As the need for freshwater increases around the world, desalination is proving to be an increasingly attractive alternative to overtaxed freshwater sources. At the beginning of 2008, there were more than 13,000 desalination plants worldwide producing more than 12 billion gal of freshwater a day.
The market is expected to more than double within the next 10 years. As new desalination plants are built and present facilities are expanded, plant operators can realize significant cost savings by employing a recent advance in online process instrumentation—wireless technology. Two of the most promising applications for wireless technology are the measurement of pH and conductivity.
Desalination Considerations
There are two basic desalination technologies: membrane separation (reverse osmosis [RO]) and distillation. Because it consumes less power than distillation, RO is often the preferred method, particularly in the western world. In the RO process, raw seawater or brackish groundwater is pumped at high pressure against a semipermeable membrane. The membrane permits water to pass through, but it blocks and rejects dissolved solids as concentrated brine. Modern RO membranes can reject up to 99% of the dissolved solids in the feedwater.
RO membranes are susceptible to chemical degradation and fouling by solids in the raw water. Pretreatment protects the membranes by removing harmful chemicals, filtering out suspended solids and controlling scale formation. (Scale forms when slightly soluble salts concentrate and eventually precipitate as the water passes through the RO module.) Fouled membranes can be cleaned, but irreversible fouling also can occur. Permanently fouled or chemically degraded membranes must be replaced. The membranes are a significant capital investment, so it is important to monitor the effectiveness of the pretreatment process carefully.
Typical measurements in the pretreatment system are pH and oxidation reduction potential (ORP). Cellulose acetate RO membranes are highly sensitive to feedwater pH and degrade rapidly in an alkaline environment. Therefore, the feedwater should be maintained at approximately pH 5 and monitored continuously. Aromatic polyamide composite membranes are more resistant to pH, often tolerating any pH between 2 and 10. They can, however, be damaged by chlorinated water.
Chlorine generally is avoided in systems using polyamide membranes, but in some cases, the risk of biological fouling is so high that chlorination is necessary. Whether chlorine is intentionally added or is already present, it must be removed, usually by treatment with a dechlorinating chemical. A convenient way of monitoring dechlorination is to measure ORP.
Another important goal of pretreatment is stopping scale formation. Scale has many sources, requiring different control strategies, but in some cases, simply adding acid to lower the pH is effective. Continuous monitoring of pH is necessary to prevent overfeeding or underfeeding of acid.
The most useful measurement in the separation process itself is conductivity. The objectionable solids in seawater or brackish water are primarily ionic, so conductivity is an inexpensive and easy way of measuring membrane performance. Typically, the conductivity of both the feedwater and permeate are measured, allowing continuous calculation of percent solids rejection by the membrane. Unexpected changes in performance immediately alert the operators to a problem.
Wireless Instrumentation
Traditionally, adding process instrumentation also meant installing an extensive network of cables to connect each transmitter to the control system. Cables, conduit, marshalling panels, engineering and installation costs often far exceeded the cost of the instrumentation itself. As new plants are built or new capacity installed, original equipment manufacturers and plant managers can eliminate this expense by specifying wireless instrumentation.
A skid equipped with wireless transmitters at the time of manufacture requires no additional wiring after installation in the plant. Because a desalination plant often contains dozens of skids, wireless technology provides a significant cost savings.
Recently, the industry’s first wireless transmitters for pH and conductivity have appeared on the market. These transmitters use the WirelessHART communication protocol, an open platform that is rapidly emerging as the de-facto industry standard in plant wireless technology. Because it is an open standard administered by the HART Communication Foundation, early adopters need not worry that their investment in wireless technology will lock them into a single supplier. The transmitters seamlessly integrate into any network of WirelessHART-compliant devices.
The solution’s protocol allows wireless transmitters to operate in a “self-organizing network.” In this configuration, every wireless device on the network acts as a router for nearby devices. This ensures high data reliability and network availability because all devices work together to identify and use the most efficient communication path for each message. The network dynamically reconfigures itself without manual intervention and without disrupting the flow of data. The large number of skids and instruments within a typical desalination plant makes for a robust wireless network.
Security is always a concern with a wireless network. The new transmitters take full advantage of the multi-layered security features present in the WirelessHART standard, which includes encryption with automatic key rotation, device authentication and data verification. External interference is mitigated by spread-spectrum broadcasts and automatic channel selection that avoids the noisy spectrum. The resulting network offers security and reliability similar to a hard-wired system.
An additional benefit of the protocol is the availability of diagnostics through the wireless signal. Its pH transmitters, for example, provide data such as slope, offset and glass and reference impedances, which can help identify a dirty or broken sensor or a sensor nearing the end of its life. Diagnostic information is relayed with process variables to the plant control system, providing operators with a quantitative means of evaluating the validity of the pH reading. This level of data sophistication can reduce the risk of an unplanned outage, while preventing unnecessary maintenance cost.
Wireless transmitters are equipped with a high-power battery module that lasts several years in most applications.
The potential of desalination continues to grow. One of the technologies that will contribute to its viability and cost-effectiveness is wireless instrumentation. The emergence of wireless pH and conductivity analyzers represents an effective step.
