By Nick Nicholas
Ultrafiltration (UF) water treatment systems have many possible combinations between the types of membrane configurations, flow patterns, aeration, and submergence. Each design consideration for a UF system has its own benefits that would work for a particular industrial/commercial application, and disadvantages that would work against it.
Therefore, when choosing a UF system for an industrial or commercial water treatment application, there are few key things to look for that can help you decide which configuration would be optimal for your application.
What is in your water?
The most obvious and significant factor that determines how you treat your water/wastewater is what is in your water. Different contaminants will need different allowances based on their size, concentration, and effects on the chemistry of the fluids.
The sizes of the smaller contaminant particles will determine the pore sizing of the selected membrane. Ultrafiltration membranes range from 0.1 to 0.01 microns. A general rule of thumb is to select a membrane with pores one-tenth of the size of the smallest particles to be filtered. This allows smaller particles to pass through the pores, but larger particles are trapped on the surface and don’t lodge within the pores. This makes maintaining the solid surface layer easier with a cross flow and also makes back washing more effective.
The concentration of solid materials in the raw water will determine a few design parameters about the configuration of the UF water treatment system. Whether the flow is cross-flow or dead-end, and inside-out or outside-in. Lower solids concentrations are ideal for dead-end, inside-out flow. Dead-end flow requires less energy to produce than cross-flow and inside-out flow has more uniform flow characteristics.
It all has to do with how quickly the membrane will build up solids on its surface layer. High loads in dead-end configurations will build up a layer quickly because every bit of organic solid remains on the membrane. Inside-out flow, especially for hollow fiber and tubular membranes, can completely clog the permeate tubes after a period of time.
On the other hand, cross-flow will carry excess solids parallel to the membrane instead of depositing them directly on its surface. This allows for longer run times in high-solids loading situations. Outside-in flows don’t have an inner circumference to clog up.
Chemicals, pH, and temperature
Different membrane materials have differing resilience to harsher effluent conditions. To keep it general, polymeric membranes are less expensive, but they can be much more susceptible to degradation in the presence of very alkaline or acidic pH’s or higher temperatures. However, ceramic membranes can handle a wider variety of conditions, but are much more expensive. There are different varieties of polymeric membranes and some can be used in more volatile conditions, but a ceramic membrane could tend to last much longer without needing replacement.
How much space do you have?
UF water treatment systems are generally more compact than other filtration methods, but there is variance between different membrane configurations.
For example, submerged systems are larger than pressurized vessels, spiral wound membranes are a compact version of plate and frame membranes. Integrated aeration systems don’t require an extra tank, and hollow fiber membranes offer greater surface areas for the same volume vessel full of tubular membranes. More compact systems are easier to integrate into preexisting treatment systems, but they may require more frequent backwashing based on water quality.
How much energy do you want to use?
The answer is likely as little as possible, but certain UF configurations have particular benefits at the expense of using extra energy. Generating cross-flow as well as the transmembrane pressure difference requires more energy than a dead-end flow system, but it has the advantage of maintaining a thinner solid layer on the membrane surface. Any recirculation of filtrate will also need more power for the necessary pumps but will improve the efficiency of the system overall.
How often do you want to backwash?
Backwashing runs the filter system in reverse to remove the solids that built up on the membrane surface. This process is necessary for proper filter maintenance but there is the associated period of downtime as well as extra clean water requirements for the back washing itself.
There are configurations that can minimize the frequency of backwashes, but they may have some associated cons that outweigh the costs for an extra cleaning round. As mentioned before, cross-flow systems increase run times, but they also have higher energy costs.
How often do you want to replace membranes?
It’s inevitable that filters will need replacing after a time. Frequent replacement can be costly, though, so it’s important to maintain the system effectively to keep it to a minimum. To do so, there are a few options to consider individually or in combination. More resilient membrane materials like specialized neutral charged polymeric or ceramic will last a long time even in volatile effluent conditions.
A proper pretreatment regimen will reduce membrane fouling. Proper pore sizing will prevent particulates from lodging within the pores themselves, making back washes more effective.
Hopefully, the information provided above is helpful in your decision-making process, but there is much more to be learned from a water treatment expert who can walk you through potential options based on your specific water analysis and any pertinent design specifications.