Published on November 7, 2007
So I’ve Found High DBPs: What Now: So I’ve Found High DBPs: What Now Presented at: KWWOA 50th Annual Conference Louisville, KY March 27, 2007 So the IDSE Found High DBPs: So the IDSE Found High DBPs This is to be expected The IDSE focused on areas in the distribution system that would have high DBPs Little to no chlorine residual High water age Influenced by tanks Etc, etc, etc But EPA gives systems 3 years to get the DBPs under control before the locational running annual average kicks in Quick Review: How Do DBPs Form?: Quick Review: How Do DBPs Form? DBPs form as a result of a disinfectant reacting with organics Example: Free chlorine + TOC = DBPs Example: Ozone + Bromide = Bromate Example: Chlorine Dioxide + TOC + Decay = Chlorite The reaction itself is dependent upon Time Temperature Water quality parameters such as pH Quick Review: Where Do DBPs Form?: Quick Review: Where Do DBPs Form? Blame it on the source water Surface water has higher organic content (TOC) than groundwater Reservoirs (or impoundments) usually have higher TOC than flowing streams Protected watersheds may have lower TOC levels than less protected ones In other states, highly colored waters have very high TOCs Florida with tannic lakes have source water TOCs over 10 mg/L Quick Review: Where Do DBPs Form?: Quick Review: Where Do DBPs Form? Blame it on the source water Higher water temperatures make the TOC + chlorine = DBP reaction go faster Summer and early fall in KY have the higher water temperatures and thus the higher DBPs Quick Review: Where Do DBPs Form?: Quick Review: Where Do DBPs Form? Blame it on the water plant Water plant operations Disinfection practices Coagulation/flocculation/sedimentation process operation Start-stop operations can result in water that sits in sedimentation basins or clearwells for extended periods of time Time can increase the TOC + chlorine = DBP reaction Quick Review: Where Do DBPs Form?: Quick Review: Where Do DBPs Form? Blame it on the water plant Water plant maintenance Excessive sludge/residuals in sedimentation basins could contribute TOC within the plant Algae in sedimentation basins can also contribute TOC through by-products or when they die off Quick Review: Where Do DBPs Form?: Quick Review: Where Do DBPs Form? Blame it on the distribution system Water age Tanks are primary culprits in increasing DBPs Number: Too many can increase water age when compared to WTP production and consumer usage Size: Large/oversized tanks can increase water age when compared to WTP production and consumer usage Location: Water moving through multiple tanks Type: Standpipes have the most issues Inlet/Outlet configuration: Common inlet/outlets use the “last in-first out” concept Mixing: Very critical and related to inlet velocity Quick Review: Where Do DBPs Form?: Quick Review: Where Do DBPs Form? Blame it on the distribution system Water line extensions/oversized mains Extending lines over many miles to serve very few customers Oversized mains to “future growth” areas can result in long skinny horizontal mains in the ground Dead ends Distribution systems that have numerous dead ends have numerous ends that could have high water age Booster chlorination Adds more chlorine to the TOC + chlorine = DBPs equation to make more DBPs Quick Review: Where Do DBPs Form?: Quick Review: Where Do DBPs Form? Blame it on the distribution system Distribution system operations and maintenance No or incorrect system-wide flushing Tanks are not inspected and cleaned regularly No working cross-connection control program Booster chlorination just because its always been used Corrosion control practices As you can see, DBPs are becoming a distribution system issue DBP Control: DBP Control We have the how and the where of DBP formation Now we need some solutions DBP Control: Source Water and Reactions: DBP Control: Source Water and Reactions The “How” of DBP formation: Can’t control temperature May not be able to control source water Surface versus ground Source water quality Produce versus purchase Can look at Different intake levels Balancing one source against another (if more than one source) Surface or ground Reservoir or river One producer or another (for purchasing systems) DBP Control: Water Plant: DBP Control: Water Plant The water plant has been the primary focus of Stage 1 with the TOC requirements for surface water systems TOC requirements remain in Stage 2 TOC removal is still key TOC removal is an art, not a science The more TOC in the source water the easier it is to remove in the coagulation/flocculation/sedimentation process The more alkalinity in the source water the more difficult it is to remove through treatment The lower the treated water pH the more easily it becomes to remove TOC DBP Control: Water Plant: DBP Control: Water Plant Improving TOC Removal Optimize the entire coagulation process (enhanced coagulation) for TOC removal Consider lowering the coagulation pH But be careful, low pH’s encourage HAA formation Lower the pH with acid addition, more alum/ferric or acidified coagulants Although PACls are great for turbidity they don’t suppress the pH and therefore don’t do much for TOC removal Try powdered activated carbon (PAC) TOC removal usually takes more PAC than for taste and odor control (can be as high as 20 mg/L) Balance with sludge production, cost DBP Control: Water Plant: DBP Control: Water Plant Disinfection practices play a key role in DBP formation Simply reduce the amount of disinfectant added Reducing pre-chlorine dosages and increasing post will help As TOC is removed, the demand for disinfectant in the plant should decrease Could result in lower post-disinfection dosages Remember to meet C-Ts Move the point of pre-chlorination to