Chlorine Residual Loss in a Long Dead-End Main
A distribution system fed from one clearwell serves a neighborhood at the end of a 4.8 km, 200 mm dead-end main. The clearwell leaves at 1.1 mg/L free chlorine and 0.6 mg/L total chlorine (chloramines formed because ammonia is present in the source). At the far end, free chlorine is 0.10 mg/L, total chlorine is 0.35 mg/L, and heterotrophic plate counts have risen. The state requires a detectable residual throughout and a minimum 0.2 mg/L free chlorine. The operator proposes raising the clearwell free chlorine to 2.0 mg/L. Recent DBP samples show total trihalomethanes at 68 µg/L against an 80 µg/L limit.
1. Evaluate the proposed fix. Will raising free chlorine at the clearwell solve the far-end residual problem?
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Probably not, and it may backfire. The system is running a chloramine residual (total chlorine exceeds free chlorine), so the residual that decays over 4.8 km is largely monochloramine, not free chlorine. Raising free chlorine at the clearwell will shift the chlorine-to-ammonia ratio, may create more free chlorine that reacts with organics, and will push THMs toward or past the 80 µg/L limit - the opposite of what the DBP data can tolerate. The far-end free chlorine of 0.10 mg/L is below the 0.2 mg/L requirement and the HPC rise signals loss of residual and possible nitrification. The real fix is to control residual where it is lost - flushing, reducing water age, or adjusting the chlorine-to-ammonia ratio and pH - not simply to raise the feed.
2. What sequence of corrective actions would you take, and how would you confirm the residual is restored without exceeding the THM limit?
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First, confirm whether nitrification is occurring: check far-end ammonia, nitrite, and nitrate, and compare with the clearwell. If nitrite is present, the chloramine residual is being consumed biologically. Actions in order: (1) increase flushing at the dead end to reduce water age and remove nitrification products; (2) evaluate the chlorine-to-ammonia ratio (target about 5:1 by weight for monochloramine) and pH (roughly 8-8.5 favors monochloramine stability); (3) consider a breakpoint or free-chlorine burn in the main only if DBP precursors allow, with a DBP sample before and after. Confirm success by far-end free chlorine at or above 0.2 mg/L sustained over several days, HPC falling, ammonia and nitrite stable, and a follow-up THM sample below 80 µg/L. If THMs rise, blend or adjust rather than continue raising chlorine.