APP-05Application
Water Quality Monitoring: pH, Turbidity and Free Chlorine from Intake to Distribution
Water quality is measured where it changes: at the intake, through treatment, in storage and where water leaves the plant. Online analyzers turn periodic laboratory checks into continuous control — dosing, filter backwash and alarms that follow the process — while flow and level give each reading its context.
| Tag | Service |
|---|---|
| AT-501 | Raw-water pH |
| FT-502 | Raw-water flow |
| PDT-503 | Filter head loss |
| AT-504 | Turbidity after filtration |
| AT-505 | Free chlorine after contact |
| LT-506 | Treated-water storage level |
01Measurement map
Measurement points along a potable-water treatment train
Each instrument sits where it is typically installed. Select one to see why it is measured, the typical technology and how its signal reaches the control system.
- Process line
- Electrical signal
- Instrument (ISA tag)
- Selected signal path
- 1PLC / SCADA
- 2Intake
- 3Coagulant
- 4Coagulation & clarification
- 5Filtration
- 6Hypochlorite
- 7Disinfection
- 8Storage
- 9Distribution
Raw-water pH
- Why it matters
- pH governs how well coagulation works and how effective disinfection will be. Measured at the intake, it shows how the source is changing and sets the starting point for pH correction and coagulant dosing.
- Parameter
- pH
- Typical technology
- Glass-electrode pH sensor with temperature compensation
- Immersion or flow-through mounting with automatic cleaning where fouling is likely
- Conductivity measured alongside as a general indicator of dissolved solids
- Integration
- 4–20 mA or a digital protocol from the analyzer transmitter to the PLC for dosing control, alarms and SCADA trends.
- Why it matters
- pH governs how well coagulation works and how effective disinfection will be. Measured at the intake, it shows how the source is changing and sets the starting point for pH correction and coagulant dosing.
- Parameter
- pH
- Typical technology
- Glass-electrode pH sensor with temperature compensation
- Immersion or flow-through mounting with automatic cleaning where fouling is likely
- Conductivity measured alongside as a general indicator of dissolved solids
- Integration
- 4–20 mA or a digital protocol from the analyzer transmitter to the PLC for dosing control, alarms and SCADA trends.
- Why it matters
- Flow paces the coagulant and disinfectant doses, so the chemicals follow the water entering the plant instead of a fixed setting. It also gives the volume treated for operating and compliance records, and shows when clarifier and filter loading is changing.
- Parameter
- Flow rate, m³/h, and totalised volume
- Typical technology
- Electromagnetic flowmeter — no pressure loss, suited to raw and treated water
- Ultrasonic flowmeter, including clamp-on for existing pipes
- Open-channel flow by level and a flume or weir
- Integration
- 4–20 mA or pulse, or a digital protocol, to the PLC for flow-paced dosing and totals in SCADA.
- Why it matters
- As a filter holds back solids, the head loss across its bed rises. Trended alongside filtered-water turbidity and run time, it shows when the filter needs backwashing, and a sudden change points to a disturbance in the bed.
- Parameter
- Head loss across the filter bed (differential pressure)
- Typical technology
- Differential pressure transmitter with connections above the media and at the outlet or underdrain
- Filter-box level measurement where the filter control scheme uses it
- Local differential pressure indicator
- Integration
- 4–20 mA to the PLC, where head loss, turbidity and run time are combined to start the backwash sequence; trended per filter in SCADA.
- Why it matters
- Turbidity after filtration is the clearest online indicator that clarification and filtration are working. A rising value calls for backwashing or points to a dosing problem — before the water reaches disinfection and storage.
- Parameter
- Turbidity, NTU
- Typical technology
- Nephelometric (scattered-light) turbidimeter
- Low-range instruments for filtered water
- Flow-through measurement with bubble elimination
- Integration
- 4–20 mA or digital to the PLC; high-turbidity alarms and filter backwash logic, recorded in SCADA.
- Why it matters
- Chlorine needs contact time to disinfect. Free chlorine measured at the contact-tank outlet shows that the dose has done its work and that a residual will carry through storage into the network. Too little risks contamination; too much causes taste complaints and by-products.
- Parameter
- Free chlorine, mg/L
- Typical technology
- Amperometric chlorine sensor with pH compensation
- Colorimetric (DPD) online analyzer
- Flow-through panel with constant sample flow
- Integration
- Analog to the PLC for dosing trim — flow-paced with residual correction — plus low and high alarms and SCADA records.
- Why it matters
- Storage level balances production against demand. It starts and stops pumps, protects against overflow and dry running, and gives operators the reserve available for distribution.
- Parameter
- Level, m
- Typical technology
- Hydrostatic level transmitter
- Non-contact radar or ultrasonic level
- Float switches as independent high and low backup
- Integration
- 4–20 mA to the PLC for pump control and alarms; reserve volume in SCADA.
02Engineering context
Why measurement matters
Raw-water quality varies with season and source, so treatment has to follow it. pH and conductivity at the intake, turbidity after clarification and filtration, and free chlorine after disinfection show whether each stage is keeping up.
Chemical dosing depends on measurement. Coagulant and disinfectant doses are paced by flow and verified by an analyzer downstream, while filter head loss and filtered-water turbidity decide when a filter is backwashed.
Wastewater treatment follows the same approach with different parameters. In biological treatment, dissolved oxygen in the aeration basin controls the blowers — often the largest energy consumer on the site — while flow, pH and turbidity are recorded at the discharge.
Operating conditions
- Medium
- Raw water, process water, treated water or wastewater — solids, fats, biological growth and chemicals all affect sensors.
- Sensor mounting
- Immersion in channels and basins, or flow-through panels fed by a sample line.
