Stage 2 DBP Exceedance at a Single High-Water-Age Site
Harlow County Water District (4,800 connections) purchases finished water from the neighboring city's 12-MGD conventional plant and distributes it through 78 miles of main, one 1.0-MG elevated tank, and a 250,000-gallon standpipe serving a dead-end industrial park. The city chlorinates with free chlorine to 1.1 mg/L at the clearwell. Stage 2 DBPR samples are collected quarterly at four approved sites. For the last four quarters, Site 4 at the standpipe has averaged TTHM 88 µg/L and HAA5 48 µg/L. The other three sites range from 52 to 61 µg/L TTHM and 35 to 44 µg/L HAA5. Free chlorine leaving the standpipe is 0.31 mg/L. Summer demand in that zone is low and the standpipe turns over about once every six days. The district's engineer proposes switching to chloramines at the city's clearwell; the city is willing but wants the numbers and the risks before committing.
1. How is the locational running annual average determined, is there a violation, and what are the reporting consequences?
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How the number is built. Each approved monitoring site has its own locational running annual average (LRAA): the average of that site's results for the current quarter and the three previous quarters. Compliance is evaluated site by site, not on a system-wide average. That is why three sites in the 50s and 60s do not offset one site at 88.
Violation. TTHM at Site 4 is 88 µg/L against an MCL of 80 µg/L, so this is a TTHM MCL violation at that location. HAA5 at 48 µg/L is under its 60 µg/L MCL.
Consequences.
- Public notice is required; a Stage 2 DBPR MCL violation is normally Tier 2, due within 30 days, with the required language on health effects and what customers should do. Confirm the tier and the required template with the Kentucky Division of Water.
- Quarterly monitoring continues; you do not get to skip a quarter because you already know the answer.
- The LRAA is slow to recover. If the next four quarters each came in at 55 µg/L, the Site 4 LRAA would still be about (88+88+88+55)/4 = 80 µg/L at the first quarter and would not clearly clear the MCL until several quarters have rolled off. Plan for the violation to persist through the next reporting year.
- The exceedance also feeds into the system's overall compliance record and any state enforcement follow-up, so start documenting corrective actions now.
2. Evaluate the chloramine proposal against alternatives. What would you recommend, and what risks come with chloramines?
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Why chloramines help. Monochloramine is far less reactive with natural organic matter than free chlorine, so it forms much less TTHM and HAA5 in the distribution system, and it holds a residual much longer in dead ends and in the standpipe. Site 4's problem is water age - six-day turnover - so a more persistent, less reactive residual addresses the actual mechanism.
Risks and costs.
- Nitrification: chloramine decays and releases free ammonia, which feeds nitrifying bacteria in the standpipe and dead ends. You will need nitrite, nitrate, and ammonia monitoring, and a plan for breakpoint chlorination or tank cycling when nitrification appears.
- Dose control: the chlorine-to-ammonia ratio (typically about 4:1 to 5:1 by weight as Cl2 to NH3-N) has to be held tightly or you create dichloramine and taste complaints, or leave free ammonia.
- Ammonia feed adds a chemical hazard and a second failure mode at the city's plant.
- Switching back to free chlorine is operationally hard, and the city becomes the decision-maker for the district's compliance.
Alternatives to compare first.
- Enhanced coagulation upstream to reduce TOC before chlorination; at 4.5 mg/L TOC this can cut TTHM formation substantially and is often the cheapest first step.
- Tank mixing, daily fill-and-draw cycling, and targeted flushing to cut water age in the industrial park.
- Lowering the chlorine dose into that zone while preserving at least 0.2 mg/L detectable residual.
Recommendation: model or pilot the chloramine option, but run the TOC-removal and water-age fixes in parallel - they may get Site 4 under 80 µg/L without committing the city to a permanent chloramine system.
3. What operational changes at the standpipe and dead-end zone would reduce TTHM at Site 4, and how would you verify they worked?
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Attack the water age, which is the root cause.
- Restore turnover: adjust the altitude valve or level controls so the standpipe fills and drains daily instead of sitting nearly static for six days. Even a partial drawdown each day displaces the oldest water.
- Install an active mixer (or a properly designed inlet/outlet arrangement) inside the standpipe to break stratification and prevent a warm, stagnant, high-demand layer from forming on top.
- Eliminate or loop the dead-end mains in the industrial park; if looping is not feasible, set up a scheduled flushing program with documented flow, duration, and residual at each flush point.
- Re-check the chlorine residual at the far end of the zone; if the district has any chlorination capability, boost residual only where needed and watch the 0.2 mg/L minimum for the whole system.
- Coordinate with the city on coagulation performance upstream so that the water entering the district has lower TOC to begin with.
Verification.
- Use the required quarterly compliance samples as the official measure, but add monthly internal TTHM/HAA5 samples at Site 4 and at the standpipe outlet so you can see a trend before the LRAA moves, which takes four quarters.
- Track free chlorine and total chlorine residual, water age estimates, tank level and turnover, and HPC or heterotrophic plate counts as process indicators.
- Set a target well below the MCL (for example, 60 µg/L TTHM) so that seasonal peaks do not put you back over 80, and report progress to the state along with the public notice follow-up.