further in the flocculation/sedimentation process KY has seen success with this in reducing HAAs There are stipulations that must be followed to move the point of pre-chlorination DBP Control: Water Plant: DBP Control: Water Plant Disinfectant practices (continued) Change disinfectants However, almost all disinfectants make some kind of by-product Example: Chlorine dioxide makes chlorite Example: Ozone makes bromate Example: Chloramines make NDMA (not yet regulated but is part of the UCMR 2 monitoring) Example: UV (no known by-products yet and leaves no residual) Chlorine dioxide and ozone are pre-disinfectants Chloramines and UV are primarily post-disinfectants Permanganate provides limited disinfection and no C-T credit DBP Control: Water Plant: DBP Control: Water Plant Disinfectant practices (continued) Change disinfectants (continued) Balance the by-products, water quality issues (such as nitrification with chloramines) with decrease in DBPs Example: In KY chlorine dioxide does not seem to reduce HAAs but is good for THM reductions Example: Chloraminated water should not mix with free chlorinated water (water goes through the breakpoint chlorination process and may result in no chlorine residual at all) An issue with purchasing systems Must have DWB approval to change disinfectants Must still meet C-Ts in the plant and have a free or total distribution system residual DBP Control: Water Plant: DBP Control: Water Plant Water Plant Operations and Maintenance Strongly consider 24/7 plant operations Reduces time at the plant that the water sits in contact with chlorine Optimize settled water turbidity Lower settled water could translate to higher TOC removals Evaluate sludge/residual management Cover basins to reduce algal growth Don’t recycle DBP Control: Distribution System: DBP Control: Distribution System Water Age and Storage Tanks Increase tank turnover: the less time in the tank the less time to react further with the chlorine and make more DBPs Monitor tank levels Control tank levels Evaluate the flow of water from one tank to another Evaluate the number and size of tanks Too many tanks can cause the water plant to operate less and thus create operational and water quality problems at the plant Too many tanks increased water age DBP Control: Distribution System: DBP Control: Distribution System Water Age and Storage Tanks (continued) Model the distribution system flow Hydraulic model Tracer study Evaluate valve operation practices Open some, close others to facilitate good flow DBP Control: Distribution System: DBP Control: Distribution System Water Age and Storage Tanks (continued) Physical Structure Consider a tank type other than a standpipe These are commonly used for pressure not storage Most are tall and skinny and water warms up quickly Common inlet/outlet configuration “Last in-first out” May encourage temperature stratification Mixing within the tank Studies have shown that mixing is critical to overcoming some water quality issues Inlet velocity Mixing systems DBP Control: Distribution System: DBP Control: Distribution System Water Lines, Mains and Dead End Areas We do need to get safe potable water to everyone and It is hard to guess at population and industry growth but Be aware that water quality will suffer if usage is low (i.e. water age is high) Flushing: Either with manpower or automatic flush hydrants Looping of mains and tanks Be careful not to create hydraulic dead ends where water in different directions meet and don’t flow DBP Control: Distribution System: DBP Control: Distribution System Booster Chlorination Take a good look at whether booster chlorination is still needed Can chlorine residuals be maintained by better/more flushing? Loop lines Evaluate if the booster station should be moved For safety and security purposes, evaluate the need for gaseous chlorine DBP Control: Distribution System: DBP Control: Distribution System Distribution System Operation and Maintenance Implement a flushing program From the water plant/master meter out Base how long to flush on water quality (such as chlorine residual) Clean and inspect tanks Broken/missing vent or overflow screens can allow creatures into the tank—creatures contain TOC and create a chlorine demand Any sediment can be stirred up during the fill/empty cycles—the sediment may contain TOC DBP Control: Distribution System: DBP Control: Distribution System Distribution System Operation and Maintenance Develop a cross-connection program Any organic introduced into the distribution system can react with chlorine to form by-products With the water plant, evaluate corrosion control strategies Anything that can clean up a distribution system without other adverse consequences can be considered Cleaner distribution systems require less disinfectant and so may lower DBPs Phosphates versus pH adjustment (lime or caustic) DBP Control: Human Factor: DBP Control: Human Factor DBPs can be formed in both the water plant and the distribution system Solving DBP problems should be a joint effort Distribution and production Producer and purchaser Distribution system evaluations and improvements will be most critical in purchasing systems DBP Control: Human Factor: DBP Control: Human Factor DBP testing is costly and the solutions can be even more so—This will be management’s area of concern Work to budget the costs THM Plus versus actual THM/HAA testing Plan for the future (5, 10, and 15 year planning) New or re-engineered tanks Looping mains to eliminate dead ends Replacing mains New sources New chemical feed/disinfection Questions?? Comments??: Questions?? Comments?? Julie W. Roney Julie.Roney@ky.gov www.water.ky.gov/dw www.epa.gov/safewater/disinfection/stage2
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