Turnover Time Versus Actual Flow in a Lap Pool
A 25 m × 12 m lap pool has an average depth of 1.6 m. The design turnover is 6 hours. The circulation pump nameplate is rated at 180 m³/h, but the installed flow meter reads 120 m³/h at normal operating pressure. The filter is a single high-rate sand unit with a design flow of 150 m³/h. The pool holds a swim meet in three days and the operator is asked whether the water quality will meet the code requirement of a 6-hour turnover. Recent readings: free chlorine 1.8 mg/L, combined chlorine 0.3 mg/L, pH 7.5, clarity good, but the deep end shows slight haze under the lights at night.
1. Calculate the pool volume and the actual turnover time at the measured flow, then determine whether the pool meets the 6-hour requirement.
Read the worked answer
Volume: 25 m × 12 m × 1.6 m = 480 m³.
Required flow for 6-hour turnover: 480 m³ ÷ 6 h = 80 m³/h.
Actual turnover at measured flow: 480 m³ ÷ 120 m³/h = 4.0 hours.
So on paper the pool exceeds the 6-hour requirement - 4.0 h is faster than 6 h. But two cautions matter for the judgement call:
- The nameplate 180 m³/h is irrelevant; only the measured 120 m³/h counts. If the meter is wrong or the pump is throttled, the real number could differ.
- The filter is rated at 150 m³/h and is being run at 120 m³/h - within rating, so filtration is not the bottleneck. However, a single filter with no redundancy means any backwash or maintenance takes the whole pool offline.
The haze under lights despite an adequate turnover suggests the problem is not turnover but filtration effectiveness or chemistry, not flow. Turnover time alone does not guarantee clarity.
2. Given the slight haze and a swim meet in three days, what would you investigate and what would you change before the event?
Read the worked answer
Turnover is adequate, so do not chase flow. Investigate in this order:
- Filter condition. Check filter pressure trend and last backwash date. A rising pressure differential or a filter that has not been backwashed on schedule passes fines and causes haze. Backwash and, if pressure remains high, consider media inspection.
- Coagulant/flocculant dosing. Haze that appears under lights is often colloidal - fine particles too small for sand alone. Verify the coagulant feed is working and at the correct dose; this is the most likely fix for 'good chemistry but hazy water.'
- Chemistry balance. pH 7.5 is fine, but check total alkalinity and calcium hardness; poor balance can cause scaling haze or cloudy water. Confirm combined chlorine stays ≤0.4 mg/L through the meet.
- Bather-load plan. A meet will spike demand. Pre-emptively raise free chlorine slightly and plan mid-event testing; have a backup plan to reduce load if ORP drops.
- Contingency. With one filter and no redundancy, schedule any backwash before the meet, not during, and confirm the flow meter is calibrated so the 4.0 h turnover figure is trustworthy.
Do not declare the pool compliant on turnover alone - clarity is a code criterion in its own right.