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Drinking Water Filtration Instrumentation: Measurement Points and Selection Guide
Filtration is one of the final particle-removal stages before disinfection or clear-water storage in a drinking water treatment plant. Reliable operation requires both water-quality and hydraulic monitoring, including turbidity, flow, level, filter head loss and backwash conditions.
Because these measurement points operate under different process conditions, instrument selection should be based on the actual medium, range and installation environment. The process diagram below shows the main drinking water filtration instrumentation points and the corresponding ARTang instruments.
Filtration Process Flow and Instrumentation
The diagram illustrates the recommended monitoring points for filtration and backwash processes. Explore each measurement area by clicking the diagram markers or the sections below to learn about suitable instruments and application considerations.
Filter Inlet Monitoring · Filtration Flow Monitoring · Filter Level and Head-Loss Monitoring · Filter Outlet Quality Monitoring · Backwash-Water Flow Monitoring · Backwash Performance Monitoring
1. Filter Inlet Monitoring
Filter inlet monitoring provides an early indication of the water quality and hydraulic loading entering the filtration stage. At this point, turbidity is used to monitor residual particles and flocs remaining after sedimentation or clarification, while inlet flow is discussed in the following section together with filtered-water flow.
Filter Inlet Turbidity
A rising inlet-turbidity trend may indicate a change in upstream coagulation, flocculation or sedimentation performance. It may also shorten the filter run because a higher solids load increases particle accumulation within the filter media.
Clarified-water turbidity is normally higher and more variable than filtered-water turbidity. Residual flocs, air bubbles, settling particles and optical fouling may therefore affect measurement stability.
Recommended Instrument
- ATS11 or ATS12 Turbidity Sensor
ATS11 is suitable for applications with higher turbidity levels or wider measurement ranges. ATS12 is designed for lower turbidity measurement, such as filtered or clarified water applications where higher sensitivity is required.
The selection should reflect the actual turbidity range, installation method and sample conditions. Direct installation may simplify the measurement arrangement, while sample-line measurement can provide more controlled conditions if representative flow and regular cleaning are maintained.
Application Note
The filter inlet, outlet and backwash-waste line may require different turbidity ranges and sensor configurations. Inlet water may contain variable flocs and bubbles, while the outlet requires stable low-range measurement and backwash wastewater may have much higher, faster-changing turbidity.
2. Filtration Flow Monitoring
Flow measurement is used to control the filtration rate, balance parallel filters and totalize filtered-water production.
Depending on the plant arrangement, the flow meter may be installed on the filter inlet, the individual filtered-water outlet or the combined filtered-water header.
Where several filters operate in parallel, flow measurement helps identify uneven distribution. Even when the total plant flow remains stable, one unit may receive a higher hydraulic load than the others, which can shorten its filter run and reduce process consistency.
Recommended Instrument
Aimag-A is suitable for conductive drinking water and provides full-bore measurement without moving parts or significant internal flow obstruction. It can support filtration-rate control, parallel-filter balancing and filtered-water production totalization.
Key Selection Considerations
An electromagnetic flow meter should be selected according to the actual minimum, normal and maximum flow rather than pipe diameter alone.
A large existing pipe may operate at a relatively low liquid velocity. If the meter is simply matched to the pipeline without checking the flow range, measurement stability may be reduced.
The water must be conductive, and the measuring tube must remain completely full. Installation points where air may accumulate or where the pipe may run partially full should be avoided. Grounding and the expected flow direction should also be confirmed before configuration.
Application Note
An electromagnetic flow meter cannot measure compressed air, another gas, non-conductive liquids or open-channel flow.
Where one pipeline is expected to handle both normal filtration flow and a much higher backwash flow, both ranges should be evaluated carefully. In many systems, separate meters provide better measurement performance for the two operating conditions.
3. Filter Level and Head-Loss Monitoring
Water level and filter head loss describe the hydraulic condition of a gravity filter. Although they are different measurements, both are influenced by the movement of water through the filter bed.
Filter operation should not rely on runtime alone. Outlet turbidity, filtration rate and filter head loss should be evaluated together when deciding whether the filter can remain in service or should enter a backwash cycle.
Gravity-Filter Water Level
The water level above the media provides the hydraulic head required to move water through a rapid gravity filter.
An abnormal level may indicate inlet or outlet valve problems, uneven filtration flow, drainage restrictions or changing resistance through the media.
Recommended Instruments
UA provides non-contact measurement and is suitable where the transmitter can be installed above the water surface with a clear acoustic path.
Foam, turbulence, internal structures and the ultrasonic blind zone should be considered when selecting the mounting position.
