Tank Turnover and Chlorine Decay at a Remote Elevated Tank
A 1.0-million-gallon elevated tank serves a pressure zone with average demand of 0.9 mgd. The tank floats on the system and is refilled by the transmission main whenever system pressure exceeds the tank's float level. Recent sampling at a dead-end tap 500 feet from the tank shows free chlorine of 0.2 mg/L, below the utility's 0.5 mg/L internal target, and two total-coliform-positive samples in the last quarter (both non-fecal, both at the same tap). The tank's inlet and outlet share a single 16-inch pipe. Tank level records show the tank swings between 92% and 96% - it rarely draws down. Water age modeling suggests the tank's average residence time is about 9 days.
1. Explain the water-quality mechanism at work and why the single-pipe tank configuration matters.
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The tank is not turning over. With a single inlet/outlet pipe, the tank fills and draws from the same connection, so the bulk of the stored water is a stagnant layer that only slowly mixes. A 92-96% swing means only the top few percent of volume is exchanged; the rest sits for days. At 0.9 mgd average demand and a 1.0-MG tank, a full turnover would take over a day, but the actual exchange is a small fraction of that - the model's 9-day age is consistent with poor mixing and a shallow drawdown.
Stagnant, warm water loses chlorine residual (decay plus reaction with sediment and biofilm), which explains the 0.2 mg/L at the tap and creates conditions where coliform can regrow in the distribution main and tank. The single-pipe design also means the tank cannot be reliably isolated for cleaning without a shutdown, and there is no separate fill/draw path to promote mixing.
2. List the corrective actions you would take, in priority order, and the monitoring that confirms they worked.
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Priority actions:
- Increase turnover: adjust the float level and pump/valve controls so the tank draws down to, say, 60-70% before refilling. This forces a real exchange and lowers water age.
- Improve mixing: install a tank mixer (or a fill-and-draw baffle/altitude valve arrangement) to break up stratification.
- Flush and disinfect the dead-end main near the tank and the tank itself if inspection shows sediment/biofilm; clean the tank interior if warranted.
- Booster chlorination or a setpoint increase at the plant if residual still decays below target after turnover improves.
- Investigate the coliform tap: resample, check for a cross-connection or a service-line issue, and review the sampling site.
Monitoring: daily tank level logs to confirm drawdown range; chlorine at the tank outlet and at the dead-end tap (target ≥0.5 mg/L); coliform sampling per the site's routine schedule plus repeat samples after any positive; water-age model rerun to confirm reduced residence time.
3. A customer calls complaining of a chlorine taste after you raise the setpoint. How do you balance the two complaints?
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Chlorine taste complaints usually appear above roughly 1.0-1.5 mg/L free chlorine at the tap, while the coliform risk appears below about 0.2-0.5 mg/L. The goal is to keep the tap residual in a band (e.g., 0.5-1.0 mg/L) rather than push the plant setpoint blindly.
Approach: fix turnover and mixing first (that raises residual at the far end without raising the plant dose), then trim the plant setpoint to the lowest value that holds the dead-end tap at target. Sample at the complaint address to see the actual residual. If taste persists at acceptable residual, advise the customer to chill the water (chlorine volatilizes) and document the complaint. Do not drop below the target just to please one customer; instead, solve the root cause - stagnation - so less chlorine is needed overall.