R O Process
Reverse Osmosis - Pros and Cons
The semi-permeable membrane used in reverse osmosis contains tiny pores through which water can flow. The small pores of this membrane are restrictive to such organic compounds as salt and other natural minerals, which generally have a larger molecular composition than water. These pores are also restrictive to bacteria and disease-causing pathogens. Thus, reverse osmosis is incredibly effective at desalinating water and providing mineral-free water for use in photo or print shops. It is also effective at providing pathogen-free water. In areas not receiving municipally treated water or at particular risk of waterborne diseases, reverse osmosis is an ideal process of contaminant removal.
The reverse osmosis process contains several downsides which make it an inefficient and ineffective means of purifying drinking water. The small pores in the membrane block particles of large molecular structure like salt, but more dangerous chemicals like pesticides, herbicides, and chlorine are molecularly smaller than water (Binnie et al, 2002). These chemicals can freely pass through the porous membrane. For this reason, a carbon filter must be used as a complementary measure to provide safe drinking water from the reverse osmosis process. Such chemicals are the major contaminants of drinking water after municipal treatment.
Another downside to reverse osmosis as a method of purifying drinking water is the removal of healthy, naturally occurring minerals in the water. The membrane of a reverse osmosis system is impermeable to natural trace minerals. These minerals not only provide a good taste to water, but they also serve a vital function in the body’s system. Water, when stripped of these trace minerals, can actually be unhealthy for the body.
Reverse osmosis also wastes a large portion of the water that runs through its system. It generally wastes two to three gallons of water for every gallon of purified water it produces. Reverse osmosis is also an incredibly slow process when compared to other water treatment alternatives.
Filtration - How does it work?
The process of filtration involves the flow of water through a granular bed, of sand or another suitable media, at a low speed. The media retains most solid matter while permitting the water to pass. The process of filtration is usually repeated to ensure adequate removal of unwanted particles in the water (Ramstorp, 2003). This type of slow filtration over a granular bed is generally known as slow sand filtration. It is the oldest method of filtration but still widely used in municipal water treatment plants today.
More modern filtration systems use carbon as the main constituent material of the filter. This carbon is compressed into a solid block form, as opposed to the more loosely structured, granular, sand filters. Such filters often include other media substances, in addition to the compressed, solid carbon. This type of water filter is known as a multimedia filter. These filters clean water through both physical and chemical processes. Physically, they perform the same function as slow sand filters, blocking the passage of unwanted materials with molecular structures that are larger than water. Chemically, the carbon or multimedia filters perform an added filtration function. Through the process of adsorption, the atomic charge of the carbon and other media encourages unwanted particles to abandon their bond with the water and chemically attach to the media (Ramstorp, 2003). The water then passes through the filter, cleansed of undesirable materials. The addition of extra media to the standard filter constitution of sand or carbon allows for more particles to chemically bond to the media, resulting in greater filter performance and efficiency.
Water is generally directed through several stages carbon and multimedia filters to ensure the removal of all unwanted materials. The first filtration stage will remove the most concentrated chemicals, like chlorine, while subsequent stages will remove smaller and more evasive chemicals, like pesticides.
The process of filtration involves the flow of water through a granular bed, of sand or another suitable media, at a low speed. The media retains most solid matter while permitting the water to pass. The process of filtration is usually repeated to ensure adequate removal of unwanted particles in the water (Ramstorp, 2003). This type of slow filtration over a granular bed is generally known as slow sand filtration. It is the oldest method of filtration but still widely used in municipal water treatment plants today.
More modern filtration systems use carbon as the main constituent material of the filter. This carbon is compressed into a solid block form, as opposed to the more loosely structured, granular, sand filters. Such filters often include other media substances, in addition to the compressed, solid carbon. This type of water filter is known as a multimedia filter. These filters clean water through both physical and chemical processes. Physically, they perform the same function as slow sand filters, blocking the passage of unwanted materials with molecular structures that are larger than water. Chemically, the carbon or multimedia filters perform an added filtration function. Through the process of adsorption, the atomic charge of the carbon and other media encourages unwanted particles to abandon their bond with the water and chemically attach to the media (Ramstorp, 2003). The water then passes through the filter, cleansed of undesirable materials. The addition of extra media to the standard filter constitution of sand or carbon allows for more particles to chemically bond to the media, resulting in greater filter performance and efficiency.
Water is generally directed through several stages carbon and multimedia filters to ensure the removal of all unwanted materials. The first filtration stage will remove the most concentrated chemicals, like chlorine, while subsequent stages will remove smaller and more evasive chemicals, like pesticides.
Water Treatment Alternatives - Reverse Osmosis
Reverse osmosis refers to a process of water purification that has been used primarily for the desalination of seawater. To understand reverse osmosis, it is first necessary to understand osmosis. Osmosis is the term for the phenomenon whereby if a semi-permeable membrane separates two salt solutions of different concentration, water will migrate from the weaker solution through the membrane to the stronger solution until the solutions are of the same salt concentration. Reverse osmosis subverts this process. It involves applying pressure to reverse the natural flow of water, forcing the water to move from the more concentrated solution to the weaker. The semi-permeable membrane is porous, allowing water to pass through, but blocking the passage of the bulkier salt molecules (Binnie, Kimber, & Smethurst, 2002). The end result is water sans salt on one side of the membrane.
