Maximizing Membrane Efficiency

Chemical Formulation

Identifying the fouling components in the feed stream is a prerequisite to developing appropriate chemical formulations for each application. Modern crossflow technology has evolved to the level where most problems associated with suspended materials have been eliminated. This is accomplished through the proper selection of the membrane geometry (e.g. open channel tubes vs. spiral wound elements when large quantities of discreet suspended solids are present) plus the specification of efficient pre-filtration/pre-treatment equipment. Thus, the foremost issue in cleaning the membranes becomes the removal of soluble materials that have adhered to or been deposited on the membrane surface.

The contaminants most commonly found in food/dairy/beverage processes include proteins, minerals, lipids, pectins, biofilms and small particulates or precipitates. Well-designed equipment will minimize the degree of fouling that occurs during production thereby facilitating the cleaning regimen.

Effective cleaning is a combination of several factors including time, temperature, dosage and turbulence. The proper selection of chemicals and the specific order of the CIP steps are critical to efficient cleaning. Restoration of acceptable water flux, limitation of membrane exposure to chemicals, and minimization of chemical costs are the drivers for successful optimization.

The most complex cleaning step is the alkaline cycle. Most protein-derived deposits are hydrophobic. Well-designed formulations of surface active agents combined with high pH are required to remove proteins and lipids from the membrane surface due to the difficulty in achieving adequate wet-out. In addition, the solution must be properly buffered to maintain the correct pH as the concentration of the alkaline base changes due to the reaction with contaminants. For this reason, pure caustic soda is seldom recommended for membrane cleaning. Often two short alkaline steps are performed in succession as opposed to one longer cycle.

Acid solutions are used to eliminate the effects of milk-based calcium fouling and water hardness. A blend of phosphoric and nitric acids has been found to be suitable for this purpose. Depending on the particular feed stream, the acid cycle may be performed before or after the first alkaline cycle. Most milk/whey plants clean daily with acid; other food plants use it only needed.

The goal of surfactants is to improve the wetability of fouling deposits on the membrane. A mixture of nonionic and anionic surfactants should provide the best performance. Specifically, nonionic compounds should be chosen with an ethylene oxide content of 10 to 12 moles as these exhibit the best lipid removal characteristics and rinsability. Rinsability is an important parameter, as many surfactants do not wash out well resulting in an undesirable reduction in water flux. Chelating agents and sodium EDTA may also be incorporated for water conditioning.

Enzymes are another choice and provide enhanced cleaning results in many applications. RO and NF processes specifically benefit from an enzyme step because chlorine is not permitted in the cleaning formulation due to limitations of the secondary polyamide membrane. Enzymes, primarily proteases, fill the void. Juice producers often benefit from enzymes as well, but in this case, a combination of pectinases, cellulases and proteases are most effective. Enzymes often require a prolonged soak, so these cycles may be designated for use only once or twice per week.

The final step in the daily CIP procedure is sanitization. Chlorine is typically used to sanitize UF and MF systems; however, a significant cleaning benefit is also derived. The downside is that chlorine is the most aggressive cleaning chemical with respect to membrane degradation. The chlorine concentration must be closely monitored and the pH of the solution controlled to 10.0 to10.5. The elevated pH provides a control on the amount of active chlorine available for undesirable oxidation.