The short answer
Filtration
A filter is controlled by holding its effluent rate or its water level with a modulating effluent valve, monitored by a head loss measurement and an individual turbidimeter on its effluent, and taken out of service for backwash when head loss, run time, or turbidity reaches its trigger. The backwash is a sequence of isolating, draining, air scouring where fitted, washing at rates that expand the bed, settling, and returning to service through a filter-to-waste period until the effluent turbidity is acceptable. Turbidity rules require continuous monitoring of each filter with recording at short intervals and define the values that trigger investigation, so the filter control system is also a compliance record.
Key points
- Each filter has a rate or level controller on its effluent valve, a head loss transmitter, and its own turbidimeter.
- Backwash triggers: head loss, run time, or turbidity, whichever comes first, and the operator can call one.
- The backwash is a sequencer: isolate, drain, air scour, wash, settle, filter to waste, return.
- Filter to waste until the effluent turbidity is below the ripening setpoint; the first minutes after a wash are the worst.
- Individual filter turbidity, recorded every 15 minutes, is the compliance record; the control system keeps it.
- Only one filter washes at a time, and the plant flow is rebalanced across the filters that remain.
What the filter does and what the controls do
A rapid gravity filter passes coagulated and settled water down through a bed of sand, anthracite, or both, and the bed captures the remaining particles. As it captures them the head loss through the bed rises, and eventually the filter either clogs or begins to pass particles, and it is backwashed. The control system has three jobs: hold each filter at the rate or level the plant needs while it runs, watch head loss and turbidity to decide when the run is over, and run the backwash sequence safely and repeatably. Pressure filters do the same in a closed vessel with pump-driven flow, with pressure and differential pressure in place of level and head loss.
Rate and level control
| Mode | How it works | Notes |
|---|---|---|
| Constant rate, constant level | A level controller on the filter modulates the effluent valve to hold the water level; the influent flow is split evenly among filters by weirs or influent valves, so each filter runs at a constant rate | The common arrangement; the effluent valve opens gradually over the run as head loss grows |
| Constant rate by flow | A flow controller on the effluent holds a rate setpoint, and the level rises over the run | Needs a flowmeter per filter; the level must be watched |
| Declining rate | Effluent valves are fixed; flow declines as head loss grows; the level rises across the bank | Simple, few instruments, less control over individual filters |
In constant level control, the loop is slow and the valve is large, so the controller is tuned gently and the valve travel is limited to avoid cycling. Filter loading rate, in gallons per minute per square foot, is the number the process engineer cares about; the control system computes it from flow and filter area and displays it. When a filter is removed for backwash, the plant flow is redistributed among the others, and a rate-of-change limit keeps the remaining filters from taking a step that shakes particles loose.
Head loss and turbidity
Head loss is the difference between the water level above the media and the pressure below it, measured by a differential pressure transmitter or by two level transmitters, and it is the fundamental indicator of a filter run. Clean bed head loss is a foot or two; terminal head loss, where the run ends, is set by the plant, often around eight feet on a gravity filter. Turbidity on each filter effluent, from an online turbidimeter with a sample flow that is itself monitored, is the measure of whether the filter is doing its job and the measure regulators watch. A filter whose turbidity rises before its head loss reaches the terminal value has broken through and is washed on turbidity.
| Trigger | Typical setting | What it protects |
|---|---|---|
| Terminal head loss | Around 6 to 10 feet of water on gravity filters | Media and underdrain; prevents air binding and a stalled filter |
| Turbidity | Effluent above a setpoint such as 0.15 or 0.2 NTU for a sustained period | Water quality; breakthrough ends the run before the head loss does |
| Run time | 24 to 96 hours depending on the plant | Biological growth and mudballs; a filter is washed on time even if it looks fine |
| Operator call | Any time | Maintenance, unusual conditions |
The backwash sequence
A backwash is a sequencer in the controller, with steps, transitions on feedback, timeouts, and a defined response to a fault. The sequence below is typical; the durations and rates come from the plant design and are setpoints on the HMI, not constants in the code.
- 1
Request and permissives
No other filter washing; wash water supply available at the tank level or pump; waste washwater capacity available; the filter effluent valve closed by the sequence.
- 2
Isolate
Close the influent valve and confirm; close the effluent valve and confirm.
- 3
Drain down
Open the drain or waste valve until the level falls to the air scour level, with a timeout.
