Replacing Conventional Water Pretreatment Processes with Membrane
The membrane filtration processes of Ultrafiltration (UF) is rapidly becoming the preferred choice for water pretreatment in industrial and municipal environments. Recent developments in membrane types and cartridge configurations have enhanced the economic viability and technical reliability in large volume applications such as surface water pretreatment and industrial waste water recycle. Membrane systems are now being used for boiler feed, cooling towers and process water in the chemical, petrochemical, power generation, semiconductor, and beverage industries. Systems may be designed for treating raw water directly from a river, lake or other surface source. The UF system is designed as a single step unit processes that replaces multiple step processes such as solids contact clarification with coagulant feeds and one or more steps of multimedia type depth filtration. For zero discharge wastewater treatment systems, water is treated directly from biological treatment and secondary clarifiers. The water is then recycled to the front end of the plant’s feed water treatment process.
Figure 1: Industrial water pretreatment and wastewater recycle using ultrafiltration and reverse osmosis membrane processes.
UF system is usually designed as a continuous process, with operation 24 hours per day. Typically units are backflushed every hour for about 30-60 seconds and periodically cleaned with chemicals restore water flux. Systems are designed with recovery rates up to 99.5%, allowing for maximum efficiency of the water treatment system. The concentrated retentate from the ultrafiltration system may be further dried and removed as a solid.
The Membrane As A Physical Barrier
The UF system provides a physical barrier between feed and product water. Membrane processes do not rely on chemical precipitation, depth filtration or biological activity as with conventional water treatment. The quality of the filtrate from a membrane filter is dependent only on the pore size of the membrane. Since the separation process is based on particle size, wide variations in feed water such as high turbidity found during flood stages of surface waters, result in no change of product water quality. Consistent, high quality water is always produced by the membrane filter.
For example, a river water system may have a feed water turbidity of 5-50 NTU during normal operation, but flood stages may result in a turbidity of 150+ NTU water due to silt and runoff. When using an ultrafilter, product water will not vary in turbidity quality, with an expected turbidity of less than 0.1 NTU in all cases. Product water quality is independent of feed water composition. This is demonstrated on river water in Figure 2.
Figure 2: Feed and permeate water quality with cartridge flux performance of a river water feed during highly turbidflood conditions.
Improving Reverse Osmosis System Performance
When used as a pretreatment step to reverse osmosis, the ultrafilter will produce a feed water quality that
significantly enhances the performance of the RO system. Reverse osmosis systems require a feed with
a low silt density index (SDI). Ultrafiltration produces a water with an SDI of less than 1.0 by
eliminating suspended solids, colloids, bacteria and particulate matter. Figure 3 illustrates the
improvement found in SDI values of a typical RO feed water. This cleaner feed water has the effect of
decreasing the RO cleaning frequency and enhancing membrane life. For example, an RO system
experiencing biological fouling with a once/week cleaning, may now be cleaned once every several
months with a change in RO membrane life from 1-2 years to 5 years or more.
Silt Density Index (SDI)
|7 6 5 4 3 2 1 0|
0 10 20 30 40
|Feed SDI Permeate|
Figure 3: Silt Density Index (SDI15) of a typical water prefiltered with ultrafiltration.
With a cleaner water feed to the RO, the design of the RO system may be significantly smaller than conventional treatment designs. Flux increases of 30-100% are found when using UF as a pretreatment step resulting in reduction in the size of the RO system for a given flow rate and therefore the cost of the RO system. Alternatively, in plants where energy consumption is the primary issue, the operating pressure of a system may be decreased, resulting in a reduction of energy costs.
Cleaner feed water to the RO also has the potential of increasing the recovery of the RO system. By removal of fouling components before the RO system, a 10-15% recovery increase is predicted assuming that the fouling species is colloidal or particulate in nature.
Pretreating Water at Low Pressures
Ultrafiltration pretreatment processes are performed at nominal pressures of 15 psig. This low pressure process is comparable to or lower than conventional treatment, where several repressurization steps are typically found. For some plants, line pressure may be the sole driving force of the filtration process. Water from the UF system may also be fed directly into an RO system with additional savings in energy.
Chemical Free Water Treatment
Most membrane processes do not require chemical addition steps to precipitate or flocculate water contaminants, making membrane processes both economically and environmentally attractive. This means a safer work environment because large volumes of chemicals do not have to be stored or utilized in the plant. Also, the concentrate from the water treatment plant is free from added components that may cause environmental discharge problems relating to increased organics, or increased disposal mass. The concentrated streams from the membrane processes are concentrated native components of the original water stream. Except for small volumes of chemicals used to clean or sanitize the membrane systems, water treatment with membranes is a chemical free process.
