Sicher bezahlen. Folgen Sie uns. Das Passwort muss mind. Darin sollte mind. Recht Steuern Wirtschaft. Erschienen: Although this section summarizes several DBF control alternatives as illustrative examples, it is not meant to provide a comprehensive discussion of this subject. A brief description of each of these four types of modifications is presented below. The TTHM formation potential may be reduced by as much as 50 percent through conventional coagulation and settling Singer and Chang, ; Summers et al.
Conventional water treatment plants that apply chlorine to raw water generally have adequate contact time for disinfection. Many water systems have eliminated or changed their pre-disinfection practices to control DBFs.
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Pre-disinfection practices involve using chemical or physical processes to remove precursors from the source water. Moving the point of disinfection after clarification with enhanced coagulation allows for greater removal of DBF precursors before disinfectant is added and also reduces the disinfectant demand of the water. When moving the point of disinfection further into the. Systems may find that seasonal use of this modification is helpful in reducing summer DBF levels, which are typically the highest. Several studies have evaluated the implications of changing the disinfection practices in water treatment plants.
Among four of the facilities, alternative disinfection strategies were investigated to evaluate the difference in DBF production from the plants' previous disinfection strategies or base disinfection conditions. The results were analyzed in three reports Metropolitan and Montgomery, ; Jacangelo et al.
Table presents the 10 potential strategies often considered for primary and secondary disinfection. Table fists the changes in DBF production observed in the four plants after eight of these new strategies were implemented. As shown in Table , employing different and more carefully selected primary and secondary disinfectants reduced the amount of DBFs produced.
In general, the results followed the characteristics of the DBFs associated with the primary disinfectant used i.
Organic oxidation products form when strong oxidants such as ozone are used. However, by carefully selecting the primary and secondary disinfectants, and avoiding long contact times and high dosages of halogens, the total DBF formation declined. It is important to note that the study did not evaluate bromate formation. Types of supporting materials include a description of the proposed change, the disinfection profile, and an analysis of how the proposed change will affect the current disinfection benchmark.
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If the State concludes that a change in disinfection practice is a significant modification, the water system must develop and submit a disinfection benchmark. The modifications listed in Sections 5. Therefore, a water system should check with the State program office for assistance in determining whether the proposed change triggers the disinfection benchmarking procedure. Water systems can refer to Alternative Disinfectants and Oxidants Guidance Manual for additional information and references on disinfectant capabilities and the potential implications of modifying disinfection practices USEPA, a.
Using the disinfection benchmarking method, the State may determine if the change in disinfection practice is acceptable e. However, there is no federal requirement for State approval of disinfection modifications.
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The current disinfection benchmark value and supporting calculations. Note that systems adding or switching to ozone or chloramines must provide the above information for both Giardia and viruses. EPA strongly recommends that systems also calculate a virus profile and benchmark if they are switching to chlorine dioxide. Specifically, water systems should calculate "modification benchmarks," based on the current operating conditions before the process change is made.
These modification benchmarks should be compared to the original benchmark to evaluate the expected inactivation level of the modified disinfection practice. The steps to calculate these modification benchmarks are as follows:.
Identify the lowest average months from the original profile i. The water system will need to assume reasonable values for the disinfectant residuals. It may also need to calculate or estimate contact times, or identify new points of disinfectant residual sampling to reflect the modification. Calculate the average of the monthly values.
This value is the modification benchmark. Compare the original benchmark to the modification benchmark. If the modification benchmark is greater than the original benchmark, the modification will likely be acceptable after consultation with the State.
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Modification benchmarks lower than the original benchmark should be evaluated by the State to determine whether the resulting level of disinfection is still considered adequate based on source water quality and watershed conditions discussed further in Chapter 6.
The system and State should discuss the reasons for any modification and whether better options exist, and assess the modification's impact on log inactivation. A detailed example of calculating the impact of changes in disinfection practices, including the comparison of original and modification benchmarks, is provided in Section 5. These situations are detailed in Chapter 6.
The disinfection benchmark can also be met by a combination of inactivation with a chemical disinfectant and an improvement in the physical removal of pathogens after consultation with the State. Consider an unfiltered system with a disinfection benchmark of 4-logs for Giardia. If this system were to implement conventional filtration and receive 2. Likewise, a utility that makes a process enhancement to improve pathogen removal could receive credit toward achieving its existing disinfection benchmark. Consider a conventional filtration plant that upgrades its process to include ultrafiltration using membranes.
