The chemical forms of chlorine used for treatment of water supply systems are usually utilized for wastewater treatment as well, with perhaps some emphasis on hypochlorite. Excessive dosages of the chemical are to be avoided.
This section consequently duplicates to a degree the coverage of the subject in Section C-4.
For storage and shipment, chlorine gas is compressed to liquid and is placed in iron cylinders or tank cars. Since the chemical is used from such containers, it is commonly referred to as "liquid chlorine." The hypochlorites of calcium and sodium release hypochlorous acid in water and have disinfecting properties essentially the same as chlorine. Because these chemicals are not wholly chlorine they are usually rated on a basis of "available chlorine" content. It is a quantitative measure of the oxidizing power expressed in terms of elemental chlorine and determined by the ability to release iodine from acidified iodide. The calculated weight of elemental chlorine required to liberate the same amount of iodine is the "available chlorine" content.
Evaluations of percent available chlorine in NaOCl solutious are expressed in either one of two ways: As percent by volume, where the grams per liter of available chlorine divided by 10 yields the desired quantity. To find percent by weight, an additional step is required, dividing the percent by volume by the specific gravity of the material.
Manufacturers and major distributors of liquid chlorine and various forms of hypochlorite are given in Section C-4.
Liquid Chlorine Feed. Chlorine is frequently added in minute dosages at times of low flow, and the equipment for applying it must be comparable to laboratory apparatus in sensitivity. It may be used for delivering the gas directly to the sewage (dry feed) or to a relatively small stream of water which then is mixed with the sewage (solution feed). The latter is the more common, giving better diffusion and greater efficiency with substantial savings.
Chlorinators are made for manual control (maintaining a uniform rate of flow at any rate set until changed by hand); automatic (varying the rate of chlorine in proportion to that of the sewage being treated); program control (varying the dosage according to a predetermined cycle); semi-automatic (starting and stopping automatically with the sewage flow, but delivering, when at all, at the rate set manually); and residual feedback control (automatic proportional to flow and chlorine demand).
The feed equipment employed for liquid and solution feed of chlorine and accessories to potable water is the same as that used in wastewater treatment. Manufacturers are listed in Section C-4 of this manual.
Storage areas should be clean, cool, well-ventilated and protected from corrosive vapors or continual dampness. Cylinders and ton containers should preferably be stored indoors, in a fire-resistant building, away from heat sources, other compressed gases, and flammable substances. It is noted that treatment operations come under The Emergency Planning and Community Right-to-Know Act. This act set certain requirements regarding hazardous and toxic chemicals such as chlorine and so forth. More information on this is found in Section C-4 of this manual.
It is rather essential to have a monitor that detects the presence of chlorine in the atmosphere (see Section C-4 of this manual).
Chlorine Flow Recorders. Manufacturers of Chlorine flow recorders are discussed in Section C4 of this manual.
A continuous record of the rate of feed, made on a standard circular chart, may be obtained by modifying a standard platform scale on which the chlorine cylinder stands. Scale manufacturers, listed in Section C-4 of this manual, furnish calibrated devices for this purpose, and scales with more sophisticated readout equipment.
Chlorine Evaporators. When chlorine is converted from liquid to gaseous form, heat is absorbed equal to the latent heat of vaporization of the chlorine. This is a cooling effect and limits the sustained gas withdrawal rate from chlorine containers - from a ton container only 450 lb may be withdrawn as gas in 24 hours. Therefore, in order to reduce the number of containers in service, prudence suggests the use of evaporators wherever chlorine usage is over 2,000 lb per day.
Hypochlorination. For feeding hypochlorite and other chemical solutions, a diaphragm or metering pump is employed. However, for handling hypochlorite, a high degree of precision is desirable, Furthermore, the equipment must be highly corrosion resistant for the wastewater treatment plant environment.
On-site generation of hypochlorite solutions by electrolytic cells from brine solutions is becoming popular, especially at wastewater treatment plants. Manufacturers of hypochlorinators and factory-built on-site hypochlorite generators are listed in Section C-4.
Application of Ozone
Ozone is generated electrically from an oxygen-containing gas through an electric discharge. Ozone reverts to oxygen at room temperature within an hour or so. For this reason, ozone storage and transfer is impracticable. Instead ozone is generated on-site. The usual feed gas is dried air or oxygen. Oxygen-enriched air is used at times. Liquid oxygen is supplied by Air Products and Chemicals, Inc., and others. Gas phase oxygen may also be utilized, but is less convenient to store.
Ozone generators are available from manufacturers listed in Section C-4. Because ozone decomposes so readily the only residual left after ozonation is oxygen. From this it is apparent that ozonation yields an effluent that is compatible with the aquatic environment of receiving waters.
In the U.S., the principal applications of ozonation are in locations where precisely this result is desired. The alternative is chlorination, dechlorination, and aeration. Ozonation is claimed by some to be cost-effective for this purpose, from small to medium size wastewater facilities. In addition, it is not suspect with regard to producing an effluent with carcinogenic possibilities flagged by EPA as of concern in the disinfection of potable water.
According to the rules and regulations issued by the U.S. Department of Labor, Occupational Safety and Health Administration, the maximum permissible concentration of ozone is 0.1 ppm. Equipment that produces ozone usually requires ducts for the exhaust air which is eventually discharged into the outside atmosphere. Various companies furnish ozone-in-air monitors, some with automatic digital readout. Microprocessor controlled ozone systems and monitoring are also available. Ozone analyzers designed to measure [O.sub.3] in water in terms of mg/L and monitor the concentration continually are useful. Companies providing these and other ozone equipment are discussed in Section C-4 of this manual.
