DBP Compliance Headroom and the Chloramine Decision
A 3.2 MGD surface-water plant uses free chlorine only, with finished-water pH 8.1, finished TOC 4.6 mg/L (raw 5.2), and long detention: about 5.5 days average in August from clearwell through a 2.0 MG standpipe and the transmission mains. Free chlorine leaving the plant is 0.9 mg/L. TTHM at the worst monitoring location by quarter was 62, 71, 78, and 85 µg/L; HAA5 at the same site was 38, 44, 49, and 53. The utility has $400,000 available and cannot build new filters this year. The state permits chloramination with written approval. Among the customers are a dialysis center and two large aquarium businesses, and last summer a neighboring system had a nitrification problem that made the board nervous.
1. Compute the worst-site LRAA and the value next quarter must stay under to avoid an exceedance. What does that tell you about timing?
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TTHM LRAA: (62 + 71 + 78 + 85) / 4 = 74 µg/L against an MCL of 80.
HAA5 LRAA: (38 + 44 + 49 + 53) / 4 = 46 µg/L against 60.
Next quarter must stay at or below: (80 x 4) - (71 + 78 + 85) = 320 - 234 = 86 µg/L, or the LRAA goes over 80.
The trend is +7 µg/L per quarter and the next quarter is late summer - warm water, higher TOC, longest detention. Projected value is about 92, so an exceedance is likely with no change.
Timing judgment: you have one quarter of headroom. State approval, design, and installation of ammonia feed realistically take that long. Waiting for the exceedance guarantees a violation and public notice, so the decision has to be made now, this month.
Where the TTHM comes from: precursor concentration (TOC 4.6 mg/L), pH (8.1 favors TTHM over HAA5), temperature, and about 5.5 days of contact. Most of the 85 µg/L is probably formed in the standpipe and mains rather than in the clearwell - which is why distribution water age and residual strategy matter more than a small plant-side tweak.
2. The board asks for options within $400,000. Compare the realistic choices and make a recommendation with conditions.
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Option A - convert to chloramines. Keep free chlorine through the clearwell for CT credit, then feed ammonia to form monochloramine for the distribution system. Ammonia feed, analyzers, and a small chemical room fit inside $400,000. Expect large reductions in TTHM and HAA5 formed in the distribution system, often well over half of the species formed there. Conditions: written state approval, ammonia safety and secondary containment, notification of the dialysis center and aquariums, and a nitrification-control plan. It does not remove TTHM already formed in the plant, so plant contact time and coagulation still matter.
Option B - enhanced coagulation and pH trim. Coagulate nearer pH 6.5-7.0 with more alum to remove TOC - 4.6 down toward 3.0 might cut TTHM by 25-40%. Costs: more alum, more caustic to hold corrosion control, more sludge, shorter filter runs, and a corrosion-control risk for lead and copper that must be monitored.
Option C - reduce water age. Mix and cycle the 2.0 MG standpipe, loop dead ends. Cheap, partial, worth doing regardless.
Recommendation: start C immediately, design and permit A for next summer because the headroom expires first, and hold B as a fallback if the state rejects chloramination. Doing nothing is the only unacceptable option.
3. If chloramines are approved, what monitoring and operational traps must be in the plan?
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Measurement: after conversion, measure and record total chlorine, not just free. A free-chlorine reading near zero with a good total reading is normal for chloramines and is not a failure - but it will alarm an operator who is not trained on it.
Ratio: monochloramine forms best around 4-5:1 chlorine to ammonia by weight. For 0.9 mg/L total chlorine, ammonia as N is roughly 0.18-0.25 mg/L - set by actual measurement, because excess ammonia is food for nitrifying bacteria.
Nitrification watch: residual free ammonia, nitrite, nitrate, and DO in the standpipe. Given low turnover in a 2.0 MG tank, that is the likely first place nitrite appears. Respond by cycling the tank and temporarily raising the chlorine-to-ammonia ratio; do not simply add more ammonia.
Distribution: keep residual above the state minimum at the farthest dead ends, check monthly at first, and keep a flushing schedule.
Regulatory and customer: continue quarterly DBP monitoring - the LRAA does not reset and credits are not automatic. If the LRAA exceeds 80, Tier 2 public notice is due within 30 days. Dialysis providers and aquarium owners need written notice of the switch, and the utility should rehearse an ammonia feed failure (fall back to free chlorine, flush, notify).