Uniform Transmembrane Pressure Microfiltration

Pressure profiles

Figure 1. Circulation systems in the MF module with uniform transmembrane pressure. (Reproduced with the permission from Reference [7]).

On the permeate side of the module, the space which is normally empty is filled with plastic grains in the UTP version as indicated in Figure 1 [7]. The high-speed circulation of the permeate causes a pressure drop inside the channels. The pressure drop on the permeate side is adjusted by the permeate pump. This results in a uniform transmembrane pressure over the whole membrane area which in turn results in an optimal and controllable utilization of the membrane area. When the membranes start to clog, the pressure/permeate valve is opened, thereby increasing the transmembrane pressure. Thus, the permeate flux at the outlet is constant. As the transmembrane pressure determines the flux, a higher transmembrane pressure leads to a higher flux. However, clogging of the membranes then takes place faster and the running time is decreased.

For polymeric, hollow fibre microfiltration membranes, the Dutch company X-flow has a patent pending concerning the use of the so-called "BACKSHOCK process", a further development of backflushing, a mode of operation in which the transport direction through the membrane is reversed by applying pressure in the opposite direction so that the feed enters the module through what is normally the permeate outlet. The backflush process is optimized both for the duration of the backflushing pressure and for the backflushing interval. The improvement in the product flow rate achieved with backflushing is mainly a function of the backflushing pressure and the interval between two backflushes. X-flow found that extremely good results could be obtained using very short backflushing times (typically 0.06 second) with a time interval of maximum of 5 seconds, preferably 1 to 3 seconds. Since the effective backflushing time is very short and the backflushing pressure is relatively high (typically 0.1 MPa over the feed pressure), the name BACKSHOCK was introduced. The loss of permeate during backshocking is said to be very low and hardly affects the net permeate flow. With this technique, quite good fluxes were obtained using very low pressures and crossflow velocities [8].

2.7 Membrane cleaning and desinfection

The cleaning and desinfection of membranes as well as the auxiliary equipment is essential, in order to maintain high fluxes, as well as to maintain a good bacteriological status. In the membrane treatment of biological materials, e.g., liquid foods, membrane equipment is usually cleaned at least once per day [9,10],

Much work has been devoted to the investigation of cleaning methods, often based on trial and error. Mechanical cleaning can be effected by introducing a high shear rate at the membrane surface. Some material can easily be removed by rinsing with water at a high rate of circulation and reduced pressure.

Chemical cleaning is essential. The method of cleaning depends, naturally, on the membrane material and its chemical and temperature resistance, as well as the composition of the process liquid and the deposits on the membrane surface or its interior. Acids are used to remove mineral deposits, e.g., 0.5% HN03. Protein deposits as well as other types of deposits are often removed with an alkaline cleaner containing 0.5-1% NaOH. Formulated cleaning agents, containing alkalis, phosphates, complex-forming agents and tensides, are often used. For some types of membranes and deposits, cleaning agents containing enzymes are used, e.g., proteases and amylases. Some kinds of surfactants can cause severe flux reduction problems and is thus very important to follow the membrane manufacturer's recommendation and not to change the cleaning agent without consulting the manufacturer. The use of cleaning agents containing unsuitable surfactants may result in having to replace the membranes in the whole plant.

Chlorine is often present in alkaline cleaning agents. These agents clean more effectively than the same agent without chlorine and also kill bacteria. Hypochlorite has a certain cleaning effect on its own, and has been successfully used for the cleaning of membranes after the ultrafdtration of potato fruit juice. Some membranes, however, cannot withstand oxidizing agents, and a degree of caution is therefore recommended. The use of chlorine in the food industry is debated and it might not be allowed in the future.

Hypochlorite, hydrogen peroxide, peracetic acid or sodium bisulphite are usually employed for the disinfection of membranes. Some inorganic membranes can be steam sterilized.

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