By Nick Nicholas
Ultrafiltration systems can be engineered and designed in several possible combinations based on the application and source water quality. There are different membrane materials, membrane shapes, flow types, and configurations. Each possible ultrafiltration technology configuration, has properties that would determine the best fit for the specific application or source water quality. Determination of a configurations best fit would depend on the following factors including, suspended solids concentration, oil/grease, color, and energy use.
In this article, we will examine and compare the pros and cons of two pairs of ultrafiltration technology configurations:
Dead-end filtration vs. cross flow filtration and submerged configuration vs. pressure vessel configuration.
Dead-end vs. Cross flow
One of the most important concepts in water treatment is fluid dynamics. Fluid dynamics can be explained briefly as how fluids flow through the membrane system. These dynamics have effects on energy use and the rate of solids buildup. Selection on flow type is dependent on concentration levels of suspended solids in the feed water.
In a dead-end flow ultrafiltration technology filtration configuration, the feed water flows perpendicular to the membrane surface. Molecules and particles smaller than the effective pore size pass through to the opposite side while larger matter builds up in a cake layer on the membrane surface.
All of the energy put into the flow is spent directly on forcing water through the membrane.
No excess solids are carried through the system all particles build up on the membrane so there is no need for recirculation.
Lack of recirculation means no extra lines, pumps, or valves are needed and no extra energy is spent on this functionality.
More frequent back washing
The continuous buildup on the membrane requires more frequent cleaning which can lead to higher operating costs.
Decreasing membrane flux
As the layer of cake on the membrane thickens, the flux through the membrane decreases as the flow is blocked by these solids.
A cross flow ultrafiltration technology filtration configuration has the feed water flow parallel to the surface of the membrane. While flowing along the length of the membrane, water and small particles pass through the membrane while some solids adhere to the membrane surface. As a result, the rest of these solids, continue down to the end of the vessel. The shear force generated by the tangential water flow, also scours the membrane and helps maintain a cleaner membrane in water sources with elevated total suspended solids levels.
The tangential flow causes a shear force that scours the membrane surface and carries away the excess solid layer.
Higher liquid removal rate
Because the solids cake layer is kept thin, the system flux is typically much more consistent.
Higher membrane lifespan
The thin cake layer and frequent cleaning by this cross flow action, keep the membrane in good working condition for longer periods of time reducing any operational chemical cleaning costs.
Because not all of the water is drawn through the membrane at any given time, recirculation is needed to prevent excess water loss.
The combination of trans membrane pressure and the force needed to move the fluid along the membrane and recirculate it, require more energy to accomplish this functionality.
Submerged vs Pressure vessel
The fluids the system is treating have to be contained somehow to be effectively treated. There are typically two predominant ultrafiltration technology configurations. The choice of configuration typically can depend on treatment application, source water solids concentration, pressure requirements, flow rates, and accessibility.
Also known as an immersed configuration, this one involves a large tank that is open to the atmosphere. A series of membranes are lowered into the tank that is filled with feed water. Vacuum pressure forces the water to pass into the membrane system and up to an outlet pipe connected to the rest of the membranes.
Because the surface of the tank is exposed to the atmosphere, operators can clearly see the membranes at work and can notice any issues by eye.
Lower pressure operation
Lower differential pressure is used to filter the water.
Operation effected at higher elevations
The higher above sea level the system resides, the lower atmospheric pressure is, which decreases the differential pressure.
Operation effected at colder feed water temperatures
When the feed water temperature drops, the viscosity of the water increases and forces the pumps to work harder to compensate.
Longer downtime for membrane replacement
When replacing membranes, the entire frame structure they are mounted to must be lifted from the tank and at that point, the system cannot be used until maintenance is finished.
This is an ultrafiltration technology configuration in which the membranes are individually housed within cylindrical tubes made of materials like PVC, PE, or PVDF. Multiples of these are mounted to skids and connected in parallel configuration. A pressure pump draws water through the membrane. These flow configurations can be operated as inside-out or outside-in filtration modes.
Wide range of operating pressures
Because it is not reliant on atmospheric pressure, pressure vessels are capable of handling upsets like temporary changes in feed water quality.
Able to operate at a higher flux which means it can process a higher volume of water in a day.
Safer, more reliable cleaning
Being in an enclosed container, fumes from any cleaning chemicals do not seep into the surrounding air.
Resistant to outside contamination
The closed design prevents the feed water, back wash water or membrane from being contaminated by outside forces.
Higher costs for housing
While a submerged system, for instance, has only one large tank, pressure vessel systems can have increased cost due to the number of individual housing units needed for the membranes.