05.03.2009 | Editor: Dr. Jörg Kempf
Scaling, fouling and oxidation are the natural enemies of reverse osmosis (RO) systems in water treatment—especially in the food and pharmaceutical industries. Proper process monitoring and control of critical parameters such as pH, ORP, conductivity, flow, temperature and differential pressure can provide advanced warning and protection of scaling, fouling or chemical attacked. If early warnings are ignored, the membrane may become irreparably damaged.
The pharmaceutical and biotechnology industries, in particular, have zero tolerance for anything other than the highest water quality. This in turn requires, first and foremost, consistent and precise process measurements. As well as ensuring water quality, proper process monitoring also cuts costs by allowing the plant to run with minimal operator intervention.
With the latest multifunctional analytical instruments and a broad range of switches, transmitters and valves, Bürkert Fluid Control Systems provides intelligent technology that protects RO membranes, enhances process efficiency and lowers operating and chemical costs. Bürkert products range from individual components to complete control systems purpose-designed for RO.
For successful RO, pre-treatment is essential to ensure that all oxidizing agents, organic and inorganic causes of fouling, and scale-forming compounds are eliminated or at least minimized. Process measurements are essential in making sure that these pre-treatment processes operate correctly at all times, as the following examples show.
Oxidation reduction potential (ORP) is critical to ensure that all chlorine and other oxidizing chemicals have been removed. If present, these chemicals can physically attack polyamide RO membrane, creating leaks through which dissolved salts can pass which will present itself by lower water quality. This type of damage is non-repairable and can result in high replacement costs. It is therefore quite common to install an ORP meter upstream of an RO plant, with an alarm to notify the plant operator when oxidizing chemicals are detected. One damaged membrane typically costs close to the cost of an online ORP meter, so a return on investment calculation can be done quickly.
The perfect pH for RO membranes is between 6 and 7. This slight acidity prevents the formation of calcium carbonate crystals on the membrane, which can result in significantly reduced recovery as well as lower water quality. For some cellulose acetate membranes, correct pH also prevents hydrolysis problems. pH transmitters like Bürkert’s type 8202 measure the pH and temperature of the incoming water. Other compact field instruments like the 8205 pH controller ensure that the pH remains within a predetermined range: a deviation from the setpoint triggers a digital dosing pump that adds acid to the RO feed water.
Conductivity is the single most important and most commonly monitored system parameter in an RO plant. The conductivity of the feed water is one of the key factors determining RO membrane flux and will significantly affect recovery rate. The higher the conductivity, the higher the osmotic pressure, and high osmotic pressure makes the RO system less efficient at a given pressure and temperature. It is important to be able to identify changes in permeate flow rate due to feed conductivity fluctuations; this helps rule out more serious problems, such as scaling or fouling. The percentage difference between the conductivity of the feed water and that of the permeate is known as the “percent reject”. This figure tells the operator what fraction of salts have been rejected, and helps to reveal the overall health of the system. In a brackish water RO system, it is common to reject 95–98% of the incoming salts. In larger systems, it is important to “normalize” measurements of permeate flow. By compensating for changes in operating pressure, temperature and conductivity, normalization reveals changes in plant performance caused by fouling or membrane degradation. In such plants, conductivity is commonly monitored on not only the feed and permeate lines, but also the concentrate line.
Temperature directly influences the membrane flux — the amount of water that permeates through an RO membrane. The higher the temperature, the higher the flux for a given conductivity and pressure, and it is common to assume that a 1 °C change in water temperature will cause the flow rate to change by 1.5%. pH, ORP or conductivity sensors that also measure temperature are cheaper than installing two separate instruments.
Feed pressure also directly influences RO performance. Comparative measurements and trending of feed, permeate, concentrate and interstage pressures ensure that the system is working at full capacity. Monitoring pressure differentials also helps the operator identify potential first-stage fouling or second-stage scaling problems. It is common to run an RO plant at various feed pressures to compensate for changes in feed conductivity or temperature. The two most common ways to control system pressure are via a feed control valve or a variable-speed high-pressure pump.
Maintaining adequate flow
Flow rate, as measured by either a Bürkert magnetic or paddle flowmeter, is another key measure of system health when expressed as percent recovery, in other words the fraction of the feed water that ends up as permeate. A change in percent recovery without a corresponding change in conductivity, temperature or pressure could be an early indicator of chemical attack, scaling or fouling of the membranes.
Maintaining adequate flow through the membrane feed channels is also the single most important factor in preventing scaling and fouling. Membrane manufacturers specify the minimum flow rates in the feed channels of the last membrane needed to ensure turbulent flow, which is important in removing concentrated minerals from the membrane surface. It is important to balance system pressure and feed flow rate; commonly, a variable-speed pump or a flow control valve are used to ensure required feed pressure, while a control valve on the concentrate line ensures adequate flow. Both of these control loops must work together to find a proper balance.
The latest two-in-one devices such as Bürkert’s pH/ORP transmitters and controllers can reduce instrumentations costs. An example would be in simple RO systems where only the feedwater pH needs to be lowered, a dosing pump combined with a compact pipe-mounted dynamic pH-transmitter/controller like Bürkert’s 8205 is sufficient.
Mini-transmitters that can perform simple on/off control or alarm functions as well as straightforward measurement can also be used to reduce overall system costs, and are especially suitable for quick refits and upgrades of simple RO processes. Apart from measuring conductivity, pH, ORP, and temperature, the Bürkert Element range of transmitters also provide on/off control. The transmitters are programmed via a removable module that can also be left in place to provide a local display.
For more complex RO systems, only a process control system that simultaneously regulates and records all the necessary para-meters, such as flow rate, temperature, pH/ORP, conductivity and pressure, can produ-ce real advantages in performance and effi-ciency.
A multifunctional control system like Bürkert’s mxControl 8620 can simplify the control of process variables in water treatment systems, in some cases to the extent of reducing control and instrumentation costs to almost half. The control system combines monitoring, control, and data recording in a compact and easily configurable unit with a display.
As an example of what a dedicated controller can do, consider a typical brackish water system operating at 75–85% recovery. If the recovery figure decreases, the controller will log every variable needed to analyze the fault. The initial focus is on conductivity then water temperature and feed pressure. “If all these parameters are within the control range, but membrane performance is still falling short of the nominal value, this may be due to scaling or fouling. This can also be quickly differentiated by appro-priate monitoring points,” explains Christof Kundel, process engineer at Bürkert Fluid Control Systems. If the first-stage pressure drop is unusually high, the reason is most often colloidal or bio-fouling. High second-stage pressure drop generally indicates scaling.
SD card slots, USB or Ethernet ports, and easily configurable PC software help the controller display, control and record many types of process data, including flowrate, pressure, pH/ORP, conductivity, O2, Cl2, water level and temperature. The controller can be connected to up to six analogue and eight digital inputs, of which four can be flow rate. Up to four analogue and nine digital outputs allow a multitude of uses, from simply retransmitting a process value, to pulse or frequency control of a metering pump. The controller can also transmit analogue outputs from one of its eight PID control blocks.
RO processes, whatever their complexity, can all benefit from state-of-the-art measurement and control systems, ranging from relatively simple transmitters/controllers to powerful multifunctional controllers. These devices enhance process efficiency and make RO systems more durable, as well as reducing instrumentation and control costs.
This article is protected by copyright. You want to use it for your own purpose? Infos can be found under www.mycontentfactory.de (ID: 287350) | Archiv: Vogel Business Media