Storage Tank Turnover, Residual Loss, and Coliform Positives
A 1.4 MGD average-day water system has one 1.0 MG elevated tank with 42 ft of usable height (about 24,000 gal per foot) and a single fixed-speed 1,200 gpm high-service pump. Night demand is about 0.6 MGD. Over the last month the tank has swung only 1.8 ft per day. Chlorine residual is 1.05 mg/L leaving the plant but 0.15 mg/L at the tank outlet and 0.05 mg/L at two dead-end hydrants three miles past the tank. TOC is 2.1 mg/L and water temperature is 26°C. Operators report the pump short-cycles every 4-6 minutes, and the tank level sits near 78% all day. The system has had two total-coliform-positive samples in the last six months (neither E. coli), and the state has asked for an assessment.
1. Compute the tank's actual turnover and explain the residual profile from plant to dead end.
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Daily movement: 1.8 ft x 24,000 gal/ft is about 43,200 gal/day, which is 4.3% of the 1.0 MG tank.
Theoretical detention: 1,000,000 / 43,200 is about 23 days.
So most of the tank is not turning over. Water sits in a warm (26°C), slightly organic (2.1 mg/L TOC) tank for weeks, and free chlorine decays roughly first-order - the decay rate roughly doubles for every 10°C rise, so 26°C stagnant water loses residual fast. That explains 1.05 mg/L at the plant decaying to 0.15 mg/L at the tank outlet and 0.05 mg/L three miles beyond.
Interpretation: the two non-E. coli total coliform hits are consistent with regrowth and biofilm in low-residual, high-water-age zones. They do not by themselves prove treatment failure, but they do point at the distribution system, and the state assessment will look exactly here. The fix is hydraulic, not more chlorine at the plant - adding chlorine upstream of a stagnant tank just makes DBPs in the tank.
2. Diagnose why the tank is not turning over and lay out corrective actions in priority order, including their constraints.
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Diagnosis - it is a control problem, not a supply problem:
- The 1,200 gpm pump equals 1.73 MGD; night demand is about 0.6 MGD. The pump is oversized for night flow.
- Cycling every 4-6 minutes means the level band is only a few tenths of a foot, so the tank floats near one level, only a shallow slice mixes, and the rest stays stagnant. That is why the daily swing is 1.8 ft even though the pump runs constantly.
Corrective actions in order:
- Widen the operating band: e.g., fill to about 90% and draw to about 55% - 14.7 ft, or roughly 350,000 gal/day of swing, giving about 3 days of turnover. Constraint: verify minimum pressure (and fire flow) at the low level before you commit; a wider band means lower pressures at the far end.
- Stop the short-cycling: VFD, impeller trim, or a small jockey pump. Check motor starts per hour against the manufacturer limit (often 6-10 per hour) and check for surge.
- Address short-circuiting: mixer in the tank, or a fill/draw arrangement that does not let inlet water run straight to the outlet.
- Flush both dead ends on a scheduled program and add them to routine residual checks; consider booster chlorination as an interim measure.
- Verify the altitude valve and check valves are working as intended.
Prediction: turnover under 5 days and residual above 0.5 mg/L at the dead ends within a few weeks; confirm with residual readings and coliform samples.
3. What does the coliform history trigger regulatorily, and how should the utility frame its response?
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Trigger: a routine total-coliform-positive sample (not E. coli) starts the RTCR assessment process - a Level 1 assessment, completed and submitted within 30 days. An E. coli positive would be Level 2. Do not treat this as a sampling error.
Response:
- Fix the cause now; the assessment is where you document that you found a storage/turnover deficiency and corrected it. That is exactly the finding the assessment is designed to produce.
- Check the state minimum disinfectant residual (commonly 0.2 mg/L free; the state sets its own number) against the 0.05 mg/L dead-end reading. The requirement applies throughout the distribution system, not at the plant tap.
- Repeat sampling within 24 hours of any positive, and increase routine distribution sampling while the problem is open.
- Review cross-connection control and storage inspection records; an unsealed or flooded tank vent can produce the same result.
- Keep records of flushing, valve adjustments, and residual readings as corrective-action evidence.
Judgment: low residuals in a high-water-age zone are a system problem. If the assessment shows an uncorrected storage deficiency, expect follow-up and possible enforcement; present the turnover calculation and the corrected valve band as your fix.