The short answer
Finished Water Storage
The clearwell holds finished water long enough for disinfection to be credited, buffers the difference between the production rate and the demand, and provides suction for the high service pumps. Its level therefore sets the plant production setpoint, has a minimum below which contact time is not met and the pumps are cut off, and has a maximum where production is reduced and the overflow alarms. Contact time is computed from the effective volume at the current level, the flow through the tank, and the baffling factor, and the control system displays it and alarms when it falls short. Backwash supply tanks and other in-plant storage are managed for availability, and all of them are watched for water age, mixing, and security.
Key points
- The clearwell level is the plant production setpoint, the high service suction, and the disinfection contact volume at once.
- Contact time falls with level; the minimum operating level is a compliance setting, not just a pump protection.
- Low level cuts off the high service pumps and alarms; high level cuts production and alarms before the overflow.
- Compute and display the contact time continuously from level, flow, and the baffling factor.
- Water age and stratification are storage problems; turnover and mixing are the answers.
- Backwash supply is a separate availability check before a wash can start.
What the clearwell does
Finished water leaves the filters and enters the clearwell, where chlorine is added or has just been added, and where it sits long enough for the disinfectant to do its work before the water is pumped to the distribution system. The clearwell also lets the plant produce at a steady rate while the demand varies through the day, and it gives the high service pumps a suction supply. Those three jobs pull in different directions: contact time wants the tank full, buffering wants room to rise and fall, and the pumps want a level that never gets low. The control settings are the compromise, and they are written down.
Contact time
Disinfection credit is computed as the disinfectant concentration at the clearwell outlet multiplied by the time the water spends in the tank, with the time taken as the time for the first ten percent of the water to pass through, which is the theoretical detention time reduced by a baffling factor that reflects how much short-circuiting the tank allows. A tank with no baffles has a small factor; a well-baffled serpentine tank has a large one. The required value depends on the pathogen, the temperature, and the pH, and comes from the regulatory tables.
CT = C × T₁₀ = C × (V_at_level × BF) / Q
- C = disinfectant residual at the outlet of the contact volume, mg/L
- V_at_level = the volume of the clearwell at the current level
- BF = the baffling factor, from a tracer study or the regulatory default for the tank geometry
- Q = the flow through the clearwell, usually the high service pumping rate
The control system computes this continuously from the level transmitter, the flow, the residual analyzer, and the temperature and pH used to look up the required value, and it displays the ratio of achieved to required. The minimum operating level is the level at which the ratio reaches one at the maximum pumping rate; below it the plant is out of compliance, so the level low alarm and the high service cutoff are set at or above it, with the cutoff protecting compliance as well as the pumps.
Level control
| Level | Action |
|---|---|
| High high | Alarm; stop or minimize production; overflow imminent |
| High | Reduce the production setpoint; alarm if sustained |
| Normal band | Production follows a slow level controller or a schedule; high service pumps run on demand |
| Low | Increase production; alarm; contact time ratio displayed and approaching one |
| Low low | High service pumps cut off; contact time not met below this level; alarm |
The production controller that follows level is slow and ramp-limited so that the treatment process sees gradual changes. The high service pumps do not follow the clearwell level at all; they follow the distribution system, and the clearwell absorbs the difference. That means the two controllers can fight: high demand drains the clearwell while production is still ramping. The band between the low and low-low levels is the buffer that gives production time to catch up, and it is sized from the maximum demand and the maximum ramp rate.
Water age and mixing
Water that sits in storage loses disinfectant residual, warms, and can stratify, with the oldest water at the top of a tank that fills and drains from the bottom. In the clearwell that is managed by keeping the level cycling, by the inlet and outlet arrangement, and by mixing where the tank is large. The larger problem is in distribution storage, where tanks that stay full for reliability turn over slowly; that is covered on the storage tank pages. In the plant, the control system tracks a simple water age from the volume and the flow and alarms a residual at the clearwell outlet that falls below the setpoint.
Other in-plant storage
- Backwash supply tank or elevated washwater tank: level measured, a minimum available volume for a full wash is a permissive for starting one, and refilling is sequenced so that it does not coincide with a wash.
- Waste washwater and equalization tanks: level for the return pumps and the recycle rate to the head of the plant, which is limited by the recycle rules.
- Chemical day tanks and bulk tanks: level, days remaining, low alarms, and secondary containment leak detection.
- Reclaim and reuse water tanks: level and the interlocks that keep them separate from finished water.
Frequently asked questions
- Why does the high service pump cutoff level seem high?
- Because it is set for contact time, not for pump suction. The level below which the clearwell cannot deliver the required contact time at the pumping rate is often several feet above the level where the pumps would lose suction. The cutoff is a compliance setting, and raising the pumping rate raises it.
- How is the baffling factor determined?
- By a tracer study, in which a tracer is injected at the inlet and its arrival at the outlet is measured to find the time for ten percent to pass, or by the regulatory default for the tank geometry, which is conservative. A tracer study usually earns a higher factor and more credit, and it has to be repeated if the tank is modified.
- Should the clearwell be kept as full as possible?
- For contact time, yes; for buffering and water age, no. Most plants run in a band in the upper part of the tank that keeps contact time comfortable and still cycles the level daily so that the water turns over. The band is a setpoint pair on the HMI, chosen from the contact time calculation and the daily demand pattern.
- What happens if the level transmitter fails?
- The contact time calculation, the production controller, and the pump cutoff all lose their input. Validate the signal, provide a second level measurement or backup floats at the critical levels, and define the fallback: production holds its last setpoint and the pumps continue only if a backup low-level switch is satisfied.
Related topics
- Disinfection ControlControlling chlorine and chloramine dosing at a water plant: the CT concept and how contact time is credited, flow-paced and residual-trimmed feed, gas, hypochlorite, and on-site generation systems, the point-of-entry residual as the compliance measurement, and the interlocks that stop a feed system from over- or under-dosing.
- High Service PumpingPumping finished water into distribution: the control modes of tank level, pressure, and flow, how they interact with the clearwell and contact time, staging and surge, the telemetry the station depends on and its fallback, energy scheduling, and the alarms.
- Tank Level ControlHow a storage tank is filled and drawn on level setpoints, how the setpoints are chosen to keep the water turning over, and what the station does when the tank stops talking.
- Elevated Tank ControlThe tank on legs that sets the zone pressure: how level equals pressure, the operating band, the altitude valve and its telemetry, turnover for water quality, filling by pumps far away, freeze protection, and what the level signal does when the tank is holding the system together.
- Radar Level MeasurementNon-contact and guided wave radar, why it handles a wet well better than most alternatives, and the installation details that decide whether it works.
- Chlorine Residual AnalyzersAmperometric and colorimetric residual analyzers: what each measures, free versus total chlorine, pH and flow dependence, sample line design, calibration against a DPD grab sample, and using the signal for feed control.
Direct contact
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