Feb 29, 20204 min read

Dead End Filtration Vs Cross Flow Filtration

An Ultrafiltration membrane is a beautiful tool available to water treatment professionals to remove Suspended Solids, Turbidity, Bacteria, viruses, colloids, silica and many more impurities present in water and other liquids.

One of the important decisions to make while using/building ultrafiltration plants is to decide the configuration of feed and concentrate flow patterns. There are two main flow configurations to choose from A) Dead End Configuration B) Crossflow Configuration, each with its own merits and demerits. These are discussed in more detail below.

Comparison between dead end and cross flow — Theway Membranes' Comparison between Cross Flow and Dead End Configuration

A) Dead End Configuration

What? - As the name suggests, dead end configuration is characterized by the feed flow coming to a dead end - i.e. the concentrate port is closed. Therefore the feed flow entering into the membrane module arrives at the membrane surface in a direction perpendicular to the surface of the membrane.

Angle of attack - This brute right angle attack on the membrane surface ensures higher flux rates across the membrane for a given feed pressure. But, in this configuration there is a higher amount of stress that the membrane surface undergoes.

Cake Formation - Also, the particulate matter that is rejected by the membrane does not have a free path to exit the membrane module through the concentrate port, but the reject particulate matter builds up on the surface of the membrane as a 'cake'. This cake is much thicker than the kind that forms when operated under cross flow. This thick cake in turn affects the flux rates across the membrane, and much lower flux rates are observed later in the filtration cycle.

Velocity of Particulate matter - To elaborate, in dead end mode, the velocity of particulate matter is brought from its feed value to zero, which results in deeper build up of particulate matter, and in many cases areas of irreversible fouling on the membrane surface.

Usage of Water/Recovery - Apparent recovery within a filtration cycle is higher in dead end mode, since there is no other direction for the feed water to go through but only across the membrane to the permeate side. There is no bleed or leak that is allowed in this mode. This means recovery can be greater than 95% easily depending on feed liquid quality. But what is regularly not talked about is

The more frequent requirement for backwash, since particulates do not exit the membrane during filtration cycle, and there is a build up of matter on the surface.
The requirement for higher backwash flux rates. (This can be seen by comparing membrane projections for an Dead end vs Cross Flow mode)
The requirement for higher flushing flux rates
The higher pressures of backwash, which are required to initially clear away the thicker cake layer formed on the membrane surface, which in many cases has more probability for causing water/fluid hammer coupled with fiber breakage.

B) Cross Flow Configuration

What? - As the name suggests, the feed flow in this case is allowed to flow along the length/ aCROSS the membrane fiber, with a small controlled bleed of feed flow on the concentrate port.

Angle of Attack - Since there is a bleed on the concentrate side, the feed flow stream, flows parallel to the membrane surface. This ensures that the membrane experiences lesser stress than experienced in dead end mode.

Cake Formation - The particulate matter rejected by the membrane has a more free path to exit the membrane during the filtration cycle going out along with controlled bleed through the reject/concentrate port. This ensures that majority of particulate matter is not deposited as a cake on top of the membrane. The cake layer that is hence formed has a much smaller thickness. Since flux rates are less affected than in dead end mode, this configuration affords the user a longer filtration cycle time, which wouldnt be possible with dead end mode.

Velocity of Particulate Matter - Presence of a controlled bleed ensures that the velocity of particulate matter is brought down only by a little from its feed velocity as compared to zero in dead end mode. This ensures that the membrane experiences much less stress and the membrane rejection layer lasts for longer duration as compared to dead end mode.

Usage of Water/Recovery - Apparent recovery within a filtration cycle is lower in cross flow mode, since there is a bleed during the filtration cycle. This bleed can be between 2-9%, which will mean a lower apparent recovery as compared to dead end mode. But what is regularly not talked about is

The less frequent requirement for backwash, since majority of particulates do exit the membrane during filtration cycle, and there is a much lesser build up of matter on the surface.
The sufficiency of lower backwash flux rates. (This can be seen by comparing membrane projections for an Dead end vs Cross Flow mode)
The sufficiency of lower flushing flux rates
The lower pressures of backwash, which in turn has lesser probability of fiber breakage. Cross flow, ensures longer, more durable lifetime of the membrane
The reduction in apparent recovery can be circumvented by routing reject back to the secondary clarifier in the case sewage or effluent treatment plants, and back to source in the case of ground or well water.