Chlorination/Dechlorination: An Environmental Solution

Transcription

1 Chlorination/Dechlorination: An Environmental Solution Presented at the 1996 IBC USA Conferences by GERALD F. CONNELL, Capital Controls Company, Inc., Colmar, PA SUMMARY: The topic of this presentation is the efficient use of chlorine in the disinfection of the effluent from the many types of waste treatment facilities. The various processes that have an impact upon the disinfection process are identified, the benefits of the process and the factors that come into play regarding the efficient use of chlorine and the provision of an environmentally sound effluent. The amount of water discharged to rivers, streams, lakes, ponds, estuaries, and other receiving bodies in the United States has increased considerably in the last 0 years. Discharge water can come from municipal waste water effluent or from process water effluents such as once through cooling water systems. The effluent from municipal and industrial waste water treatment plants may contain a great deal of contaminant depending upon the level of treatment used at the facility. The facility design and the operation of the facility to remove these contaminants is important to the amount of chlorine required. In other words, the amount of chlorine used is a direct function of the type and effectiveness of pretreatment. The amount of disinfectant is considerably reduced as the level of treatment (primary, secondary and tertiary) is increased. It is important to recognize that with each succeeding step in the process train additional contaminants are removed. The reduction of chlorine demand that results from each step reduces the amount of chlorine feed to manageable levels both economically and environmentally. All too often we in the waste treatment industry have fed more and more chlorine since it was often considered that chlorine was the panacea for all plant operating problems. The truth is that the opposite is the case. Excessive chlorine addition has created situations that contributed to unwanted levels of chlorine in receiving streams, killed fish, upset the ecosystem and added unwanted chloramines and other potential Disinfection ByProducts (DBPs) to downstream intakes for drinking water plants. As we became more experienced in plant operation and design, we also became more aware of the needs to chlorinate effectively, efficiently and with specificity and not chlorinate with a broad brush. The reduction in chlorine dosages, the latest developments in mixing, the improvements in residual measurement and advances in chlorine control have made for a more environmentally sound discharge into each receiving body of water. Industrial cooling water might be expected to contribute little in the way of impurities since few chemicals are added to once through cooling water. Normally, the only chemical addition considered would be an algicide that is used to maintain the heat transfer efficiency and to aid the performance of the process cooling equipment. Chlorine is used extensively for this purpose. The nature of a cooling water system is such that the intake structure, piping and heat exchangers act as a breeding ground for Asiatic clams, zebra mussels, and other forms of aquatic life. Chlorine is used to reduce the impact that these organisms have upon the performance of the system. Discharges operate under various regulations. The Environmental Protection Agency (EPA) has established regulations that govern discharges from wastewater plants and cooling water users. States also have established their individual needs. In general, states require a maximum level of 0.0 mg/l of chlorine in the waste water effluent discharge. Some states are lower. Cooling water systems at installations such as electrical power plants are guided by regulations that permit a maximum of 0.2 mg/l in the effluent during a two hour time period every 24 hours. Let us look at the benefits obtained with proper use of the equipment available on the market today and briefly review the chemistry involved. In the disinfection process chlorine reacts with water to form hypochlorous acid. Eq. (1). (1) Cl 2 + H 2 O HOCl + The hypochlorous acid formed can dissociate depending upon the ph, to form hypochlorite ions. Eq. (2). (2) HOCl OCl + H

