Instruments and controls for water and wastewater plants

A treatment plant is controlled by what it can measure. Dosing follows pH and ORP, aeration follows dissolved oxygen, backwash follows pressure drop and a tank pump follows level. Good instruments let the plant run safely with fewer operators, hold the outlet inside its limit and give the records that consent and audits need. Poor or badly placed instruments give false readings and cause the very upsets they are meant to prevent. Tag numbering and symbols on our P&IDs follow ISA 5.1, and every plant has an interlock list.

pH

A glass electrode measures hydrogen ion activity against a reference electrode. Used for neutralisation, coagulation, biological tanks and treated water. Needs regular cleaning and calibration against buffer solutions, and the electrode ages with use.

ORP (redox)

A metal electrode measures oxidising or reducing strength in millivolts. Used for chromium reduction, cyanide destruction, chlorination and biocide control in cooling towers.

Conductivity and TDS

Measures dissolved salts. Used on RO permeate and feed, cooling tower blowdown control, DM plant and boiler water. Contacting cells suit clean water, and inductive (toroidal) sensors suit high conductivity and fouling liquids.

Turbidity

Light-scatter measurement of cloudiness in nephelometric turbidity units. Used on filter effluent, UF permeate and treated water.

Dissolved oxygen (DO)

Optical sensors measure oxygen through a luminescent layer and need less maintenance than older electrochemical probes. Used to control blowers and keep the biological tank in the target range. See blowers for aeration.

MLSS, suspended solids and sludge blanket

Optical or ultrasonic sensors measure solids concentration in aeration tanks and sludge lines, and the sludge level in clarifiers.

Residual chlorine

Amperometric or colorimetric analyzers measure free or total chlorine in treated water, for reuse lines, disinfection control and ahead of RO.

COD, BOD and TOC

Online analyzers estimate the organic load in effluent. COD analyzers use chemical oxidation, TOC analyzers oxidise carbon and measure the CO2, and UV absorbance is used as a quick indicator. Often required for outlet monitoring.

Ammonia, nitrate and phosphate

Ion-selective or colorimetric analyzers for nutrient control and compliance, mainly in plants with nitrogen and phosphorus limits.

Sampling

Automatic samplers collect composite samples for lab tests at the inlet and outlet.

Electromagnetic flow meters

Measure conductive liquids with no moving parts and no pressure loss. Suited to sewage, effluent, sludge and clean water, with suitable liner and electrode materials.

Ultrasonic flow meters

Clamp-on or inline types that measure by transit time. Convenient for retrofit and for large pipes.

Vortex and variable-area (rotameter) meters

Vortex meters suit clean liquids, steam and air. Rotameters give a simple local reading for chemical and air lines.

Open-channel flow

A flume or weir with an ultrasonic level sensor measures flow in open channels and at plant inlets and outlets.

Coriolis meters

Measure mass flow directly, used for accurate chemical dosing and high-value liquids.

Ultrasonic level

A non-contact sensor that measures the distance to the liquid surface. Suited to tanks and sumps. Foam, vapour and condensation can disturb the reading.

Radar level

Radar is less affected by vapour, foam and temperature, and suits difficult tanks, though it costs more.

Hydrostatic (submersible) level

A pressure transmitter in the liquid reads the level from the water head. Simple and dependable in sumps and wells.

Float and conductive level switches

Low-cost switches for pump start-stop, high and low alarms and dry-run protection.

Vibrating fork and capacitance switches

For chemical tanks, sludge and powders.

Pressure gauges and transmitters

Local gauges and 4 to 20 mA transmitters on pump discharge, filter inlet and outlet and membrane stages. Diaphragm seals protect them from sludge and corrosive liquids.

Differential pressure

Pressure drop across filters, cartridge filters and RO stages triggers backwash or cleaning, and shows fouling early.

Temperature

RTD and thermocouple sensors for process, heat exchangers and pump protection.

UV transmittance

Used with UV disinfection to confirm the water is clear enough for the dose.

Gas detection

H2S, methane (LEL), oxygen and chlorine detectors for sewage, sludge and chemical areas, with alarms and interlocks.

Motor control centres and panels

Power and control panels with starters, protection and indicators. Variable frequency drives control blower, pump and mixer speed and save energy. Soft starters limit starting current.

PLC and HMI

A programmable controller runs the process logic, interlocks, sequences and alarms. A local HMI screen shows the plant status and set points.

SCADA and data logging

Central screens, trends and reports for larger plants and for compliance records.

Communication

Analog 4 to 20 mA signals and digital protocols such as Modbus connect instruments, drives and the controller.

Remote monitoring

Remote access and alerts let a plant be watched, and faults found, without being on site.

Online effluent monitoring

Some industries in India must install online effluent monitoring connected to the pollution control board server. Check your consent and our discharge norms guide.

Range and accuracy

Choose a range that suits normal operation, with enough margin for upsets.

Wetted materials

Match the sensor, liner and seal materials to the liquid, including chemicals and temperature.

Fouling and cleaning

Choose sensors that can be cleaned or are self-cleaning where oil, grease or solids foul them, and fit isolation valves for removal.

Installation

Straight pipe runs for flow meters, positions that give a representative sample, and protection from sun, vibration and moisture.

Ingress and area rating

IP rating to the site and hazardous area rating where gas may be present.

Signal and power

Output signal, protocol and supply that match your control system.

Calibration and spares

Plan for calibration, sensor replacement intervals and spare sensors. A simple sensor you can maintain beats a better one left uncalibrated.

We select instruments with the plant or on their own, to your specification and approved make list, and supply panels, PLC logic and commissioning. We can also audit an existing plant and propose the instruments and controls it lacks. Send your plant details. See also mechanical equipment and operator checklists.