. Aeration by submerged or spray aerators also diminishes the residual chlorine concentration.
The chlorine used for disinfection is available in three forms: liquified compressed gas, calcium hypochlorite or sodium hypochlorite, and chlorine bleach solutions. Liquid chlorine is shipped in pressurized steel cylinders with sizes typically 100 and 500 lb; one-ton containers are used in large installations. There are two types of chlorine dispensing systems: direct feed and solution feed. The first involves metering dry chlorine gas and conducting it under pressure to the water. Solution-feed systems meter chlorine gas under vacuum and dissolve it in a small amount of water, forming a concentrated solution which is then applied to the water being treated. At 20° C, 1 volume of water dissolves 2.3 volumes of chlorine gas (about 7,000 mg/1). At concentrations of total chlorine below 1,000 mg/1, none of the gas exists in solutions as Cl2; all of it is present as HOC1 or dissociated ions. Calcium hypochlorite is a dry bleach which is available in granular and tablet forms. Calcium hypochlorite is relatively stable under normal conditions; however, it can undergo reactions with organic materials. It should be stored in an isolated area. Sodium hypochlorite is available in liquid form. It is marketed in carboys and rubber-lined drums for small quantities. Sodium hypochlorite solutions are highly corrosive, unstable, and require storage at temperatures below 85° F. Sodium hypochlorite can either be delivered to the site in liquid form in 500 - 5,000-gallon tank cars or trucks, or manufactured on site. It is normally sold at a concentration of 12 percent to 15 percent by weight of available chlorine. It can be manufactured
The main component in a chlorine gas feed is the variable orifice inserted in the feed line to control the rate of flow out of the cylinder. The orifice basically consists of a grooved plug sliding in a fitted ring. Feed rate is adjusted by varying the V-shaped opening. Since a chlorine cylinder pressure varies with temperature, the discharge through such a throttling valve does not remain constant without frequent adjustments of the valve setting. Also, conditions on the outlet side vary with pressure changes at the point of application. Therefore, a pressure-regulating valve is used between the cylinder and the orifice, with a vacuum-compensating valve on the discharge side. A safety pressure-relief valve is held closed by
Chlorine feeders can be controlled either manually or automatically based on flow or chlorine residual, or both. In manual mode, a continuous feed rate is established. This is satisfactory when chlorine demand and flow are relatively constant and where operators are available to make adjustments. Automatic proportional control equipment is used to adjust the feed rate to provide a constant preestablished dosage for all rates of flow. This is accomplished by metering the main flow and using a transmitter to signal a chlorine feeder. An analyzer located downstream from the point of application is used to monitor the chlorinator. Combined automatic flow and residual control maintain a present chlorine residual in the water that is independent of the demand and flow variations. The feeder is designed to respond to signals from both the flow meter transmitter and the chlorine residual analyzer. For hypochlorite solutions, positive-displacement diaphragm pumps (either mechanically or hydraulically actuated) are used. The hypochlorinator consists of a water-powered pump paced by a positive-displacement water meter. The meter register shaft rotates proportionately to the main line flow and controls a cam-operated pilot valve. This in turn regulates water now discharged of hypochlorite that is proportional to the main flow. Admitting main pressure behind the pumping diaphragm balances the water pressure in the pumping head. The advantage of this system is that the pump does not need electrical power. The hypochlorite dosage can be manually adjusted by changing the stroke length setting
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