Gregory Nadsady

RO Desalinators & PLCs Provide Fresh Water to Remote Areas

Reverse osmosis plants increasingly are being installed in outlying and remote areas. By providing water they can sustain life and/or provide irrigation to previously remote areas without readily available fresh water sources.

One such reverse osmosis plant recently was manufactured for a client operating in West Africa. The unit was requested to supplement—and eventually replace—the current distillation units operating on the island.

Energy consumption of the watermaker is kept in check through the use of a PEI energy recovery turbine. The unit produces up to 280 cubic meters (about 80,000 gallons) per minute depending upon the feed conditions. The area seawater feed conditions vary depending on the season. During optimum conditions, the seawater conditions are 38,000 parts per million (ppm) of dissolved solids and a temperature of 22° C. The upper limit of the seawater feed conditions are 46,000 ppm and 26° C. Regional storms often contribute to poor feed water conditions. As a result, salt water high rejection membranes were installed in the unit to produce high quality water given the operating characteristics.

The operating conditions of the unit are monitored from an adjacent office in the complex and also from a satellite consulting office operating in Continental Europe. To achieve this level of control and support from such a remote location, a special monitoring system was integrated into the unit using a programmable logic controller (PLC) and a PC workstation connected to the Internet.

The basic mechanical layout is shown in Figure 1. In order to closely monitor the operation of the unit, sensor hardware was installed in the unit (numerical identifications are referenced to Figure 1).

The items listed with Figure 1 are continuously monitored by the PLC, which provides access to this operational information. These parameters are useful in determining the exact conditions in which the system is operating. The PLC is connected to a remote workstation via an ethernet connection; the PC workstation reads this operational information at regular, set intervals based on the resolution and density of data desired. Such data can accumulate rapidly and deplete memory; therefore, careful data management is required. This data subsequently is used to plot trends using a data management software (in this case, Microsoft Excel). Among the trends produced are feed water salinity and temperature; feed water turbidity; filtration stage differential pressures; positive displacement pump oil temperature; collective product flow; ultraviolet sterilizer disinfecting output; and individual as well as aggregate permeate salinity.

The trends mentioned are used for both maintenance and historical performance. These trends permit the operator to pinpoint slow degradation such as media conditions, filter elements and reverse osmosis membranes as well as diagnose current and/or future operating conditions. This evaluation always is taken in the context of the immediate operating conditions of the unit.

The workstation also is used as a server and is connected to the Internet. The server is connected to a website, which permits access from another computer terminal anywhere in the world. Using this connection, all of the operational information and trends that are described above are available for review and analysis by a consultant or other such expert. Trends such as permeate flow information vs. time are downloaded and totaled to find the volume of water produced. The operating client is then billed for that amount of water over a given time period. This also permits membrane cleaning operations to be requested as required, which are judged on the product water flow and salt content and the history of feed water conditions (i.e., temperature, salinity, turbidity). These membrane cleanings are, of course, vital to the life of the membranes.

Operation of reverse osmosis desalination units requires the support of an adequate infrastructure; deliveries of maintenance components and other necessities are vital to the operation of the unit. Obtaining replacement and maintenance components often are projects unto themselves that drive operation costs up and discourage potential customers. Therefore, parts and consumable items must be inventoried on site or ordered sufficiently in advance of when they might be required because of the extended delivery time and high cost of expedited deliveries. Continual monitoring of the system from an intermediate location allows the unit to be evaluated by specialists who are not on site. It also permits identification of minor problems and correcting them before they cause a major system failure.