As fresh water supplies dwindle, the use of sea water becomes more attractive. New technologies are making reverse osmosis (RO) a more effective and cost-efficient method for desalination. Utilizing an effective pretreatment system to improve the quality of the feed water traveling through the RO membranes will aid in improving the performance of your membrane system.
RO refers to a type of water purification process that uses semipermeable membranes that block passage of particles, bacteria, and colloids, and through which water molecules diffuse freely in relation to dissolved constituents (such as dissolved salts, silica, boron, and many organic impurities). Because of this special property, under the influence of an applied hydraulic pressure and at a pressure differential higher than the osmotic pressure of the source water, the membranes allow passage of water while excluding passage of water impurities.
Evoqua Water Technologies is a global leader in helping to improve the world’s water resources. Water Online spoke with Alain Silverwood of Evoqua to understand how the right pretreatment process can reduce costs while improving water quality.
What unique characteristics of sea water reverse osmosis (SWRO) desalination plants make pretreatment of the water important?
Due to the introduction of new technologies, along with an increase in demand, the use of SWRO membranes has become a cost-effective solution. Today, SWRO membranes have become the preferred technology for those looking to utilize sea water as a source for drinking water.
Due to the characteristics of sea water, RO membranes may be exposed to solids and organics that foul the membranes. It is no surprise that our climate is forever changing; recent findings have shown that this change in climate has triggered multiple red algal blooms in various regions. Algal blooms, like other particulates found in the water, are very troublesome for RO membranes. Such contaminants must be removed prior to reaching the final desalination membranes, because if they are not you run the risk of the membrane failing.
What are some of the common causes of premature membrane failure?
As discussed above, membrane failure can be induced by various types of solids. Some of the more common causes are diatoms, also referred to as brown algae. Diatoms can be spine-shaped, and they can eventually puncture the membranes due to the high operating pressure required for RO applications. Algal blooms also introduce organic matter into the system, some of which remains on the membranes, nourishing the growth of bacteria. The end result is the creation of an environment that is conducive to biofouling, which is very troublesome and difficult to manage, not to mention costly.
What is the main objective of pretreatment to a reverse osmosis (RO) membrane system?
The primary objective of pretreatment to any RO membrane system is to make the feedwater compatible with the membrane by removing suspended solids and organics. Different technologies are used in pretreatment applications. Some pretreatment methods prefer the use of chemicals to remove organic matter, while others include a filter capable of removing fine particulate. Treatment methods can also be defined based on the characteristics of the water. For instance, a different treatment may be used for polymeric matter associated with bacterial contamination or total organic carbon.
The Silt Density Index (SDI) is a global measurement of the fouling capacity of water. RO membranes typically require feedwater with an SDI below 3.0, ultimately requiring an efficient pretreatment system to be put in place. We have encountered pretreatment systems that make use of multi-media filtration (MMF) technology. However, due to MMF systems not being able to filter fine particulates under 5 microns in size, they often cannot yield an SDI below 3.0. As a result, over time, fouling of the RO membranes will become a regular occurrence.
Choosing the right pretreatment technology that meets the needs of your water characteristics is of extreme importance. This is why laboratory analysis and onsite pilot testing is often conducted to evaluate the efficiency of the technologies to ensure proper pretreatment.
What is the value or return on investment (ROI) in utilizing a pretreatment system?
Inadequate membrane pretreatment results in high chemical cleaning costs, increased downtime, and permanent loss of performance with reduced membrane life. This is particularly true in SWRO desalination plants with open intakes where water quality parameters fluctuate. In many cases, high salinity can also significantly reduce the efficiency of coagulation and flocculation.
Having a membrane pretreatment system that meets your needs will help in:
- Reducing operating and maintenance costs
- Increasing membrane life
- Producing high quality water with a larger flow rate
- Eliminating an unexpected treatment failure, ultimately reducing downtime
What makes cross-flow microsand filtration (CMF) so unique in its pretreatment capabilities?
Cross-flow microsand filters consist of pressurized vessels using fine silica sand and cross-flow hydraulics. CMF typically operates at rates from 30 to 60 m/h while achieving submicronic particle removal. They are pressurized mono-media filters operating as dead-end filters. What makes this technology so unique is its filtration performance, removing fine particulate down to submicron levels, while providing a small and compact footprint. The technology’s modular design enables the possibility to add multiple units at later date, horizontally and/or vertically. Beachfront property comes at a high expense, so a solution that reduces the need for further construction and acquisition of land can make future expansion a simpler task.
The CMF technology was recently used during a pilot study in Egypt near the Red Sea. The technology was proved and validated by a third party to remove turbidity and total suspended solids more efficiently than other traditional technologies. The traditional MMF system reduced turbidity to 0.5 NTU, whereas the CMF technology reduced the level of turbidity to 0.07! MMF pretreatment produced SDIs between 3 and 4, but never below 3. However, CMF SDI values averaged 2.3. All parameters tested were lower than traditional pretreatment technologies. To learn more about the project review the white paper study.
What makes CMF different from current media filtration technologies?
Other filters have a cutoff filtration diameter of 20 to 30 microns. Even when using a chemical additive, the best result is removal of particles down to 10 microns in size. CMF technology provides a cutoff diameter that is so fine it removes particlulate as small as 0.5 microns in size. The CMF technology is a highly efficient filter, removing those fine particles that rapidly foul the RO membrane.
Can existing facilities be modified to add CMF for pretreatment, even with limited space?
Yes, absolutely! CMF pretreatment uses high-velocity filtration. The system is designed with skidded equipment for use in a small footprint. A CMF system footprint is 75 percent smaller than other media filtration systems. For example, an application requiring four MMF filters would only require one CMF filter to achieve the same flow. Ultimately, you could retrofit a water treatment plant with fewer units and still increase your flow within the same footprint. The treatment plant could then handle higher flow rates and still provide improved water quality.
In what way can pretreatment using CMF improve plant operations and maintenance?
The CMF study conducted at the Red Sea demonstrated that the energy required to operate the filters is equal to traditional MMF filters. But, with the CMF technology being capable of filtering fine particulate, less energy is needed to run the RO membrane system over a longer period. Some chemical feed components can be eliminated or reduced, as the CMF technology allows you to consume less chemicals as the chemicals work more efficiently in a cleaner environment. Also, replacement of the cartridge filters protecting the RO filters is reduced, often by a factor of two.