Solids Capture Rate
The percentage of feed solids retained in the thickened product rather than lost to the liquid stream.
Solids capture rate = (Feed Solids − Filtrate/Subnatant Solids) ÷ Feed Solids × 100, a direct measure of thickening efficiency independent of final cake or float concentration.
A high final solids concentration with a low capture rate means the process is concentrating what it retains well, but losing a meaningful share of solids back to the plant's liquid stream.
Lost solids in filtrate or subnatant return to the head of the plant, adding hidden loading that shows up as unexplained BOD or TSS increases elsewhere.
Track capture rate alongside final concentration; chemical conditioning (polymer) dose adjustments often improve one at the expense of the other, so both need monitoring together.
Worked example: feed solids of 0.9%, filtrate solids of 0.08% — capture rate = (0.9 − 0.08) ÷ 0.9 × 100 ≈ 91%, a solid result for most mechanical thickening equipment.
Well-run gravity thickeners and DAF units typically achieve 90-98% capture rate; a sustained result below 85% usually points to an underdosed or poorly activated polymer rather than a mechanical fault.
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