Posted: June 28, 2009     Author:

More Reliable, Affordable. Seawater desalination by reverse osmosis

Seawater Desal: More Reliable, Affordable.

For many years, desalination was regarded as a process applicable only to the Middle East and a few island communities with limited access to potable water sources. With the first generation of desalination equipment, which relied on a thermal-based, evaporative process, the cost of water produced through desalination was extremely high. By the mid-1970s, a new process technology, reverse osmosis (RO), became available to produce quality potable water from seawater or other brackish sources.

NOTE: See original article at http://www.waterefficiency.net/elements-2010/pace-of-change.aspx

The Rapid Development of Seawater Reverse Osmosis
At that time, the process was still in its infancy, and the first seawater reverse osmosis (SWRO) plants were of low capacity, typically producing less than 100 cubic meters of fresh water per day. The cost of each cubic meter was approximately $2.40. By 1990, as individual plant capacity had more than doubled, with plants exceeding 40,000 cubic meters each day, the unit cost of water was reduced to about $1.40 per cubic meter. (Table 1 shows the change in the typical cost of water production in the next 25 years.)

The reduction in the unit cost was achieved partly by economies of scale, but mainly due to improvements in membrane technology and the introduction of energy recovery devices (ERD). ERDs enable the use of some of the previously untapped pressure energy in the brine or waste stream from the SWRO module. By 2000, plants with capacity of up to 100,000 cubic meters per day were producing water at approximately $1.20 per cubic meter. Continuing improvements in ERD technology led to further reductions to below $1 per cubic meter.

A plant in Ashkelon, Israel, which started production by 2005, had a nominal capacity of 340,000 cubic meters per day and the initial published cost of water below $0.50 per cubic meter. Since Ashkelon, many more plants have been proposed with capacities in the range of 100,000 to 500,000 cubic meters per day. Recently, the typical unit cost of water, quoted for new large plants has increased to a range of $0.65$0.85 per cubic meter. (Table 2 shows how the cost of a cubic meter is calculated on a percentage basis.)

The recent increase is largely due to the growing cost of energy and the cost of key construction materials at the different sites around the world. Despite these challenges, the unit cost of water can be further reduced by lowering the capital expenditures (CAPEX) and the operating expenses (OPEX); the latter by reducing the amount of energy used in the SWRO process. These further reductions in CAPEX and OPEX are expected to come from improvements in membrane technology, improvements in ERD unit efficiency, and further optimization of the SWRO process.

Future Developments

Reductions in the pressure required by SWRO are expected to come from higher productivity membranes that will enable more flow for the same expenditure of energy. Also, the use of higher temperature feed water, such as the cooling streams from costal power plants, could help to reduce the pressure needed to operate an SWRO system. Improvements to membranes and pre-treatment processes will also allow SWRO systems to be operated at higher recovery rates. Increasing the recovery rate will allow more fresh water to be produced from the same capital investment in intake structure, pre-treatment plant, and infrastructure.

Back in 1980, a typical SWRO system would convert 25% to 35% of the feed seawater into potable water. A modern plant, such as Ashkelon, is able to convert 45% of the feed into product, and it is possible for plants to convert 55% to 60% in the future. A plant able to operate at 50% recovery rate will produce twice as much fresh water from the same quantity of feed water as a plant operating at only 25%. It is becoming possible to double the output of an existing older plant for relatively little additional CAPEX.
Improvements in Membrane Productivity

One of the enabling factors in increasing system recovery has been the improvement in the productivity of SWRO elements, as depicted in Table 3. In 1985, a typical SWRO element would produce approximately 4,000 US gallons per day of fresh water and would reject 99.4% of the salt. Currently, a typical element produces roughly 8,000 gallons and rejects 99.8% of the salt under the same conditions. In a period of only 20 years, productivity has doubled, while the amount of salt able to pass through the membrane has been reduced by a factor of three, decreasing from 0.6% to 0.2%. This decrease in salt passage allows modern SWRO plants to meet current World Health Organization and European Union guidelines for drinking water in a single-stage process.

Two-stage or multiple-stage processes may still be required to meet specific limits on some ions, such as chloride, if necessary for compatibility with old distribution system piping; or boron, if low levels are required to allow the water produced to be used for irrigation of crops such as citrus fruits. The need for a multi-stage process generally increases both the CAPEX and OPEX of the process.

