What is the difference between desalination and reverse osmosis

After the pretreatment system, a pump pressurizes the feedwater salinity and recovery rate. Typical seawater ranges between 28, to 35, ppm total dissolved solids TDS. The feedwater then enters the membrane system where organic and inorganic contaminants are separated into a product water stream and a concentrated reject water stream.

Optimum performances are obtained by hydraulically balancing velocities and flows of water along and through the membrane surfaces. A properly designed plant, from the aspects of feedwater source, pretreatment and hydraulic balancing, will reduce organic, inorganic and biological membrane fouling tendencies, thereby minimizing membrane-cleaning requirements. To say that a membrane plant will never need cleaning is not an accurate statement. Thus, all land-based plants should be designed with an in-place cleaning system that also will act as a sanitizing system when needed.

On shipboard systems, especially small yachts and boats, there is very limited space, so membrane-cleaning systems are not sold with the desalination system. Instead, membranes generally are replaced each year. The two most basic individual components in a seawater reverse osmosis system are the high pressure feed pump and the RO membranes. These components comprise the heart of any RO system and require careful selection and application for successful operation.

There are two types of high pressure pumping units on seawater RO systems: centrifugal and positive displacement PD plunger pumps.

Because plunger pumps operate at much higher efficiencies, these most often are the pumps of choice for plants less than , gpd and where high-energy costs exist. In larger plants, the centrifugal pumps are used most often because these pumps may approach 80 percent efficiency, are less costly and require less maintenance. However, the majority of RO desalination systems are in the 1, to , gpd capacity range.

PD pumps are most common due to the lack of availability of higher pressure, low flow centrifugal. Plunger pumps produce large output pressure variance pulsation due to their reciprocating action, which translates to vibration.

This vibration not only is potentially damaging to the pump but to all other system components as well especially plumbing, instrumentation and the systems framework. In order to minimize vibration damage to system components, the pump requires a discharge pulsation dampener and, in some cases, a suction stabilizer depending on the acceleration head attributed to systems feed plumbing.

Another important factor is pump speed in RPM. The slower the pump speed, the less vibration transfers. Mechanical design for vibration isolation also is key to minimizing vibration damage from the pumping system. Because seawater RO pumps can generate pressures in excess of 1, psig, it is recommended that a safety switch, in combination with a pressure relief valve, be incorporated in the design.

Severe damage or injury could occur if the pump pressure exceeds material strengths of the RO design. Membranes are the largest single consumable cost factor in RO desalination Therefore, increasing membrane life will contribute significantly to lowering operating cost.

Fouling is a direct result of either an inadequate feed source or pretreatment equipment. As previously mentioned, installing the correct feed source and corresponding pretreatment system is critical in minimizing foulants. It also was mentioned that a well-type feed source normally is the best source for reducing colloidal content in the feedwater.

However, in some circumstances, either because of well development cost or lack of permeability of the ground structure, well systems are not feasible and surface intake systems will be required. When surface water is utilized as a feed source, pretreatment systems can become extensive in order to reduce feedwater colloid load. Even then, stormy weather conditions will make operation of the RO plant ill advised due to increased turbidity of the feed source.

With ED, water quality is not affected by reducing energy. With RO, consistent water quality is dependent on a certain high pressure to pump and filter feed water through tiny membrane pores, regardless of how much salt is being removed. You can change pressure, but you get a fairly similar output. You have to design to account for the noise, vibration, and pressure. According to Buzzell, such demanding equipment requirements for RO take their toll. ED is driven mostly by system feed water, not pumps, which helps to reduce lifecycle costs.

Not just less expensive, but also simpler. Buzzell explained:. Our flow rate and product water would remain the same. The system would just apply more energy to the modules that are treating the increased flow.

Finally, the tunability of ED allows operators to run at higher power and reduce their physical footprint; or, conversely, to increase the number of modules and keep energy consumption low. Without this discharge, the concentration of dissolved salts in the feedwater would continue to increase, requiring ever-increasing energy inputs to overcome the naturally increased osmotic pressure.

Figure 16 illustrates the basic components of a reverse osmosis system. Pretreatment: The incoming feedwater is pretreated to be compatible with the membranes by removing suspended solids, adjusting the pH, and adding a threshold inhibitor to control scaling caused by constituents such as calcium sulphate.

Pressurization: The pump raises the pressure of the pretreated feedwater to an operating pressure appropriate for the membrane and the salinity of the feedwater. Separation: The permeable membranes inhibit the passage of dissolved salts while permitting the desalinated product water to pass through.

Applying feedwater to the membrane assembly results in a freshwater product stream and a concentrated brine reject stream. Because no membrane is perfect in its rejection of dissolved salts, a small percentage of salt passes through the membrane and remains in the product water.

Reverse osmosis membranes come in a variety of configurations. Two of the most popular are spiral wound and hollow fine fiber membranes see Figure They are generally made of cellulose acetate, aromatic polyamides, or, nowadays, thin film polymer composites. Both types are used for brackish water and seawater desalination, although the specific membrane and the construction of the pressure vessel vary according to the different operating pressures used for the two types of feedwater.

Stabilization: The product water from the membrane assembly usually requires pH adjustment and degasification before being transferred to the distribution system for use as drinking water.

The product passes through an aeration column in which the pH is elevated from a value of approximately 5 to a value close to 7. In many cases, this water is discharged to a storage cistern for later use. Source: O. Buros, et. During the last 15 years, this capacity has continued to increase as a result of cost reductions and technological advances.

RO-desalinated water has been used as potable water and for industrial and agricultural purposes. Potable Water Use : RO technology is currently being used in Argentina and the northeast region of Brazil to desalinate groundwater.

New membranes are being designed to operate at higher pressures 7 to 8. Agricultural Use : Greenhouse and hydroponic farmers are beginning to use reverse osmosis to desalinate and purify irrigation water for greenhouse use the RO product water tends to be lower in bacteria and nematodes, which also helps to control plant diseases.

Reverse osmosis technology has been used for this type of application by a farmer in the State of Florida, U. In some Caribbean islands like Antigua, the Bahamas, and the British Virgin Islands see case study in Part C, Chapter 5 , reverse osmosis technology has been used to provide public water supplies with moderate success. During the eighteen-month period between January and June , the Antigua plant produced between 6.

In addition, the major resort hotels and a bottling company have desalination plants. On Tortola, there are about 4 water connections serving a population of 13 year-round residents and approximately visitors annually.

In , the government water utility bought million liters of desalinated water for distribution on Tortola. On Virgin Gorda, there are two seawater desalination plants. Both have open seawater intakes extending about m offshore.

These plants serve a population of 2 year-round residents and a visitor population of 49 , annually. There are connections to the public water system on Virgin Gorda. In , the government water utility purchased 80 million liters of water for distribution on Virgin Gorda. In South America, particularly in the rural areas of Argentina, Brazil, and northern Chile, reverse osmosis desalination has been used on a smaller scale.

Fewer plants have had long-term operational problems. Assuming that a properly designed and constructed unit is installed, the major operational elements associated with the use of RO technology will be the day-to-day monitoring of the system and a systematic program of preventive maintenance. Preventive maintenance includes instrument calibration, pump adjustment, chemical feed inspection and adjustment, leak detection and repair, and structural repair of the system on a planned schedule.

The main operational concern related to the use of reverse osmosis units is fouling. Fouling is caused when membrane pores are clogged by salts or obstructed by suspended particulates.