Because ultrafiltration has been demonstrated to achieve greater than 6-logS of Giardia removal, the existing Giardia disinfection benchmark could be reduced by an amount deemed acceptable by the State AWWARF, The remainder of the existing disinfection benchmark could be accomplished with chemical disinfection.
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The examples include process changes that may accomplish the goals of controlling DBF levels and disinfection benchmarking. This section does not discuss major process changes, such as alternative primary disinfectants, since they require extensive engineering evaluation! As discussed previously, the system should only implement significant changes to a disinfection practice after careful consideration and consultation with the State. In most circumstances, the system should seek the assistance of a qualified professional engineer to develop and implement a process change. Table lists the important raw water characteristics, while Table describes the important unit processes of Plant A.
This system applies chlorine to the raw water for disinfection to achieve at least a 0. Since chlorine and alum are both acids, the pH is reduced from about 7.
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This results in a concurrent decline in SUVA. The plant examines making four modifications to its disinfection practices to control DBFs. These modifications include: 1. Installing chloramination to provide residual disinfection 3. Moving the point of chlorine application after settling possibly a seasonal change 4. Improving hydraulic characteristics of clearwell. Because the system is not exempt from enhanced coagulation requirements, it must achieve TOC removal requirements as stated in Table Because waters with greater alkalinity and lower TOC concentrations are more difficult to coagulate, performance requirements in these categories are lower than for other categories.
Based on Table , these conditions require the utility to remove 45 percent or more TOC through the coagulation and settling process as an annual average refer to the Guidance Manual for Enhanced Coagulation and Enhanced Precipitative Softening for additional information USEPA, g.
This alum dose reduces the TOC from 5. This is equivalent to 26 percent removal [5. Practicing enhanced coagulation in settled water is expected to result in the following DBF concentrations in the distribution system Table The disinfectant residual achieved by a given dose is a function of contact time and disinfectant demand of the water, among other factors. Because TOC exerts a disinfectant demand, the disinfectant residual will be greater when practicing enhanced coagulation for the same chlorine dose.
The addition of alum to water decreases the pH of the water. This drop in pH with enhanced coagulation may adversely impact corrosion in the distribution system and should be mitigated appropriately. The drop in pH actually improves disinfection, because chlorine is more effective at inactivating Giardia at lower pH. Acids, such as hydrochloric acid, are used in treatment plants to lower pH levels to enhance coagulation and improve filter performance. Table indicates the improved disinfection occurring due to enhanced coagulation and'disinfection of settled water.
The system also maintains a disinfection level above its current benchmark. Table lists the important raw water characteristics for this plant, while Table describes the important unit processes of Plant B. This system applies chlorine to the raw water for disinfection and maintains a detectable residual throughout the distribution system.
The effects of both,chlorine and alum on pH is evident in the decrease in pH levels from about 7. This results in a concurrent decline in UV absorbance. Improving hydraulic characteristics of the clearwell. Based on Table , these conditions require the utility to remove 45 percent or more TOC through the coagulation and settling process as an annual average.
This alum dose reduces the TOC from 5,0 to 3. Practicing enhanced coagulation results in the following DBP concentrations in the distribution system Table Because TOC exerts a disinfectant demand, the disinfectant residual will be greater for the same chlorine dose. The drop in pH actually improves disinfection, however, since chlorine is more effective at inactivating Giardia at lower pH.
Table indicates the improved coagulation occurring due to enhanced coagulation. The system considers switching to chloramines for a secondary disinfectant in order to reduce DBF levels. This system is considering the application of free chlorine to its raw water, with application of ammonia to the suction line of the high service pumps. This allows disinfection using free chlorine, while quenching the free chlorine residual with ammonia to limit formation of regulated DBFs in the distribution system. The use of chloramines by this system will not affect its primary disinfection because ammonia is applied following the clearwell.
Therefore, the disinfection level listed in Table for enhanced coagulation 1. Chloramines will effectively control DBF formation in the distribution system.
For systems that exceed DBF MCLs within the plant, rather than the distribution system, ammonia would need to be applied prior to the clearwell for effective DBF control. For this system, application of ammonia at the suction line of the high service pumps after clearwell allows disinfection levels to be maintained while further controlling DBFs. For this system, the TOC level declines from about 5.
Moving the point of chlorine application from raw water to settled water results in DBF formation shown in Table The chlorine dose is not changed from the baseline condition which is 4.