- Fouling
- The main maintenance driver; automatic cleaning by air, water or wiper is common in raw water and wastewater.
- Chemistry
- pH shifts the balance of free-chlorine species and so affects chlorine readings; temperature, salinity and barometric pressure affect dissolved-oxygen readings. Analyzers commonly compensate for them.
- Environment
- Outdoor installations exposed to sun, rain and temperature swings; shelters or sun shields protect transmitters.
- Compliance
- Discharge permits and supply standards decide which parameters must be monitored and recorded.
03Measurement parameters
What is measured, where, and why
The parameters typically measured in this application, with the location, the reason and the technologies commonly used. The right selection always depends on the process conditions.
| Tag | Parameter | Location | Why it matters | Typical technology |
|---|---|---|---|---|
| AT | pH | Intake, after coagulant dosing, before disinfection | Coagulation and disinfection effectiveness | Glass-electrode pH sensor with temperature compensation |
| AT | Conductivity | Intake and treated water | Indicator of dissolved solids and source changes | Contacting or inductive (toroidal) conductivity sensor |
| FT | Flow | Raw-water inlet, plant outlet | Flow-paced dosing and volume records | Electromagnetic, ultrasonic, open-channel |
| AT | Turbidity | After clarification and after each filter | Clarifier and filter performance, backwash control | Nephelometric turbidimeter |
| PDT | Filter head loss | Across each filter bed | Backwash timing and filter condition | Differential pressure transmitter |
| AT | Free chlorine | Contact-tank outlet, plant outlet, network points | Disinfection maintained into the network | Amperometric sensor or colorimetric analyzer |
| LT | Level | Treated-water storage, sumps, channels | Pump control and reserve | Hydrostatic, radar, ultrasonic |
| AT | Dissolved oxygen | Aeration basin — wastewater treatment | Blower control and aeration energy in biological treatment | Optical or membrane electrochemical sensor |
ATpH
- Location
- Intake, after coagulant dosing, before disinfection
- Why it matters
- Coagulation and disinfection effectiveness
- Typical technology
- Glass-electrode pH sensor with temperature compensation
ATConductivity
- Location
- Intake and treated water
- Why it matters
- Indicator of dissolved solids and source changes
- Typical technology
- Contacting or inductive (toroidal) conductivity sensor
FTFlow
- Location
- Raw-water inlet, plant outlet
- Why it matters
- Flow-paced dosing and volume records
- Typical technology
- Electromagnetic, ultrasonic, open-channel
ATTurbidity
- Location
- After clarification and after each filter
- Why it matters
- Clarifier and filter performance, backwash control
- Typical technology
- Nephelometric turbidimeter
PDTFilter head loss
- Location
- Across each filter bed
- Why it matters
- Backwash timing and filter condition
- Typical technology
- Differential pressure transmitter
ATFree chlorine
- Location
- Contact-tank outlet, plant outlet, network points
- Why it matters
- Disinfection maintained into the network
- Typical technology
- Amperometric sensor or colorimetric analyzer
LTLevel
- Location
- Treated-water storage, sumps, channels
- Why it matters
- Pump control and reserve
- Typical technology
- Hydrostatic, radar, ultrasonic
ATDissolved oxygen
- Location
- Aeration basin — wastewater treatment
- Why it matters
- Blower control and aeration energy in biological treatment
- Typical technology
- Optical or membrane electrochemical sensor
04Field → Control → Supervision
From the field to the control room
The same application seen as layers of one system — and the Spaaronn capability that engineers each layer.
Field
Measure and analyse at the process
Instrumentation
Flow, level and filter differential-pressure instruments with the mounting, cleaning and protection the water environment demands.
- Flow, level and DP selection
- Mounting and cleaning arrangements
- Calibration and verification routines
Analytical
pH, conductivity, turbidity and chlorine analyzers chosen for each point — and dissolved oxygen where wastewater is treated — with sample handling where immersion is not practical.
- Analyzer selection per point
- Flow-through panels and sample lines
- Sensor maintenance plans
Control
Decide and act on the signals
Automation & monitoring
Flow-paced dosing with residual trim, filter backwash sequences, pump control and alarms in the plant PLC.
- Flow-paced dosing with residual trim
- Backwash from head loss and turbidity
- Pump control and alarms
Supervision
See, record and report across the plant
Integration
Quality, flow and level data brought into SCADA for supervision, alarms and the records operations and compliance depend on.
- Supervisory views across the treatment train
- Compliance records and reports
- Remote sites and telemetry
05Project sequence
Typical project sequence
How this application is typically taken from requirement to a supported, running system.
- 01
Define monitoring objectives
Which parameters the process, the supply standard or the discharge permit requires, and where.
- 02
Survey the treatment train
Process stages, existing instruments, sample access and the control system in place.
- 03
Select analyzers and mounting
Measuring principle, immersion or flow-through, cleaning and protection for each point.
- 04
Engineer control and integration
Dosing and backwash control, alarms and SCADA records engineered into the plant system.
- 05
Commission and maintain
Verification against laboratory results, cleaning schedules and sensor maintenance.
Next steps
Where to go next
- 01IndustryWater & WastewaterFlow, level and water-quality measurement with pump-station and plant automation.
- 02ResourceFlow Measurement Technology SelectionElectromagnetic, vortex, Coriolis, ultrasonic, DP and thermal mass — matched to the fluid.
- 03ResourceLevel Measurement Methods for TanksHydrostatic, DP, radar, ultrasonic, guided-wave and point-level methods compared.
Engineering enquiry
Monitoring water quality online?
Share the treatment stages, the parameters you need to monitor and how data reaches your control room today. We can help select the analyzers and engineer the integration.