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Key Sensors and Alarms in Reverse Osmosis Plants

Modern industrial and commercial reverse osmosis water treatment plants are fully automated and staffed only in case of malfunctions, alarms or membrane cleaning. Sensors play a major role in the reverse osmosis control process.
Reverse Osmosis key sensors and alarms

Table of Contents

1. Instrumentation and automation in modern reverse osmosis plants

Modern industrial and commercial RO plants are fully automated and staffed only in case of malfunctions, alarms or membrane cleaning.

  • Modern automation delivers more consistent water treatment than manual control
  • Labour savings: intervention is only required in rare cases
  • Eventually all RO systems will face problems. Earlier detection is possible thanks to all information stored and available from different sensors
  • Quicker response in case of emergency

Reverse Osmosis plants should allow a combination of manual, semi-automatic and fully automatic controls. These modes will facilitate commissioning, testing and troubleshooting when required.

2. Critical alarms in reverse osmosis systems

The alarm system is part of the automatic control system of any Reverse Osmosis plant and will be responsible for informing the operator about any abnormal situation. Old acoustic alarms have been replaced by SMS or emails with the critical details.

The most important alarms in any Reverse Osmosis unit will include the following scenarios:

  • Low pressure on the feed side
  • High pressure on the concentrate valve
  • Recovery value exceeded
  • Over-temp in feed water
  • Oxidant leak detected
  • Permeate conductivity level exceeded
  • Pump malfunction

3. Key sensors in reverse osmosis plants

3.1 Flowmeters

One of the most significant measurements in Reverse Osmosis plants is flow. Flow values are needed for the RO inflow as well as the permeate and concentrate flows. It is also required to control the dosing pumps that add chemicals to the water feed and to control the recirculation rate in small and medium Reverse Osmosis systems.

There are a wide variety of commercial flow meters in the market. The most common are the Ultrasonic and the Turbine meters:

  • The ultrasonic flow meters use sound pulses and transducers to determine the flow from the speed of sound in the fluid. Typical accuracies are in the range of 0.5%.
  • The turbine flow meters are electromechanical devices that convert the rotary motion of a bladed propeller into electrical pulses. Typical accuracies are in the range of 2%. These meters are susceptible to dirt and solids, which is a drawback for other water treatment applications.

Generally, the accuracy of most flow meters will depend on the upstream and downstream straight-pipe configuration: the best piping configuration to have is an upstream and downstream long run of straight pipe. Obviously, options are limited in most skid-mounted or containerised reverse osmosis systems, however, this is something that must be considered before deciding the place of these devices.

3.2 Pressure Transmitters

Pressure transmitters sense pressure variations through a piezoresistive strain gauge. Accuracy is typically better than ±0.2 percent of full scale and these are commercially available for absolute or differential pressure references.

Reverse Osmosis unit should have pressure transmitters installed at the output of the high pressure pump and, when applicable, at the input of the low pressure pump. Whenever cavitation is a concern, a pressure transmitter should also be available at the inlet of the high pressure pump.

It is also important to record the pressure just before the concentrate valve as this value will provide valuable trans-membrane pressure losses.

Most pressure transmitters available in the market come with standard 0-10V or 4-20mA analog outputs that can be connected to the PLC. However, some brands are supplied with proprietary communication protocols that can only be read by their own transducers. Obviously that means a more complex, expensive and prone to failure option that is not ideal.

3.3 Temperature Transmitters

Water temperature plays a major role in reverse osmosis, with a big impact on the membrane performance.

Increased water temperature reduces water viscosity. This means an increases in membrane permeability. Part of this benefit is reduced by the increase of osmotic pressure with temperature, however the overall effect is usually positive: as a rule-of-thumb, the production (permeate) increase is around 3% for every 1°C of temperature rise.

It must be noted that an increase in water temperature is also linked to a more loose membrane structure, which translates into an increase in salt passage (worst permeate water quality).

Another important consideration of the feedwater temperature is that warmer water tends to foster biological growth and therefore RO membrane fouling. This reduces/degrades the membrane permeability and increases the need for chemical pre-treatment and cleaning.

For all these reasons, feedwater temperature is a key parameter of every Reverse Osmosis plant. Similarly to pressure sensors, most temperature transmitters come with standard 0-10V or 4-20mA analog outputs that can be connected to any PLC.

3.4 Conductivity Sensors

It is neither practical nor possible to measure all the anions and cations dissolved in a water stream hence, in practice, TDS concentration is monitored by measuring the electrical conductivity (EC) of the water.

Electrical conductivity is expressed in microsiemens per centimeter (μS/cm) and the ratio between TDS and EC usually varies in a range between 0.67 and 0.70.

TDS of the feedwater is the most important parameter in any Reverse Osmosis desalination plant for two main reasons:

  • It is the main driver for the osmotic pressure, so it will determine the minimum feed pressure to be supplied by the high pressure pump and the overall RO power demand.
  • It will also determine the permeate (product) water quality.

In seawater desalination applications, feedwater salinity is quite predictable and consistent, however most reverse osmosis desalination systems are fed from quite variable industrial or ground water. In these circumstances, the use of a PLC controlled and motorised concentrate valve is critical for the optimal performance of any reverse osmosis system.

3.5 ORP sensors

Reverse Osmosis membranes are damaged by exposure to oxidants. Even very low levels of chlorine or other oxidants in the feed stream can result in irreparable oxidation damage of the membranes.

When the feed water is pre-treated with chlorine or any other strong oxidants, those oxidants need to be removed to prevent the membrane oxidation and degradation.

Watercore always recommends the use of ORP sensors installed at the RO water inlet. In normal operation the ORP sensor should be reading values not higher than 350mV and an alarm should advise in cases where those limits are surpassed.

As the oxidant effect of Chlorine is strongly catalyzed in the presence of transition metals such as iron and manganese, the maximum ORP value should be even more conservative in their presence.

3.6 PH sensors

Mineral deposits (scaling) is one of the most common and challenging problems in Reverse Osmosis. Low-solubility salts such as Calcium Carbonate, Magnesium Carbonate, Calcium sulfate and silica reach their solubility limit as the RO plant increases the recovery.

Those salts in the feed water reach the point at which their solubility is exceeded and begin forming crystals that deposit and accumulate on the membrane surface decreasing its permeability.

Acids can convert carbonate ions into soluble bicarbonate ions and carbonic acid, and ultimately into carbon dioxide. Hence, dosing acids and maintaining the PH below 8 (the actual PH value needs to be calculated for each particular system and water characteristics) will reduce the risk of scaling.

The PH control can only take place with the use of PH sensors. These sensors are based on a very simple chemical process: a glass membrane electrode and a reference electrode are used to measure pH. When they are submerged in water an electrical potential, proportional to the H+ ion concentration, appears between the sensor and the reference electrode.

4. Is it possible to upgrade an existing reverse osmosis system to fit all these features?

Upgrading existing Reverse Osmosis plants with modern motorised valves, PLC control and instrumentation is quite common in the water treatment industry. In most cases, depending on the system complexity, when access is good and pumps, membranes, vessels and frame are in good condition, RO units can be upgraded and re-commissioned in less than two weeks.

Don’t hesitate to contact us for an assessment.

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