The semi-permeable membranes for reverse osmosis treatment are generally constructed from polyamide-based materials. These materials are resistant to biological degradations but are subject to chemical attacks from chlorine.
Reverse osmosis has been used as a method of purification for ground and surface fresh water, in addition to its role as a desalinating agent. Working with such water sources creates some problems for the reverse osmosis system. Because of the very small pore sizes involved in the membrane, it is vital that ground and surface water is adequately pre-treated prior to the reverse osmosis process. Depending upon the hardness of the water involved, scaling of the membrane is likely to occur. If the concentration of the calcium or magnesium in the water (the chemicals that determine water’s hardness) is at a high enough level where the chemicals are insoluble, it will create a hard mineral on the inside of the membrane, rendering it impotent (Vigneswaran & Visvanathan, 1995).
Reverse osmosis refers to a process of water purification that has been used primarily for the desalination of seawater. To understand reverse osmosis, it is first necessary to understand osmosis. Osmosis is the term for the phenomenon whereby if a semi-permeable membrane separates two salt solutions of different concentration, water will migrate from the weaker solution through the membrane to the stronger solution until the solutions are of the same salt concentration. Reverse osmosis subverts this process. It involves applying pressure to reverse the natural flow of water, forcing the water to move from the more concentrated solution to the weaker. The semi-permeable membrane is porous, allowing water to pass through, but blocking the passage of the bulkier salt molecules (Binnie, Kimber, & Smethurst, 2002). The end result is water sans salt on one side of the membrane.
The semi-permeable membranes for reverse osmosis treatment are generally constructed from polyamide-based materials. These materials are resistant to biological degradations but are subject to chemical attacks from chlorine.
Reverse osmosis has been used as a method of purification for ground and surface fresh water, in addition to its role as a desalinating agent. Working with such water sources creates some problems for the reverse osmosis system. Because of the very small pore sizes involved in the membrane, it is vital that ground and surface water is adequately pre-treated prior to the reverse osmosis process. Depending upon the hardness of the water involved, scaling of the membrane is likely to occur. If the concentration of the calcium or magnesium in the water (the chemicals that determine water’s hardness) is at a high enough level where the chemicals are insoluble, it will create a hard mineral on the inside of the membrane, rendering it impotent (Vigneswaran & Visvanathan, 1995).
2.3. Membrane processes
2.3.1. Reverse osmosis
Reverse osmosis (R0) is the most widely used process for seawater desalination. R0 process
involves the forced passage of water through a membrane against the natural osmotic pressure to accomplish separation of water and ions.
Technology Description
A typical R0 system consists of four major subsystems:
ƒ pre-treatment system
ƒ high-pressure pump
ƒ membrane modules
ƒ post-treatment system
Feed water pre-treatment is a critical factor in the operation of an R0 system due to membranes sensitivity to fouling. Pre-treatment commonly includes feed water sterilization, filtration and addition of chemicals in order to prevent scaling and biofouling. The post-treatment system consists of sterilization, stabilization and mineral enrichment of the product water. The pre-treated feed water is forced by a high-pressure pump to flow across the membrane surface. RO operating pressure varies from 17-27 bar for brackish water and from the 55-82 bar for sea water. Part of the feed water, the product or permeate water, passes through the membrane, removing from it the majority of the dissolved solids.
The remainder together with the rejected salts emerges from the membrane modules at high pressure, as a concentrated reject stream (brine). In large plants the reject brine pressure energy is recovered by a turbine, recovering from 20% up to 40% of the consumed energy.
Two types of R0 membranes are used commercially. These are the spiral wound (SW) membranes and the hollow fiber (HF) membranes. SW and HF membranes are used to desalt both seawater and brackish water. The choice between the two is based on factors such as cost, feed water quality, and product water capacity.
Due to the R0 unit operation at ambient temperature, corrosion and scaling problems are
diminished in comparison with distillation processes. However, effective pre-treatment of the feed water is required to minimize fouling, scaling and membrane degradation. In general, the selection of the proper pre-treatment as well as the proper membrane maintenance are critical for the efficiency and life of the system
A large number of R0 plants has been installed for both seawater (SWRO) and brackish water
(BWRO) applications. The process is also widely used in manufacturing, agriculture, food
processing, and pharmaceutical industries. 32% of the total R0 units installed capacity is found in the USA, 21% in Saudi Arabia, 8% in Japan, and 8.9% in Europe.
R0 units are available in a wide range of capacities due to their modular design. Large plants are made up of hundreds or thousands of modules which are accommodated in racks. Also, very small units (down to 0.1 m3/d) for marine purposes, for houses or hotels are available.
13 Manufacturing
The main membranes manufacturers are in USA and Japan. Concerning RO systems manufacturing 23% are produced in the USA, 18.3% in Japan and 12.3% in Europe.
Economics
As a general rule, a seawater R0 unit has a low capital cost and significant maintenance cost due to the high cost of the membrane replacement. The cost of the energy used to drive the plant is also significant. The major energy requirement for reverse osmosis desalination is for pressurizing the feed water. Energy requirements for SWRO have been reduced to around 5 kWh/m3 for large units with energy recovery systems, while for small units this may exceed 15 kWh/m3. For brackish water desalination, the energy requirement is between 1 to 3 kWh/m3.
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