After a 01:30 backwash last night, filtered effluent turbidity decreased from 0.11 to 0.05 NTU, but the clearwell grab at 02:00 showed HPC jumping from 28 to 190 CFU/mL with a steady [redacted]/L free chlorine (DPD). Has anyone tied this kind of transient to biofilm sloughing or short-circuiting, and did ATP or a rhodamine tracer confirm the contamination pathway?
@OP, that “01:30 backwash” likely sent a hydraulic pulse that shook loose clearwell biofilm — turbidity down while HPC spikes is classic snow‑globe stuff. Try a rhodamine tracer during the next backwash and compare ATP pre- vs post-clearwell to confirm the pathway; curious whether your filter‑to‑waste or baffling is enough to damp that surge.
I’m with @brown89 on sloughing, but check the clock against your clearwell t10; the 02:00 grab might be catching a lazy-river shortcut. Quick win: drop a small rhodamine WT slug at the filter effluent and watch the clearwell outlet with a handheld fluorometer; “early breakthrough = short-circuiting”, clean curve points to biofilm (then swab a baffle for ATP)?
Quick check: pull a 02:00 duplicate and run HPC on both unfiltered and 0.45 µm–prefiltered splits; if the filtered count collapses, you’re looking at floc‑shielded detritus that chlorine can’t hit immediately (Russian‑doll effect). Small caveat: rule out the tap — swab/plate the clearwell sample line or grab from a sterilized port; if you run rhodamine, @amewill’s t10 point will tell you fast whether short‑circuiting is in play.