- 4
Air scour
Start the blower and open the air valve for the design time; on some filters this is combined with a low-rate water wash.
- 5
Water wash
Stop the air, open the washwater valve, and ramp the wash rate to the design value that expands the bed by 20 to 30 percent; hold for the design time, or until the waste turbidity falls below a setpoint. Surface wash or a high-rate step where fitted.
- 6
Settle
Stop the wash, close the washwater and waste valves, and wait for the bed to settle.
- 7
Filter to waste
Open the influent valve, open the filter-to-waste valve, and run until the effluent turbidity falls below the ripening setpoint or the maximum time elapses.
- 8
Return to service
Close the filter-to-waste valve, open the effluent valve, and hand the filter back to its rate or level controller with a ramp.
Compliance monitoring
Under the surface water treatment rules in the United States, conventional plants monitor turbidity on the combined filter effluent at least every four hours with limits on the fraction of readings above set values, and they monitor each individual filter continuously with values recorded at least every 15 minutes. Readings above defined values on an individual filter for defined durations trigger reporting, a filter profile, a self-assessment, or a comprehensive performance evaluation. The control system is the recorder: it stores the individual filter turbidity at the required interval, flags the exceedances with the filter and the time, and keeps the record for the retention period. A turbidimeter with a lost sample flow or a failed instrument is a monitoring gap the system must alarm.
The operator view
- A filter bank overview: each filter with its state, rate, level, head loss, turbidity, run time since wash, and volume since wash.
- A filter detail: the valves and their positions, the controller, the trends, the wash history, and the sequence step when washing.
- Backwash controls: start, hold, resume, abort, and the supervisor advance, with the current step, the step timer, and what the step is waiting for.
- Setpoints: terminal head loss, turbidity triggers, run time limit, wash rates and durations, filter-to-waste turbidity and time, all with limits.
- Reports: unit filter run volume per run, wash water used, turbidity compliance summaries, and exceedances.
Frequently asked questions
- Why filter to waste after a backwash?
- A freshly washed bed passes particles for the first minutes until it ripens, and the effluent turbidity spike after a wash is often the highest turbidity a filter produces all run. Sending that water to waste until the turbidimeter shows it has cleared keeps the spike out of the clearwell and out of the compliance record.
- What is unit filter run volume and why track it?
- The volume of water filtered per square foot of filter area between washes. It combines rate and run length into one number that says how productive a run was, and a falling value over months means the media, the coagulation, or the wash is deteriorating. Compute it from the flow total and the area at each wash and trend it.
- Can two filters wash at once?
- Usually not: the wash water supply and the waste handling are sized for one, and two filters out at once overloads the rest. The sequence permissive checks that no other wash is in progress and queues requests. A plant designed for two simultaneous washes says so in the functional description.
- How is head loss measured on a filter?
- With a differential pressure transmitter between the water above the media and the effluent below it, or by subtracting a pressure below the underdrain from the level above the bed. Either way it is the drop across the media, in feet of water, and it should read near the clean bed value right after a wash; a higher reading after a wash means the wash was inadequate or the taps are plugged.
Related topics
- SequencersStep-based control for operations that always run in the same order: backwash, pump and valve startup, batch makeup, membrane cleans. What every step needs, three ways to implement one, handling faults and power loss mid-sequence, and what the HMI shows.
- Turbidity MeasurementNephelometric turbidimeters for filter effluent and raw water: how 90-degree scattering is measured, the methods and the NTU, low-range versus high-range instruments, bubbles and the debubbler, formazin calibration and verification standards, and the compliance record the instrument exists to produce.
- Differential PressureThe differential pressure transmitter as an instrument: what it measures, static and overrange ratings, the three-valve manifold and the order of operations that protects the cell, and the applications from filter head loss to membrane TMP, flow, and level.
- Raw WaterControlling the front of a surface water plant: intake and screens, low-lift pumping, the raw water flow that paces the whole plant, the online quality instruments that warn of a changing source, and the alarms and ramp limits that keep the train stable.
- Finished Water StorageStorage inside the plant: the clearwell that provides contact time and high service suction, backwash supply tanks, and how their levels drive production and pumping. The contact time calculation and the minimum level it demands, water age, and alarms.
- PermissivesThe conditions that must be true before a device may start: how permissives differ from interlocks and trips, the list a pump or feeder usually needs, coding them so the HMI can say why a start was refused, and the rules for time delays and bypasses.
Direct contact
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