Industrial applications may use one of two main types of filtration methods: dead-end filtration or cross-flow filtration. Cross-flow microfiltration is often used along with reverse osmosis and ultrafiltration to provide enhanced levels of filtration. Understanding the differences between these filtration methods can help determine which system to use for a given application.
Filtration Systems Overview
Here is an overview of the features of cross-flow and dead-end filtration.
Cross-Flow Filtration System
In cross-flow filtration systems, the process fluid to be filtered moves tangentially across the membrane filter instead of into the filter. This enables the filter cake to wash away during filtration, preventing blockage and helping the system maintain consistent flow levels over a longer period of time. The process is either cross-flow microfiltration or ultrafiltration, depending on the pore size of the filter membrane.
Dead-End Filtration System
Dead-end filtration separates particles from liquids flowing vertically into the filter. The filtered materials develop into a cake on the filter’s surface, clogging the membrane relatively quickly.
To overcome the resistance of the membrane, dead-end filtration systems require a booster or vacuum pump that can supply the force needed to create the appropriate flow rate. Unlike cross-flow filtration systems, dead-end filtration is intermittent since the membrane must be cleaned or replaced after set intervals to maintain optimal performance.
Differences Between Cross-Flow & Dead-End Filtration
Cross-flow and dead-end membrane filtration differ in several key ways, including:
Filtering method — In dead-end filtration, the feed flow comes to a dead end after filtration. Cross-flow filtration means the feed flow moves across the filter, and the concentrate port allows a controlled bleed of feed flow.
Liquid flow direction — In dead-end filtration systems, the liquid flow and filtering directions are the same since the flow moves directly vertical through the filter. In cross-flow filtration, the process flow moves parallel to the filter.
Angle of attack — Deadend filtration systems attack the membrane surface at a right angle that allows for higher flux rates on the entire membrane. However, this leads to higher stress levels on the membrane surface. In cross-flow filtration systems, the flow stream is parallel to the surface of the membrane due to a concentrate-side bleed, which reduces the amount of stress on the membrane.
Cake formation — When the membrane in a dead-end filtration system traps particulates, they form a cake and build up over time. Also known as filter cake, this buildup is thicker in dead-end filtration systems than in cross-flow filtration, where the cake is naturally flushed away by the tangential flow. As a result, cross-flow filtration systems tend to operate with more efficiency and reliability.
Velocity of feed flow — The velocity of particulate matter in dead-end systems goes to zero from its feed value, correlating to thicker filter cake accumulation. The controlled bleed in cross-flow systems results in a minimal reduction of the velocity of particulate matter, which applies less stress to the membrane and boosts overall performance.
Change in liquid flow — In dead-end filtration systems, liquid flow through the membrane changes relative to time. As time goes on, the thickness of the filter cake increases due to particle buildup. This buildup causes flow through the membrane to slow. Cross-flow filtration uses a constant water flow that separates and discharges particles, slowing the buildup of a filter cake and maintaining a more constant flow rate.
Pressure on filter media — Over time, the filter cake on the surface of dead-end filtration systems thickens and places greater pressure on the filter media, requiring higher backwash flux rates. The cross-flow filtration system leads to a thinner filter cake, which reduces the pressure difference and the amount of pressure required for backwashing.
Usage of water/recovery — Dead-end filtration systems see a higher apparent recovery since the flow liquid cannot travel anywhere else other than through the membrane. Additionally, these systems don’t allow for any leak or bleed, yielding a recovery of about 95%. The apparent recovery with cross-flow filtration is not as high due to the 2-9% bleed.
Advantages of Cross-Flow Filtration Over Dead-End Filtration
Dead-end filtration offers certain benefits for simple, small-scale operations, but cross-flow filtration is better for a wider variety of industrial applications. The benefits of cross-flow filtration systems include:
The ability to remove buildup from the membrane’s surface
No need for a filter aid
Improved filter media lifespan due to lack of buildup and filter cake
Prevents irreversible fouling of the membrane
If you want to further improve the performance of cross-flow filtration systems, implement these techniques:
ATF — Alternating tangential flow (ATF) relies on a diaphragm pump for production to dislodge blocked particulate matter from the filter membrane and prevent irreversible fouling.
CIP — Following periods of extensive use, clean-in-place (CIP) systems eliminate fouling from filter membranes. CIP systems often use various substances, including alkalis, acids, and reactive agents. If these systems use sodium hypochlorite or bleach, you will need to remove this agent from some membrane plant feeds. Bleach oxidizes membranes and causes damage over time that will eventually require a replacement membrane. However, most systems use acids or caustics for CIP, which will prevent degradation with the right temperature and pH levels.
Concentration — Enabling permeate flow will reduce the volume of liquid. The system will concentrate particles that are larger than the membrane’s pores by retaining them.
Diafiltration — Diafiltration may occur following concentration in bioprocessing applications. This process helps eliminate permeate components from the filtered material by applying fresh solvent to replace the volume of the permeate. The rate should be the same as the permeate flow rate, allowing for a constant flow volume.
PFD — Process flow disruption (PFD) offers an effective alternative to backwashing. It involves eliminating the transmembrane pressure by closing off the permeate outlet for a limited time, which helps prevent fouling, albeit to a lesser extent than backwashing.
Filtration Systems from Membrane System Specialists
Need a high-quality filtration system for your application? Membrane System Specialists, Inc. offers design and manufacturing services to produce a custom filtration system for your application. We’ll help you decide on the right system, whether you need a cross-flow filtration or a dead-end filtration system.