Lower Capital and Installation Costs
Ultrafiltration systems are competitive and often less expensive than conventional treatment using clarifiers and multimedia filters. Membrane filtration systems are modular in design and require minimal on site installation investment. Compared to conventional systems which require more extensive civil work, foundations, concrete work, and tankage, membrane systems are factory fabricated for quick and easy on-site installation. A typical UF/RO system requires 25-30% of the footprint space of a conventional treatment plant, and is significantly lighter in weight. This results in a major on-site cost savings, particularly in plants where space is at a premium or capital for site expansion is limited.
Figure 4: Reduced space requirements for an ultrafiltration system with cleaning and backwash skids compared to conventional filtration systems.
Reduced Operating Costs
Ultrafiltration pretreatment cost less to operate than conventional treatment. Operating and maintenance costs of a conventional RO prefiltration system with clarifiers and multimedia filter are typically between $0.30-$0.50 per 1000 gallons of treated water. This compares to ultrafiltration O&M costs of $0.08 to $0.16 per 1000 gallons. The UF treatment system saves costs by reducing manpower, and eliminating coagulation and flocculation chemicals. Full automation of the membrane system allows for minimal on-site monitoring by plant operators and membrane replacement costs are minimal due to the long life of a membrane cartridge (typically 5+ years).
High Removal Efficiencies
Both ultrafiltration and microfiltration membranes are designed to remove a wide range of contaminants from surface water and industrial waste waters, but because of its improved rejection properties, ultrafiltration is typically selected over microfiltration. Ultrafiltration membranes have pore sizes of approximately 0.02-0.03 microns and are used because of their ability to almost completely retain both suspended and colloidal particles. Essentially all silt, clay, particulate matter, colloidal silicates, insoluble iron, and microorganisms are removed by the ultrafiltration process. The table below describes typical retention properties of a 10,000 MWCO and 100,000 MWCO ultrafiltration membrane. Product water quality typically has no detectable suspended solids, a turbidity of less than 0.1 NTU, and an silt density index of less than 1.0 SDI units. Typical retention values are found in figure 6.
Figure 5 Rejection values of a PM10 and PM100 MWCO membrane for a typical water filtration process.
The high level of removal for colloidal silicates and colloids of iron, manganese, aluminum and other metals make ultrafiltration an ideal pretreatment to reverse osmosis, nanofiltration, ion exchange, and other water treatment processes where colloids may present a problem with downstream unit operations Ultrafiltration processes exceed that achievable with conventional clarification and filtration.
Membrane filtration also allows for almost complete removal of microorganisms such as bacteria, algae, fungus and viruses. Ultrafiltration produces the highest level of microorganism removal and is the membrane of choice for most potable and industrial processes with high levels of microorganism contamination. For industrial RO pretreatment processes, the high microorganism removal of a UF membrane minimizes biofouling at the RO system.
Low Fouling Membranes
RomiPureTM Hollow fiber membrane products are selected for pretreatment processes because of their low fouling membrane structure. The membrane morphology is an anisotropic or asymmetric structure that minimizes internal plugging or fouling. From Figure 7, the tight internal pore structure on the inside surface of the hollow fiber resists particulate plugging of the membrane. All particles are retained on the surface of the membrane and are excluded from the membrane matrix. This means that particles in the water stream are easily removed from the membrane with backwashing techniques and do not result in irreversible plugging of the membrane structure.
Figure 6 Scanning Electron Micrograph (SEM) of a hollow fiber membrane
RomiPure membranes are made from chemical resistant polysulfone or polyacrylonitrile polymers. These polymers are designed for exposure to high levels of chlorine (250 ppm), caustic (pH 12) and acids (pH1.5), which may be used for cleaning the membrane cartridges.
Membranes are designed in different pore diameters to accommodate varying suspended solids levels in water streams. For post treatment applications, a 20 mil fiber has the advantage of high membrane areas (up to 744 ft2/cartridge). For pretreatment applications, 30 and 35 mil diameter products are selected, depending on the particulate loading of the feed water. For high yield backwash recovery systems, extremely dirty feed waters and some waste water systems, 43 mil diameter fibers offer the ability to concentrate the retained particles to very high solids levels. Figure 8 illustrates various lumen diameters (ID) for RomiPure hollow fiber products.
Figure 7. Various available fiber diameters for a hollow fiber membrane. Cleanable and Backflushable Cartridges
RomiPure cartridge performance is restored with normal cleaning and backwash cycles. Several times a day, the membrane system is backwashed with clean permeate water to remove particles that may have accumulated on the membrane surface. Cleaning of the membrane with chemicals such as caustic, chlorine and acids is performed infrequently (typically ever 1-3 months). These techniques are designed to restore membrane fluxes to100% of the design value of the system.
Figure 8. RomiPure Hollow Fiber cartridges mounted vertically on a system. Cartridge housing are clear polysulfone.
Integrity Checked Membranes
All RomiPure cartridges can be tested on line for leaks or damaged membranes. The clear polysulfone housing allows for a visual air leak detection technique that identifies the leak down to a specific fiber. If a leak is detected, cartridges may be isolated and removed from the system. With a simple on-site repair technique by plant operators, cartridges may be repaired and returned to operation.