Over 20 years, the energy required to produce one cubic meter of desalinated water has decreased significantly. In 1985, 68 kWh would have been required. By 2000, a typical value would be around 4.5 kWh, and, today, the value is in the range of 23 kWh [Mickols et al., 2004]. It is necessary to understand that all the costs of water production and the energy consumption values above are only for the production of the water at the plant. They do not account for the capital and operating costs of pipelines or other distribution systems to get the desalinated water to the consumer.

SWRO and the Environment

As the size of individual SWRO plants and the concentration of desalination plants of all types in some regions have increased, so has the potential impact that such plants have on the environment. Many issues have been raised about the positioning of major civil engineering infrastructure projects in coastal locations. Apart from the industrialization of coastal areas, the most
common objections relate to the energy consumption and the potential effects of the intake and brine disposal on the ocean.

The energy issue is, of course, a valid one, but the opposition to some recent desalination projects proposed in such diverse locations as London, California, and Western Australia, seems to be based on outdated power consumption figures. In a modern SWRO system, the actual energy usage is 23 kW per hour per cubed meter, rather than 6 kW per hour per cubed meter required by older systems. Unfortunately, the public and, in some cases, expert perceptions of the power consumption of SWRO lag behind the real situation, as evident in Table 4. (see original article)

Undoubtedly, the removal of salt from seawater requires a certain amount of energy. But how is this energy demand in relation to other activities? Lets look at driving. A typical family size car driven for two hours will use approximately 100 kWh of power. In desalination terms, that 100 kWh of power would produce about 30 cubic meters of fresh water, the equivalent of the needs of a typical family of four for about one month.

The issues involving the effects of intake structures and the disposal of the brine are somewhat more complex. As far as the intake, the concerns generally relate to the possible entrainment and destruction of fish or other marine life in the plant. Another concern is the possible modification or creation of currents in the vicinity of the intake that may affect the immediate and extended environment. For many proposed large plants, advanced hydraulic modeling seems to be able to provide good data on the potential impact of currents, and careful design of the intake structure, combined with suitable screens, appears able to minimize the risk to marine life.

The concerns about the potential impact of the brine include: salinity, temperature, chemical additions, pH, and oxygen depletion. The relative importance of specific issues may change with the location and the conditions of the environment into which the brine is to be discharged.
A brine discharge from a typical SWRO plant can be expected to have a salinity that is between 1.5 and two times greater than the bulk seawater around it. However, it is not actually adding any additional salt to the environment. The salt is contained in half the volume, so the total mass of salt returned to the sea in the brine is almost exactly the same as that removed from the sea in the feed stream.
In a pre-treatment process, the seawater feed stream may be injected with a low dose of chlorine to help keep the intake structure clean. A coagulant, such as ferric chloride, and a polyelectrolyte may also be added to assist with the flocculation and removal of particles on the filters downstream. Generally, the dosing of such chemicals is closely controlledpartly on cost grounds, but also because the excess can result in the carry forward of iron and/or polyelectrolyte into the actual SWRO system, where they can foul the membrane.

An SWRO plant needs to be cleaned periodically to remove fouling due to bio-film formation in spaces between the membrane surfaces or to dissolve salts from the brine solution concentrated during the RO process. The bulk of the cleaning chemicals, usually based on a caustic detergent or relatively weak acid solution, is collected and treated before discharge to the drain. However, some traces may still be washed through the RO plant.

Much environmental interest has centered on the recently commissioned SWRO plant located on Cockburn Sound in Kwinana, Western Australia. Cockburn Sound, due to geographic features, is sensitive to the possible effects of stratification due to differences in density, salinity, temperature, and oxygen content of the brine stream from the SWRO plant. After extensive studies, the authorities allowed the discharge of the plant brine into Cockburn Sound, but set strict limits on the process, including how quickly it had to be mixed and diluted to the same conditions as the bulk seawater around it.
The SWRO plant began operations in October 2006. To date, the measures taken to diffuse and mix the brine meet the criteria for the discharge, and there have been no reported adverse effects on conditions in Cockburn Sound. In other locations, co-locating new SWRO plants near existing power facilities and blending the brine flow into the cooling water discharge from the power plant has minimized the potential effects of brine disposal. It seems that, in most cases, brines from SWRO plants can be discharged back into the sea with minimal impact, provided adequate care is taken.

Other Opportunities for Membrane Technology

In addition to desalination, membrane technology offers the potential to improve the quality of water from other sources. Membranes can be broadly categorized by their ability to reject specific ions, molecular weight compounds, or particle sizes. Generally, membranes are grouped into three families: reverse osmosis (RO), nanofiltration (NF), and ultrafiltration/microfiltration (UF/MF).

