The Water Quality Assn. (WQA) shared highlights of its...
Desalination technology has been around for the better part of the last century. Many countries, municipalities, armed services and ships have the need to produce fresh water by desalination because of their lack of natural sources of fresh water. Desalination technology has brought fresh water and hence industrial and commercial development to areas of the world that otherwise might have remained unproductive. Not only has development been enhanced by this technology but, more importantly, the health and welfare of many people have been improved by the supply of sanitary fresh water supplies.
The first desalination systems were probably ones developed by the navy. These systems utilized distillation technology and, most times, were operated by waste heat from the ship engines. Distillation-type desalination systems require large amounts of energy to produce fresh water and, because of these, only where low cost energy is available is distillation economically feasible for applications.
In the late 1960s, reverse osmosis (RO) was developed for desalting saline water supplies. This process is based on the principal of osmosis and requires a membrane barrier to separate salts from water. Because RO technology required considerably less energy to operate than distillation, it was considered to be the technology that would make desalination much more attainable to the world’s water scarce areas. Indeed, RO for water purification has become widely utilized not just for drinking water applications but for high purity industrial process applications such as manufacturing of electronic components, pharmaceuticals, chemicals, boiler feedwater, medical applications and industrial and municipal wastewater recovery systems as well. However, in the early days of RO commercialization, historical design and operations information wasn’t available and, as with any new technology, many problems arose. Therefore, in the initial years of RO membrane plant application, many owners and operators experienced costly downtime attributable to lack of applications experience by engineers, vendors, etc. As both membranes and applications engineering became more advanced with time, RO systems were installed and operated with greater success. Today, with the necessary understanding of feedwater source conditions, pretreatment needs and high quality system engineering, RO can be applied to almost any ground or surface water desalination/purification application. Advancements in membrane manufacturing and applications engineering have made RO the leading process in the worldwide water desalination market.
The definition of desalination generally is considered to be the production of fresh water from seawater. However, the term also is commonly used by engineers to describe desalting of slightly to moderately saline waters normally referred to as brackish. For the remainder of this discussion, we will focus on desalination as defined by seawater desalting.
Principles of RO
Desalination by RO requires the use of an osmotic membrane (i.e., one that allows water to pass through it at much higher rates than dissolved salts). Osmotic membranes occur naturally in living organisms everywhere. The osmotic membrane also is referred to as a semipermeable membrane because of its capability to allow some constituents to pass through it while holding back others.
The osmosis phenomenon in nature is one where a dilute solution is transported across a semipermeable membrane toward a concentrated solution on the other side. The process of RO is just the opposite of osmosis and is illustrated in Figure 1. In osmosis, the solvent water passes through the membrane until the pressure difference across the membrane is equal to the osmotic pressure (approximately 350 psig for a fresh water/seawater interface). In the RO desalination process, a pressure greater than the osmotic pressure applied to the saline water will cause fresh water to flow through the membrane while holding back the solutes (salts). The higher the applied pressure above the osmotic pressure, the higher the rate of fresh water transports across the membranes.
System Design Consideration
When designing a seawater RO system, many factors must be considered. First and extremely important is the feedwater source. Seawater either will be drawn from a surface water supply, a beach header-lateral face well system or a borehole well system. Typically, seawater well systems are preferred because they provide a low turbidity feedwater requiring less pretreatment. However, even well systems produce varying seawater quality depending on the origin of the strata they are constructed in. Once the best feedwater source (i.e., the system that requires the least amount of pretreatment) is selected and characterized, both chemically and physically, the pretreatment system must be designed to create the optimum conditions for membrane operation and performance. With a high quality feedwater (one with low colloidal, microorganism, organics and iron content), a seawater RO system needs only five micron cartridge filtration for pretreatment. Lower quality feedwater may require much more pretreatment including any and all combinations of the following
• infection coagulation
• multi-media filtration
• sequestering and dispersant chemical feeds
• five micron cartridge filtration.
After the pretreatment system, a pump pressurizes the feedwater salinity and recovery rate. Typical seawater ranges between 28,000 to 35,000 ppm total dissolved solids (TDS). However, some saline surface waters are much higher such as the Red Sea at 40,000 to 45,000 ppm 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 simple illustration is provided in Figure 2. System water recovery rates range from 20 to 40 percent for small to large systems, respectively, for typical 36,000 ppm feedwater.
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 RO 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 (88 percent vs. 50 to 75 percent), these most often are the pumps of choice for plants less than 150,000 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 1,000 to 100,000 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 1,000 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. Membranes most often require replacement because of reduced capacity, which in most cases is attributable to colloidal and/or biological fouling. 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.
Colloidal fouling tendency of an RO membrane is relative to the feedwater’s Salt Density Index (SDI), an ASTM method for measuring small particulate/colloidal loading in water. Typical borehole well systems that have been constructed correctly produce SDIs less than 3.0 and many times less than 1.0 Header/lateral beach wells have produced SDIs less than 3.0. However, it is advisable to expect a range of 2 to 5 for beach well systems. Surface water will produce SDIs much greater than 5 and, with an excellent pretreatment system, this can be lowered to 3 to 5. However, adverse weather conditions and upsets in clarification systems can make these intake systems unreliable. Intake systems, pretreatment and SDI must be stressed because membrane life and warranties are directly correlated to these parameters. Spiral-wound membranes have a maximum allowable operating SDI of 5.0, which is why they often are chosen.
The last item needing discussion relative to plant equipment longevity is corrosion of materials. Systems should be constructed with plastics and nonferrous components wherever possible. All low-pressure components should be constructed from PVC, fiberglass or plastics in general. High-pressure components that require the use of metals should be of an acceptable alloy to their location and application in the system. Seawater environments are highly corrosive, and only the highest quality of material is acceptable. As an example, welded pipe should not be any grade lower than super austenitic stainless steel.
Seawater RO system equipment capital costs range from $10/gpd production capacity to $2/gpd capacity for systems ranging from 500 to 1.0 million gallons per day.
Intake and pretreatment costs can be substantial when considering a surface water source adding anywhere from $0.50 to $2.00 per gallon to the system capital cost where pretreatment costs predominant. Intake and pretreatment costs for a well system will add anywhere from $0.20 to $0.80 per gallon where intake costs predominate. Systems that can utilize a well-feed source that produces a SDI of 3 or less are the least capital-intensive systems.
Operating costs will vary most predominately with varying power cost for the system as identified earlier. Typical operating costs will range from $2/Kgal of product to $8/Kgal of product for the land-based systems. When energy recovery equipment is incorporated, overall operating costs can be reduced by roughly 20 to 30 percent. Operating cost includes system consumables such as power, filter cartridges, chemicals, RO membranes and miscellaneous mechanical maintenance items and does not include labor or plant capital cost amortization.
An RO desalination system marketplace is justifiably anywhere there is a lack of adequate fresh water supplies, insufficient brackish water for lower pressure brackish RO operation and a good source of available seawater. This tells us that anywhere there is growth and development in water shortage areas, there is a candidate for desalination by RO. Historically, the world market has been in the equatorial zone, arid environments and island coastal communities. Remote resort developments have lead the way with this technology, but as quality water sources become more and more scarce, both industry and municipalities are recognizing the need for RO desalination. In California, Florida, The Caribbean, Central and South America, the Mediterranean, Middle-East and the Pacific Rim (i.e., anywhere there’s an ocean and a need), RO desalination is a viable resource for the world’s fresh water production requirements.