2 Organisms exposed to either of these chemical forms (hypochlorous acid and hypochlorite ion) are attacked by cell wall penetration and subsequent disruption of the reproductive system so that within a short time the coliform level has been reduced to < MPN/0 ml. The presence of particulate matter that entrap various organisms may slow the process but does not stop it during the exposure time. Chlorination is very forgiving of process upsets that may cause concern with other disinfection techniques. At the end of the disinfection process (less than 30 minutes) the residual chlorine remaining is reacted with sulfur dioxide. (Eq. 3, 4,, 6). (3) Cl 2 + H 2 O + SO2 HCL + H 2 SO 4 (4) NH 2 Cl + SO 2 + 2H 2 O NH 4 Cl + H 2 SO 4 () NHCl 2 + 2SO 2 + 4H 2 O NH 4 Cl + HCl + 2H 2 SO 4 (6) NCl 3 + 3SO 2 + 6H 2 O NH 4 Cl + 2HCL + H 2 SO 4 The reaction with SO 2 (or its solution forms of sulfite and bisulfite) is rapid and complete regardless of the chlorine form as free or combined chlorine. Now that the chemistry has been established, the processes prior to the disinfection step must be used effectively. Primary (removal of settle able and floatable solids) secondary (removal of dissolved materials and organisms) and tertiary (filtration/ adsorption) treatment are important and even necessary to efficient, proper and effective disinfection practices. In other words, the cleaner the water, as it arrives at the disinfection step, the better the result will be. There will be less turbidity and solids to upset the disinfection process as well as a reduction in chlorine demand. Table 1 identifies typical reduction levels of coliform in each process step. Water Quality Table 1 Unit Process Coliform Levels (2) Total Coliform Fecal Coliform Influent Primary Secondary 6 4 Secondary 4 3 Nitrified 4 3 Nitrified Filtered 4 3 Disinfection

3 Table 2 identifies design and operating disinfection dosages required after each treatment step. Table 2 Disinfection Dosage By Process Process Design Operation Raw 20 1 Primary 1 Secondary Tertiary 23 The previous information establishes the need for good waste plant performance. Now let us see the equipment used in the chlorination/dechlorination process. Chlorine Analyzer Of all the chlorination/dechlorination practices being used, the rapid and accurate measurement of the chlorine residual is most important. Of all the measurement techniques used, amperometric measurement is the simplest and most widespread. The ability of the current design of amperometric analyzers to measure free or total chlorine, to handle waters of various qualities, to be unaffected by color changes and to require only a daily calibration check, is of utmost importance. The confidence level of the operator is raised and this frees him for other duties at the plant. In addition, the ability to use the amperometric device to measure chlorine (oxidizing agent) or sulfite (reducing agent) provides a vital link in the control process (Figure 1). The analyzer measures a current flow between two electrodes (copper and gold) that is directly proportional to the chlorine residual. Interferences caused by dissolved oxygen are relieved through a unique vent and entrained particulate matter are drained with a easily accessible plug. Frequency of draining is site specific. Residence time of the water sample in the analyzer is several seconds and response time to change is instantaneous with change to a 0 percent level measured in less than two minutes. The residual chlorine reading is limited more by the time for the sample to reach the analyzer from the sample collection point. All this is more critical when the signal is to be used for more than as an indicator; that is, for control purposes. Figure

4 Mixing Mixing chlorine disinfectant with the water to be treated is a necessity for good disinfection and adequate control. Previously employed mixing techniques generally provided a diffuser in the contact channel and an externally driven propeller type mixer. Newly designed induction mixers improves the mixing with the effluent mass. In addition, the current induction mixer simplifies the process needs by improving safety and reducing chemical consumption (Figure 2 and 3). A study has shown that reduction of anywhere from 20% to 40% of chemical usage can be expected. Data from several plants identifies the results (Table 3). Table 3 P lant Process F low (MGD) P re Mix (PPD) Post Mixing (PPD) Percent Savings Activated Sludge Extended Aeration Tertiary Filtration Savings will depend on local costs. Plants using chlorine in ton containers at a cost of 20 cents per pound will have savings ranging upward from $.40 per day depending upon plant treatment. Table 4 provides daily and yearly savings. Table 4 P rocess Type F low (MGD) D aily Savings ($) Yearly Savings ($) Activated Sludge ,796 Extended Aeration ,176 Tertiary Filtration ,60 Figure

