By Ben Movahed, President, WATEK Engineering Corporation
Membrane treatment technologies have gained significant interest in the last three decades for treating non-traditional water sources to meet more stringent water quality standards. Technological advances in membrane have decreased costs significantly, making them more economically comparable to other treatment options.
The most common types of membranes are low-pressure membranes including microfiltration (MF) and ultrafiltration (UF) for water and wastewater treatment, and Membrane Bioreactor (MBR) for wastewater treatment. High pressure membranes such as nanofiltration (NF) and reverse osmosis (RO), and non-pressure, electric potential driven membrane called Electro Dialysis Reversal (EDR) are used for desalting and contaminant removal. The following is a quick overview of these various membrane technologies.
Low Pressure Membranes
MF and UF membrane filtration technologies have emerged as viable options for addressing the current and future drinking water regulations related to the treatment of surface water, groundwater under the influence, and water reuse applications for microbial and turbidity removal. MF / UF membranes are effective at removing particulate matter and have 5+ log removal capabilities for Giardia and Cryptosporidium. Most MF / UF systems operate with high recoveries of 90 – 98%. Full-scale facilities have demonstrated the efficient performance of both MF and UF as feasible treatment alternatives to conventional granular media processes. Both systems have been shown to exceed the removal efficiencies identified in the Surface Water Treatment Rule such as Cryptosporidium oocyst, Giardia cyst, and turbidity. MF and UF membranes are most commonly made from various organic polymers such as cellulose derivatives, polysulfones, polypropylene, and polyvinylidene fluoride (PVDF). Physical configurations include hollow fiber, spiral wound, cartridge, and tubular.
In response to a changing economic climate, MBR is commonly viewed as an option for the retrofit, expansion and upgrade of aging infrastructure to meet new nutrient limits or increase plant capacity. MBR technology is also ideally suited for an array of municipal and industrial water reuse applications such as irrigation, aquifer replenishment, wetlands development, industrial process water, boilers and cooling systems. MBR systems provide this high effluent quality in a greatly simplified process, requiring only headworks, biological process, membrane filtration, and disinfection to meet the most stringent water quality standards. In comparison, conventional process requires additional primary treatment, secondary clarifiers, Enhanced Nutrient Removal and media filtration in order to obtain the same effluent characteristics. Additionally, there are ever increasing regulations related to pathogens, viruses and other constituents of concern, which are effectively removed by MBR, but not typically reduced to desirable levels by conventional treatment processes.
High Pressure Membranes
RO membrane technology has been successfully used since the 1970s for brackish and seawater desalination. A lower pressure RO technology, NF, also known as “membrane softening,” has also been widely used for treatment of hard, high color, and high organic content feed water. NF also has a high rejection capability for divalent ions such as iron, calcium, magnesium, etc. Reverse osmosis systems are also utilized for removal of inorganic contaminants such as radionuclides, nitrates, arsenic, and other contaminants such as pesticides and viruses.
RO is a physical separation process in which properly pretreated source water is delivered at moderate pressures against a semi-permeable membrane. The membrane rejects most solute ions and molecules, while allowing water of very low mineral content to pass through. This process also works as an absolute barrier for cysts and viruses. The process produces a concentrated reject stream in addition to the clean permeate product. By-product water or the “concentrate” may range from 10% to 60% of the raw water pumped to the reverse osmosis unit. For most brackish waters and contaminant removal applications, the concentrate is in the 10-25% range, while for seawater, it could be as high as 50%.
A non-pressure, electric potential driven membrane, EDR, has also been widely used for removal of dissolved substances and contaminants. In EDR, a variation of electro-dialysis, the electrical potential applied by the electrodes is periodically reversed. This causes the direction of migration of the ions to reverse, and switches the functions of the flow channels so that the dilute channel becomes the concentrate channel and vice versa to flush the membrane spacers. Thus, EDR tends to reduce the buildup of scale and foulants on membrane surfaces.
Selecting a Membrane
Choosing the right membrane process is vital when it comes to the long-term performance and success of a membrane plant. Some of these membranes are not chlorine or oxidant tolerant, and depending on the raw water chemistry, there is a potential for scaling and fouling. This significantly reduces the life of the membrane and increases production and membrane replacement costs. These are the biggest disadvantages of membranes and should be carefully considered for every project.
Pilot plant testing offers the best method for evaluating the feasibility of a membrane application for a specific water supply and comparing the applicability of several membranes. For plants utilizing groundwater or groundwater under the direct influence of surface waters (GUDI), the design parameters are typically well known and there is not much concern with fouling and cleaning. However, surface waters tend to vary in temperature, chemical composition and organic/metal/solids loading seasonally and during storm events, making most sources unique. Through pilot testing, the source water can be assessed and a focused and tailored design can be created, resulting in a more reliable and efficient facility.
Although most membrane manufacturers are capable of recommending an applicable membrane configuration, the engineer provides a custom design and coordination of the entire treatment system, which also includes the membrane modules/skids/cells, feed pumps, chemical feed systems, pretreatment systems, energy recovery devices, cleaning systems, and instrumentation and control systems. A knowledgeable design engineer serves as a single source of contact, responsible for coordinating all these systems and various individual components required in a treatment plant. This coordination is crucial to the compatibility of all related systems and the smooth implementation of the project.
WATEK Engineering Corporation is a full-service water and wastewater engineering firm with capabilities ranging from feasibility studies to complete engineering and design as well as testing and training for advanced water and wastewater treatment facilities. The company was formed in 1995 and in the last 17 years, the primary focus of the firm has turned to engineering and design of membrane filtration and desalination systems. WATEK engineers have been involved in and are focused on the study and design of various sizes (up to 100 MGD) and types of membrane technologies for groundwater/surface water, softening, brackish water/seawater desalination and contaminant removal. In fact, membrane systems engineering is what they do best. WATEK engineers can provide clients with a cost effective, practical and reliable solution.
For more information contact:
WATEK Engineering Corporation