AE12 measures level through hydrostatic pressure and is installed directly in the water.
It may be more suitable where overhead installation is difficult or where foam, structural obstructions or false echoes make ultrasonic measurement less reliable. Liquid density, venting, cable design and long-term submerged conditions should be confirmed.
Application Note
UA depends on obtaining a stable echo from the water surface. AE12 depends on correct density compensation, atmospheric venting and reliable submerged installation.
The choice should therefore be based on the actual basin conditions rather than a general preference for contact or non-contact measurement.
Filter Head Loss
Filter head loss indicates the resistance developing across the filter media as retained solids accumulate.
A rising head-loss trend may be used as one of the conditions for initiating backwashing. However, low head loss does not necessarily confirm that the filter is operating correctly.
Media channeling, media loss, damaged underdrains or internal bypass may allow particles to pass through without creating a large increase in hydraulic resistance. This is why head loss should always be evaluated together with outlet turbidity.
Recommended Instrument
AED23 compares the pressure at the high- and low-pressure measurement points across the filter and provides a continuous differential-pressure signal.
Key Selection Considerations
The transmitter may need to resolve a relatively small differential pressure while both sides remain subject to a higher static pressure.
Selection should therefore consider not only the differential-pressure range, but also static-pressure capability, measurement resolution, tapping locations and impulse-line arrangement.
Incorrect pressure tapping, trapped air, blocked impulse lines or an unsuitable measuring range can produce unstable or misleading readings.
Application Note
For an open gravity filter, differential-pressure measurement should only be used where the high- and low-pressure points are hydraulically valid.
Some gravity-filter designs may be monitored more reliably using a combination of water-level and outlet-pressure measurement rather than a conventional differential-pressure arrangement.
4. Filter Outlet Quality Monitoring
Filter outlet turbidity is the main water-quality measurement used to verify the performance of each filter and detect particle breakthrough.
A rising turbidity trend may indicate media channeling, internal bypass, media loss, hydraulic overloading or incomplete stabilization after backwashing.
Recommended Instrument
- ATS12 Low-Range Turbidity Sensor
Filtered water normally has much lower turbidity than filter inlet water or backwash wastewater. The main requirement is therefore stable low-range measurement rather than a very wide measuring range.
ATS12 can provide a continuous measurement signal to the control system for high-turbidity alarms, filter-performance comparison, filter isolation and filter-to-waste control.
The sensor provides the measurement value. The final control action is carried out by the plant control system.
Why Individual Filter Monitoring Matters
Where several filters operate in parallel, turbidity should normally be measured at each individual filter outlet before the water streams are combined.
If the measurement point is installed only after several outlets are blended, low-turbidity water from the other filters may dilute the abnormal signal from one unit. This can delay breakthrough detection and make it difficult to identify which filter requires attention.
Key Selection Considerations
Reliable low-range turbidity measurement requires a representative measurement point, stable sample flow, minimal bubble interference and regular cleaning and calibration.
The sensor range should match the expected filtered-water turbidity rather than the higher inlet or backwash-waste range.
Application Note
After backwashing, outlet turbidity may rise temporarily while the filter stabilizes.
The filter may therefore remain in filter-to-waste mode until both water-quality and hydraulic conditions return to the approved operating range.
5. Backwash-Water Flow Monitoring
Backwash-water flow controls the hydraulic intensity used to clean the filter media.
Too little flow may leave retained solids within the filter bed. Too much flow may increase water consumption, expand the media excessively or contribute to media loss.
The required backwash flow is normally much higher than the regular filtration flow and may operate only during a short cleaning cycle. The flow meter should therefore be selected for the backwash range rather than the normal service flow.
Recommended Instrument
Aimag-A Electromagnetic Flow Meter
Aimag-A is suitable for clean conductive backwash water and can support backwash flow control, cycle verification and water-use totalization.
Key Selection Considerations
The meter should be sized according to the minimum and maximum backwash flow, expected velocity and pipe diameter.
The line must remain completely full during measurement. Rapid valve operation, pump start-up or poor pipeline arrangement may introduce air into the pipe, creating unstable readings even though the water itself is suitable for electromagnetic measurement.
Where reverse flow may occur, the required flow direction, output polarity and totalization logic should be confirmed before transmitter configuration.
Application Note
A meter capable of detecting reverse flow does not automatically mean that forward and reverse flow should be accumulated in the same totalizer.
The plant control requirement should determine whether separate directional totals or only one primary flow direction is needed.