5 Figure 2A Figure

6 In addition safety in plant operation clearly is enhanced with the presence of chemical induction systems by the minimization of gas pressure lines, the elimination of solution lines and the use of gas vacuum lines, water booster pumps and water line. The need for solution diffusers is no longer required and external mixing devices are not needed. Controllers Control of chlorine residual is impacted primarily by variations in flow. Thus any chlorine residual control scheme must be able to receive a flow signal. Combination of the flow with the actual residual from a set location in the contact chamber by use of an electronic controller provides good disinfection and minimizes excess chlorine addition. The use of an electronic controller permits adjustments for time changes caused by the fixed sample location. This allows sampling at one location where the most effective residual control can be attained. The controller is programmable to use one sample location regardless of flow anticipates residual level change and accounts for the impact on the process. Key to the controller s ability to function properly is the location of the analyzer as near as possible to the sampling point. Time lag in the process is caused by the length of time from disinfectant addition to the sampling point (T1), the time from the sampling point to the analyzer (T2), the time from the sample reaching the analyzer to full scale response (T3) and the time from the control valve to the point of addition (T4). (Figure 4). The controller adjusts for variations in T1. The installation mechanics must consider the impact of T2 by minimizing the sample transfer time and the impact of T4 by locating the control valve as close to the injection point as possible. If these conditions are addressed properly, the control scheme will perform as desired. The keys to reduced chlorine consumption, improved control and efficient operation are: 1. Improved mixing. 2. Reduced sampling times. 3. Efficiently operating chlorine analyzer. 4. Flexible controller. Figure

7 Dechlorination Dechlorination control needs are the same as chlorination s control needs reliable analyzer, rapid chemical transfer, dedicated controller and improved mixing. With the rapid reaction and good, immediate mixing to speed the process, the reduction or dechlorination reaction requires less time and a smaller contact volume. In fact, many dechlorination systems use the contact chamber discharge well or an effluent outfall line. The analyzer for measuring the removal of chlorine to a level near zero or certainly below 0.0 mg/l is a variation of the standard chlorine analyzer (the Renton system) adapted to measure the extremely low levels of chlorine required. The Renton system provides for an offset of the chlorine zero to permit a low level measurement. Without the offset, low levels or, on occasion, zero chlorine makes the analyzer susceptible to algae and slime growth that fouls analyzer and affects performance. An example of the Renton system is shown in Figure. The controller used in the chlorination feed system has the ability in the dechlorination system to be used in feed forward, feed back, cascade control (Figures 6 and 7) in addition to the previously used compound loop control. The controller has the ability to respond to rapid reaction of dechlorination as well as slow the reaction of disinfection. Once again mixing using an induction type mixer provides the rapid addition and dispersion within a short time frame. The induction mixer is an asset to a system with short circuit tendencies. Two states have looked at adjusting their mandated contact times when induction mixing systems are provided. The dechlorination systems are used to maintain effluents at or below the 0.0 mg/l levels. Keys to success once again. 1) Mixing 2) Control 3) Analysis RENTON SYSTEM FIGURE The following summarizes the best practices for an environmentally sound chlorination/dechlorination system. 1. Clean effluent produced by a tertiary treated waste requires a reduced chlorine dosage. 2. A well analyzed and controlled chlorine feed provides a system that has as little excess chlorine present as possible. 3. A well mixed effluent and chlorine feed using CHLORAVAC induction mixers can reduce the overall contact time and, therefore, reduce the potential for appearance of DBPs such as chloramine. 4. Minimal chlorine feed reduces the cost of chlorine and the cost of the reducing agent.. Proper location of the effluent analyzer and the control valve in the chemical feed line reduces lag time and improves the control logic. 6. The best indicator of disinfection performance is continuous chlorine residual analysis. 7. The best control schemes operate continuously and are not sampling periodically. 8. Safety of operation is excellent, down time at installations are less than 0.1 percent and performance meets design expectations. 9. All vacuum systems permit containment of the chemicals in one location should local codes require containment system (Emergency Kit from The Chlorine Institute or scrubbers).. Although cooling water effluent was cited as a contributor of chlorine compounds, the measurement and control was not cited in detail. The problems associated with cooling water discharge are similar with some distinct differences; such as: a. Flow is generally constant. b. Demand, due to the mass involved, is fairly constant. c. Control is less complicated. d. Treatment is usually performed in the outfall line and not in a contact tank. e. The use of a vacuum induction mixer may be limited due to the nature of the system flow

8 Figure Compound Loop Control Design improvements may be made without notice. Represented by: Figure 6 Chlorine/Ammonia Feed Systerm De Nora Water Technologies 3000 Advance Lane Colmar, PA 1891 ph fax web: mail: info.dnwt@denora.com Registered Trademark All Rights Reserved. OCT