Ultrafiltration (UF) uses a pressurized membrane to achieve TSS (total suspended solids) removal in wastewater within a size exclusion range of about 0.01 to 1 µm. This is a critical regime for the screening of particulate, bacteria, viruses, oils and grease. UF is therefore an essential step for many processes, including municipal wastewater treatment and pre-treatment for reverse osmosis.
The majority of UF membrane installations are polymeric which imposes some important technical limits. Operation at high temperature, exposure to high/low pH, high TSS concentration, or oils and grease may restrict the longevity and performance of polymeric membranes or make them unsuitable altogether. A typical polymeric membrane module consists of thin hollow fibres (imagine a handful of hollowed-out spaghetti) with flow channels 0.2–3 mm in diameter. Failure of polymeric ultrafiltration membranes occurs when these fibres are damaged or blocked, especially likely in severe applications.
Ceramic UF membranes are a solution in the treatment of a wide range of highly impaired wastewaters such as highly turbid water, oil and grease in produced water and slurries. Usage of ceramic membranes has historically been restricted to especially challenging conditions, largely on the grounds of cost. Recent advances in ceramic membrane manufacturing and availability mean that they are now a commodity item, widely available at a lower cost than in the past. For challenging waters, ceramic membranes can now be a much more viable economic choice.
A typical ceramic membrane is composed of a sintered material such as alumina or zirconia (imagine a porous rock that can take a real beating). Unlike polymeric materials, ceramics are stable at high temperatures, will not swell and deform in the presence of oil and grease, and can repeatedly withstand harsh chemical cleans. In addition, commodity ceramic membranes are extruded with flow channel openings 2–8 mm in diameter, making them less susceptible to blockage in high TSS applications. A ceramic membrane is highly robust and—with the correct automated processes—can recover from performance degradation to maintain operations.
Image Credit | 4ewip [CC BY-SA]
Image Credit | Wikiwayman [CC BY-SA]
Polymeric (left) and ceramic (right) membranes
To keep ceramic membranes operating at their best, it is important to employ a combination of automated-cleaning methods. The first defense is to achieve high crossflow, minimizing the stagnant zone or boundary layer near the membrane surface. Higher crossflow, or velocity at the membrane surface, will wash particulate away. There are two crossflow velocities: operational; and a forward flush—whereby velocity is dramatically increased for a short duration by 2-3× to wash away debris. This can require very high flow rates and energy consumption.
Saltworks has developed a novel hydraulic system that minimizes energy consumption and back pressure while maximizing velocity at the membrane surface. Our forward flushes are done in a smart way, to increase the crossflow to wash away any particulate while minimizing the operational impact on the system.
Backwashing is another cleaning method which incorporates hydraulic pressure to push particulate off the membrane surface. Chemical cleaning is comparatively expensive and used less frequently. Nevertheless, we build-in and automate all of these systems.
Saltworks’ roboticized XtremeUF performs cleaning in an intelligent and responsive manner, based on continuous online monitoring of membrane performance. The system selects from multiple levels of cleaning cycles to enable the membrane to maintain flux without irreversibly fouling. We clean only when and how it is necessary. This maintains performance and uptime while minimizing energy consumption.
UF performance is quantified by a measure of filtrate flux: a normalized unit that represents the filtrate flow rate per unit area of membrane. The two most common flux units are LMH (L/m2/h) and GFD (gallons/ft2/day). A typical polymeric membrane will operate in the range of 40–80 LMH (24–47 GFD). A ceramic membrane will operate in the range of 50–1000 LMH (29–588 GFD). Pinpointing where an application will fall on this spectrum is a function of particle size, oil/organics type, concentrations and operating pressure. Lower flux applications will require more membrane area and more energy to meet the same nominal filtrate flow rate. Using experimental data, Saltworks has developed a performance map of expected flux ranges for different industries.
Saltworks focuses on treating the toughest waters. We identified a gap for XtremeUF on the basis of three observations: significant demand for treating waters too challenging for polymeric UF; reduction in ceramic membrane prices; and an opportunity to apply our know-how to implement intelligent automation in cleaning.
To produce an XtremeUF system, we package commodity ceramic UF membranes into well-engineered and intelligently automated systems that can take slurry concentrations to new levels.
We needed XtremeUF to meet the following criteria:
The choice of construction materials of pipework and pumps is critically important for the performance and longevity of UF systems. For high chloride waters, corrosion-resistant materials are required. Saltworks selects and incorporates the correct materials for your application. This may include Stainless Steel 316L, Super Duplex Stainless Steel, 6 Moly Alloy, Titanium or even non-metallics such as CPVC. For short-lived or cost-sensitive projects, this may include an economic trade-off to accept some corrosion, especially with known replacements.
Enhanced Oil Recovery (EOR) maximizes reservoir production by injecting water to push out more oil. While reinjecting the water may be desirable, the water becomes saline and picks up particulate during the process. The water may also have been treated with polymers to increase its viscosity, to push out even more oil.
This is a prime example of where traditional polymeric UF membranes don’t fit, while ceramic membranes do. XtremeUF can produce clean filtrate—removing oils, grease, polymers and particulate—from produced water. This allows the injection/reinjection of high-quality water into a reservoir undergoing enhanced oil recovery, reducing the potential for reservoir plugging. It can also facilitate the ocean discharge of produced water, by removing all of the above prior to discharge.
Here you can read about a pilot test where an XtremeUF system was dispatched to a live oil field in an EOR scenario, and its reliable operations were successfully demonstrated.
Can the XtremeUF help you to meet your targets? Is ceramic or polymeric filtration right for you? Our Saltworks experts are ready to test your water to establish feasibility and indicate site performance. Contact us to learn more.
You can also see our spec sheet to see where polymeric membranes stop and ceramic UF membranes start.
Request More Information Download Spec Sheet Ultrafiltration (UF) filters particles with a size range of about 0.01 to 1 µm. This includes oils, grease, precipitated by-products, bacteria, viruses, and suspended
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