We run monochloramine at 4.7:1 Cl2:N (by weight) and see the residual drop from about 2.0 to about [redacted]/L between 1–4 a.m. as pH slips from 7.9 to 7.6. Are you adjusting ammonia setpoint, trimming hypochlorite, or adding alkalinity to buffer that swing — and which readings do you trust most in the moment (DPD free/total vs online NH3)?
I’d add a small sodium bicarb feed to hold pH ≥7.8 overnight rather than chasing the Cl2:N — “don’t chase the night with chlorine.” In the moment I trust DPD total and use online NH3 for trend; if your clearwell RT is short, couple a tiny hypo trim to pH as a fallback — try a 5–[redacted]/L as CaCO3 bump 11 p.m.–4 a.m. and see if the residual flattens with the pH hold. Is your NH3 analyzer temp‑compensated?
, we saw the same overnight slide — we stopped it by biasing the ratio down from “4.7:1” to about 4.5:1 only during “1–4 a.m.”, keeping hypo flat and nudging NH3 3–5% so mono doesn’t drift to di- when pH hits about 7.6; in the moment I trust DPD total over online NH3 (ours wanders with temp). Small caveat: it only stuck after we added a static mixer and about 2–3 min post-blend contact before the analyzer — do you have that much mix/CT right now?
Building on @jackson30, I’d tie the ratio to pH instead of the clock: keep about 4.7:1 when ≥7.8 and ramp toward about 4.55:1 as pH drifts to about 7.6, with a small deadband and a 10–15 min filter on the residual signal so you’re not playing whack‑a‑mole at 2 a.m. For readings, an amperometric total‑Cl with a glycine‑scrubbed channel flags free chlorine creep, and I only pull DPD mono/di as the tiebreaker — do you have pH upstream of the first mix point to drive the schedule?