Ultraviolet radiation has long been known to have microbiocidal properties. Equipment is usually provided for thin-film applications. Plant scale demonstrations have indicated that this is cost-effective and has adequate penetration to maintain low focal coliform counts in wastewater effluents. There is some indication that UV radiation may be more effective at eliminating certain bacteria and viruses than other methods of disinfection. However, it is important to note that certain substances can absorb UV light and suspended solids can do the same, as well as shielding microorganisms from the light. Also, photoreactivation (where microorganisms exposed to UV radiation and then exposed to sunlight will increase their number as much as ten times) must be taken into account when developing bacterial standards. Designing several banks of UV light tubes can reduce the effect of some of these problems. Although the UV light tubes can last a long time, they must be maintained, cleaned, and replaced as needed.
Manufacturers include Atlantic Ultraviolet Corp.; Capital Controls Company, Inc.; Fischer & Porter Co.; Hydro-Aerobics, Inc.; Katadyn Products, Inc.; Trojan Technologies Inc.; Ultra-Hyd; UV Waterguard Systems, Inc.; U V Water Purification Co.
Bromine Chloride & Sulfur Dioxide
Among the alternatives to chlorine as a disinfectant that appears to hold promise is bromine chloride (BrCl), which, although acting in a manner similar to chlorine, is indicated to have advantages over the latter. Among those claimed in field trials on wastewater effluent are higher efficiency in the ammoniated (bromamine vs. chloramine) forms. Also, bromamines are less stable than chloramine making them less hazardous to marine life, although the breakdown of BrCl to bromide can increase trihalomethane formation. Depending on the nature of the effluent and the receiving stream it is possible that brominated organic compounds could form that would be toxic to fish and aquatic insects. It is considered less toxic to handle in storage and feeding because of its low vapor pressure.
It can be stored and delivered in pressurized tanks as in the case of chlorine and the same handling equipment can be used. Retrofitting may require a different vaporizer and different feeder, though it is injected in a similar manner.
Sulfur dioxide (S[O.sub.2]) has been used as a bactericide and disinfecting agent in the food processing industry for many years. Research conducted at Utah State University has pointed to the use of S[O.sub.2] for disinfecting wastewater effluent. Reportedly the use of S[O.sub.2] meets many of "the nation's most stringent disinfection requirements" without producing adverse reactions or undesirable compounds. The S[O.sub.2] not consumed in the disinfection process can be recycled into the system. Information and technical data is available from P.B.&S. Chemical Co.
Hydrogen Sulfide Poisoning
One of the paramount concerns in the operation of any facility is the safety and protection of the workers who must necessarily carry out the daily routines of control, inspection, maintenance, and repair. With the expansion of wastewater collection and treatment systems there has been an unfortunate increase in the number of injuries and deaths to workers, especially in areas where hydrogen sulfide gas is present. It has become increasingly apparent over the last several years that workers who have died in manholes, sewers, and other chambers, have not died from asphyxiation, but from hydrogen sulfide poisoning. An atmospheric concentration of 300 ppm is considered lethal and can be produced by as little as 2 mg/I. of dissolved sulfide in wastewater.
Reliable means of detecting [H.sub.2]S must be utilized whenever the presence of the gas is suspected. "Sniffing" the air with one's nose is not enough as the gas quickly paralyzes the body's respiratory system. Breathing apparatus, safety harnesses, ventilation blowers, and other equipment are described elsewhere in this manual and should be used in all potentially hazardous areas. Section D-9 of this manual discusses confined entry safety procedures.
In addition to being a lethal gas and potentially very corrosive, [H.sub.2]S is a cause of increased septicity and results in poor settling, increased bulking, and possibly severe odor problems in the surrounding community.
Control and reduction of the problem of [H.sub.2]S can be effected by the introduction of hydrogen peroxide ([H.sub.2][O.sub.2]) into the system. The use of [H.sub.2][O.sub.2] to eliminate [H.sub.2]S also has the beneficial effects of removing odors associated with the gas and preventing the corrosion of certain kinds of pipes and fittings from sulfurous and sulfuric acids. These aspects are described in Section D-7. Stronger oxidizing agents, such as chlorine, are available, however, it is not very selective in the materials it oxidizes and it does not have a very long life cycle. Hydrogen peroxide is selective and does have a relatively long life cycle.
Preventive measures, either improved system design or chemical treatment can eliminate hydrogen sulfide or reduce it to minimal levels. Proper use of hydrogen peroxide technology can aid municipal management in improving worker safety and community health, and help reduce maintenance or rehabilitation costs while improving sewerage system performance.
Firms providing hydrogen peroxide and related technical services include Ashland Chemical, Inc.; Degussa Corp.; E.I. DuPont De Nemours, Inc.; FMC Corp., Chemical Products Group; Harcros Chemicals Inc.; Peroxidation Systems Inc.; P.B.&S. Chemical Co.; Solvay Interox; and Vulcan Peroxidation Systems.
Companies supplying equipment for the detection of hydrogen sulfide include CHEMetrics, Inc.; Control Instruments Corp.; Enmet Corp.; Gas Tech Inc.; Heath Consultants, Inc.; Houston Atlas; Industrial Scientific Corp.; LaMotte Chemical Products; Lumidor Safety Products; National Draeger, Inc.; Neotronics N.A. Inc.; Sensidyne, Inc.
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|Title Annotation:||Water Pollution Control|
|Date:||Apr 15, 1995|
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