Dead-End Membrane Module
When dead-end filtration takes place, all the water that enters the membrane surface is pressed through the membrane. Some solids and components will stay behind on the membrane while water flows through. This depends on the pore size of the membrane. Consequentially, the water will experience a greater resistance to passing through the membrane. When feed water pressure is continual, this will result in a decreasing flux. After a certain amount of time the flux has decreased to such an extent, that the membrane will need cleaning.
Dead-end management is applied because the energy loss is less than when one applies a cross-flow filtration. This is because all energy enters the water that actually passed the membrane. The pressure that is needed to press water through a membrane is called Trans Membrane Pressure (TMP).
The TMP is defined as the pressure gradient of the membrane, or the average feed pressure minus the permeate pressure. The feed pressure is often measured at the initial point of a membrane module. However, this pressure does not equal the average feed pressure, because the flow through a membrane will cause hydraulic pressure losses.
The main disadvantage of a dead end filtration is the extensive membrane fouling and concentration polarization. The fouling is usually induced faster at the higher driving forces. Membrane fouling and particle retention in a feed solution also builds up a concentration gradients and particle backflow (concentration polarization). The tangential flow devices are more cost and labor-intensive, but they are less susceptible to fouling due to the sweeping effects and high shear rates of the passing flow.
During cleaning of a membrane, components are removed hydraulically, chemically or physically. When the cleaning process is performed, a module is temporarily out of order. As a result, dead-end management is a discontinuous process.
The length of time that a module performs filtration is called filtration time and the length of time that a module is cleaned is called cleaning time. In practice one always tries to make filtration time last as long as possible, and apply the lowest possible cleaning time.
When a membrane is cleaned with permeate, it does not have a continuous production of water. This results in a lower production. The factor that indicates the amount of production is called recovery.
The most commonly used synthetic membrane devices (modules) are flat plates, spiral wounds, and hollow fibers. Flat plates are usually constructed as circular thin flat membrane surfaces to be used in dead-end geometry modules.