Direct membrane cleaning

Liberman, B. (2004) Methods of direct osmosis membrane cleaning online for... 

The techniques and chemicals described in Chapter 13.2 apply directly to on-site or Clean-In-Place (CIP) membrane cleaning. 

Mores W.D. and Davis R.H., Direct observation of membrane cleaning via rapid backpulsing. Desalination 146 2002 135-140. 

The main cost factors are represented by equipment depreciation, membrane replacement, and electric power consumption. The other costs (man power and water consumption) are of minor influence. Membrane filtration plants equipped with a low level of automation require only a few hours of attention and direct surveillance by the operator per day. Direct surveillance of membrane filtration equipment is necessary at times when batches need to be changed or during membrane cleaning. The electric power consumption is associated with the use of pumps to move the viscous yeast slurry over the membrane surface, and its estimation is relatively straight forward. 

Whole-cell, hollow-fiber MBR are still under development. Despite their significant potential they have, so far, found only limited application for biochemicals production. One of the reasons is that cleaning of the hollow-fiber membranes is difficult, especially when whole-cell biocatalysts are immobilized in the small fibers. The mass transfer between the nutrients and cells has also to be taken into consideration and enhanced. Immobilizing the biocatalysts in porous beads, instead of directly on the membrane, may tend to avoid some of these problems, and to simplify membrane cleaning. The concept of using MBR as bioartificial organs is technically very attractive the various MBR under development, however, must still be validated with clinical results. One can expect, however, that their development will follow the success of artificial kidneys, which are currently employed worldwide. 

Microbial fouling is best dealt with before biofilm becomes mature. Biofilm protects the microorganisms from the action of shear forces and biocidal chemicals used to attack them. Microbes can be destroyed using chlorine, ozone, ultraviolet radiation, or some non-oxidizing biocides (see Chapters 8.2.1, 8.2.2, and 8.1.8). An effective method to control bacteria and biofilm growth usually involves a combination of these measures. Specifically, chlorination or ozonation of the pretreatment system, followed by dechlorination to protect the membranes, or UV distraction followed by periodic disinfection with a non-oxidizing biocide used directly on the membranes to keep the membranes clean. 

A.2 Cross-Flow, Dead-End Configurations Microfiltration and UF systems are operated in two possible filtration modes. Figure 6.10 shows the cross-flow configuration in which the feed water is pumped tangential to the membrane. Clean water passes the membrane while the water that does not permeate is recirculated as concentrate and combined with additional feed water. To control the concentration of the sohds in the recirculation loop, a portion of the concentrate is discharged at a specific rate. In dead-end or direct filtration, all the feed water passes through the membrane. Therefore, the recovery is 100%, and a small fraction is used periodically for backwash in the system (5-15%). 

The inhaent flux decUne in many membrane filtration processes requires procedures to reverse the decline (for the reasons discussed above). These membrane cleaning opaa-tions can involve back-fiushing (reversal of flow direction) at... 

The filter usually has an endless cloth, traveling intermittently between the plates via roUers, to peel off cakes. Unfortunately, if the cloth is damaged anywhere, the whole cloth must be replaced, which is a difficult process. Each time the filter cloth zigzags through the filter, the filtering direction is reversed this tends to keep the cloth clean. Most of these filters incorporate membranes for mechanical expression, and cakes sometimes stick to the membranes and remain in the chamber after discharge. Some vertical filters are available with a separate cloth for each frame. The cloths maybe disposable and such filters are designed to operate with or without filter aids. 

Most solution-cast composite membranes are prepared by a technique pioneered at UOP (35). In this technique, a polymer solution is cast directly onto the microporous support film. The support film must be clean, defect-free, and very finely microporous, to prevent penetration of the coating solution into the pores. If these conditions are met, the support can be coated with a Hquid layer 50—100 p.m thick, which after evaporation leaves a thin permselective film, 0.5—2 pm thick. This technique was used to form the Monsanto Prism gas separation membranes (6) and at Membrane Technology and Research to form pervaporation and organic vapor—air separation membranes (36,37) (Fig. 16). 

Economic Yield Both in a high-value protein separation and in a low-value commodity concentration, economic yield is vital. Economic yield is defined as the fraction of useful product entering the process that leaves it in salable form. The yield equations used in the industry focus on retention, so they deal only with direct losses through the membrane. These losses result both in direct (product not sold) and indirect costs from a waste stream whose disposal or subsequent use may be more expensive when it is contaminated by macrosolute. There are additional indirec t losses, mainly product left in the equipment, particularly that left adhering to the membrane. Costs of cleaning and disposal or this indirect loss, while hard to measure, are usually higher than the cost of product lost through the membrane. 

Articles which are to he discharged from the clean room (or elsewhere) to the aseptic area must he sterilized. To achieve this they should be transferred via a double-ended sterilizer (i.e. with a door at each end). If it is not possible, or required, that they be discharged directly to the aseptic area, they should be (i) double-wrapped before sterilization (ii) transferred immediately after sterilization to a clean environment until required and (iii) transferred from this clean environment via a double-doored hatch (where the outer wrapping is removed) to the aseptic area (where the inner wrapper is removed at the workbench). Hatchways and sterilizers should be arranged so that only one side of the entry into an aseptic area may be opened at any one time. Solutions manufactured in the clean room may be brought into the aseptic area through a sterile 0.22-/im bacteria-proof membrane filter. 

---