6. Backwash Performance Monitoring
In addition to backwash-water flow, air-scour flow, pump or blower pressure and backwash-waste turbidity can be monitored to evaluate whether the cleaning system is operating as intended.
Air-Scour Flow
Air scour helps loosen deposits within the filter media before or during water backwashing.
Compressed-air measurement is affected by gas pressure, temperature, operating velocity, straight-pipe conditions, vibration and condensate. The meter should therefore be selected according to the actual air-scour conditions rather than pipe size alone.
Recommended instruments: VF Vortex Flow Meter or ATF Thermal Mass Flow Meter
VF is suitable where air velocity is sufficient and the installation provides adequate straight pipe with limited vibration. Low velocity, heavy vibration and condensate may reduce measurement stability.
ATF is more suitable where direct gas mass-flow measurement, lower flow or wider turndown is required. Gas composition, moisture and deposits should be confirmed because they can affect thermal measurement.
The choice between VF and ATF should be based on the actual gas type, flow range, pressure, temperature and installation conditions.
Pump or Blower Pressure
Pressure monitoring helps verify the operating condition of the backwash-water pump, air blower and associated pipework.
Low flow combined with abnormal pressure may indicate valve restriction, blocked distributors, excessive pipeline resistance or changes in pump or blower performance.
Recommended instruments: AE11 Pressure Transducer or AEA16 Pressure Transmitter
Both models can provide continuous pressure signals for equipment and pipeline monitoring. The final choice should reflect the required pressure range, accuracy, local display, communication function and installation environment.
Pressure pulsation, vibration and overpressure should also be considered.
Backwash-Waste Monitoring
Backwash-waste turbidity reflects the solids being removed from the filter media during cleaning.
The turbidity trend can help operators observe the progress of the backwash cycle, but one universal endpoint should not be applied to every plant. The expected profile depends on raw-water conditions, filter media, accumulated solids and the selected backwash strategy.
Recommended instrument: ATS11 Turbidity Sensor
ATS11 is generally suitable for the higher and more rapidly changing turbidity found in backwash wastewater. Selection should consider the expected turbidity range, solids loading, bubbles, fouling risk and cleaning method.
Where backwash-wastewater flow also needs to be measured, Aimag-A may be used for general water with low-to-moderate solids, while Aimag-S may be considered for higher-solids or more demanding slurry service.
Both flow meters require conductive liquid and a completely filled pipe. Solids concentration, fibers, abrasion, minimum velocity and wetted materials should be confirmed before selection.
Drinking Water Filtration Instrument Selection Summary
|
Measurement Point |
Recommended Instrument |
Key Selection Considerations |
|---|---|---|
|
Filter inlet turbidity |
ATS11 or ATS12 |
Turbidity range, flocs, bubbles and installation |
|
Filtration flow |
Aimag-A |
Actual flow range, conductivity, velocity and full-pipe condition |
|
Gravity-filter water level |
UA or AE12 |
Blind zone, foam, turbulence, density and mounting |
|
Filter head loss |
AED23 |
Differential pressure, static pressure and tapping arrangement |
|
Filter outlet turbidity |
ATS12 |
Low-range stability, representative sampling and cleaning |
|
Backwash-water flow |
Aimag-A |
Maximum flow, full-pipe condition and flow direction |
|
Backwash-air flow |
VF or ATF |
Velocity, pressure, temperature, moisture and vibration |
|
Pump or blower pressure |
AE11 or AEA16 |
Pressure range, pulsation and overpressure |
|
Backwash-waste monitoring |
ATS11; optional Aimag-A or Aimag-S |
Turbidity, solids, fouling and abrasion |
FAQ
Blending can dilute an abnormal turbidity signal from one filter. Individual outlet measurement provides earlier detection of filter breakthrough and makes filter-to-filter comparison possible.
Use the model whose verified range matches the actual clarified-water turbidity. ATS11 is generally suitable for higher or more variable turbidity, while ATS12 may be considered for lower ranges. Bubbles, residual flocs, cleaning and installation conditions should also be confirmed.
No. Aimag-A and Aimag-S measure conductive liquids in completely filled pipes. Backwash air requires a gas flow meter such as the VF vortex flow meter or ATF thermal mass flow meter.
UA is preferred for non-contact measurement when a clear acoustic path is available. AE12 may be more suitable when overhead installation is difficult or when foam, obstacles or false echoes make ultrasonic measurement unreliable, provided submerged installation is acceptable.
VF is suitable when the air velocity, straight-pipe length and vibration conditions meet vortex-meter requirements. ATF may be more suitable for direct gas mass-flow measurement, lower flow or wider turndown. Gas composition, moisture and deposits must also be considered.
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