RO membranes are the tightest membranes. They are typically characterized by the degree of rejection of sodium chloride. Most RO membranes will reject more than 99.5% of this mineral, and SWRO membranes can reject 99.8%.
In addition to rejecting monovalent ions, RO membranes provide a barrier to most naturally occurring and synthetic organic compounds, as well as pathogens and viruses. As an extreme example, RO membranes are used by the military worldwide to purify water after any use of nuclear, biological, or chemical weapons on the battlefield. In civilian applications, RO is used to upgrade saline aquifer water. In Bahrain, for example, RO is used to treat water from three aquifers, each differing in salinity and other impurities. In other regions of the world, RO is used to remove general salinity, specific ions, or compounds from otherwise unusable water sources.

Nanofiltration membranes are used to reject divalent salts, such as magnesium sulphate, naturally occurring organic compounds, and color components in water. A large NF plant located at M鲹-sur-Oise outside Paris reduces the concentration of total organic carbon in the water sourced from the river Oise to help prevent the formation of trihalomethane (THM) compounds created when the water is disinfected with chlorine before being put into supply. In addition to this, the NF membrane provides a barrier to any pesticides that leach into the Oise from agricultural land, and also improves the aesthetic attributes of the water, such as hardness and color. NF enables the removal of microorganisms, pathogens, and viruses, while operating at lower pressures and, therefore, lower power demand, than RO membranes.
UF and MF membranes are generally used in water treatment as a means to ensure removal of microorganisms, such as cryptosporidium, as well as pathogens and some high molecular weight compounds.

Ultrafiltration/microfiltration is also used as a pre-treatment stage to NF or RO systems on brackish and seawater systems.

The use of membranes for desalination, or for the upgrading of other water sources, is not restricted to industrialized nations. Many developing nations have been among the pioneers and early adopters for both seawater and brackish water treatment.

Reverse Osmosis is basically a simple process requiring conventional pumping, piping, and instrumentation equipment. RO installations also scale up fairly simply. In the early days of RO development when it was a relatively expensive process, RO was typically used in the developed countries as a means of producing high-grade, high-value industrial water. However, some of the first uses of RO for drinking water were pioneered by countries like Abu Dhabi, to provide reliable water supplies for remote villages and settlements around their borders with Saudi Arabia. These plants were, in fact, mounted on trailers, and the pumps driven by diesel engines. As the cost of RO was reduced, commercial applications started to develop in areas such as Sharm el-Sheikh, Egypt. The development of this region for tourism led hotels to install SWRO systems to provide reliable supplies of safe, good-quality water for their kitchens and guest amenities.

One of the first large-scale applications for RO in the treatment of municipal wastewater was developed at the Madras Refinery Limited (MRL) site, in India. To ensure a reliable source of water throughout the year, MRL invested in a pretreatment plant and an RO system to take the effluent from a local municipal waste water treatment plant and convert it into high-grade process water. This allowed the plant to operate through the year without tapping into limited supplies of fresh water. With reduced capital and operating costs of membrane systems, more developing regions should be able to use the process to augment existing water supplies.

While the cost of desalinating both brackish water and seawater has reduced significantly for any community located by the sea, suitable river, or aquifer, the high cost of distributing that water to more remote locations remains a significant problem. In many regions, laying a pipeline to take water from a plant on the coast to a remote community inland is either impractical or cost-prohibitive. Membrane processes may, however, offer small communities with access to low-grade quality water the opportunity to upgrade that water to be fit for human consumption, in an affordable and sustainable way.
A Small Communities Packaged Plant concept posits that a simple, robust method of water treatment based on membrane filtration could be developed. A simple UF/MF process may be sufficient to upgrade a biologically contaminated source, such as a river or surface water, into water suitable for domestic use. For locations where the available water sources may be high in general salinity or a specific ion, or contaminated with harmful organisms, a second membrane process such as NF or RO could be employed.
Today, many communities around the world do not have access to a reliable supply of clean water. As the relative cost of water production continues to fall, it most likely will bring the technology into a range that can be afforded by the less-industrialized and less-developed nations to provide access to safe drinking water for more people. Additionally, the strong growth in membrane applications is generating large investments in membrane research and manufacturing capacity for the anticipated future demand.

Posted from

Elements 2010

The Pace of Change
Seawater desalination by reverse osmosis

By Ian Lomax, Markus Busch