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Maximizing Membrane Efficiency
Optimization of the cleaning cycle has a high return on investment for food, dairy and beverage producers.
by Brad Milne
Crossflow membranes are widely used in the production of milk and whey products, wine, vinegar, gelatin and fruit juices and are utilized for concentration, clarification and fractionation - each of which requires specific membrane polymers and pore sizes.
Proper cleaning and disinfection of crossflow systems must be tailored to the particular process feed stream contaminants and to the geometry and material composition of the membrane. Different membrane materials have varying levels of tolerance to pH, temperature and chemical exposure. The cleaning regime should also account for variations in water quality.
Sanitation regulations require that food plant operators clean membrane systems on a frequent basis - typically once per day - a potentially expensive and time-consuming process that can harm the membranes and diminish their life. Establishing an efficient and cost-effective cleaning regime is a critical aspect for all food and dairy applications.
The leading suppliers of membrane cleaning chemicals provide broad ranges of products. These fall into six basic categories: Alkaline compounds, mineral acid blends, surface active agents, enzymes, disinfectants and preservative solutions. As many as four chemical clean-in-place (CIP) steps are normally required to adequately clean and sanitize a membrane plant. Using the proper formulation of ingredients and operating conditions, this process can be accomplished economically with repeatable results.
The first step in establishing a cleaning protocol is to understand the requirements and limitations of the membrane in use. The materials of construction (including backing, feed spacer, permeate support, glues and epoxies), configuration and pore size are all factors in developing suitable cleaning regimes.
The most common membranes in use in the food industry are constructed of polyethersulfone (PES) and polyvinylidene fluoride (PVDF). These are durable polymers that can withstand pH levels from 2 to 13 and temperatures to 75ºC. They are partially hydrophilic making them suitable for most ultrafiltration (UF) and microfiltration (MF) food applications.
Nanofiltration (NF) and reverse osmosis (RO) membranes have PES substrates, but they also have a secondary thin-film composite membrane comprised of a polyamide or similar material with specific chemical limitations that must be considered.
Crossflow membranes are manufactured in three configurations, tubular, hollow fiber and spiral wound. In addition, membrane products vary from open pore-size MF membranes to tighter NF and RO products. In all cases, the primary design criteria for cleaning is the provision for adequate crossflow velocity to sweep the membrane surface clean and provide sufficient fluid contact to all sections of the membrane (feed and permeate sides) plus associated piping.
Depending on the specific configuration and the given application, the cleaning flow velocities may be greater than or less than the corresponding processing conditions. The optimal cleaning regime consists of an adequate shear rate to effectively scour the membrane surface combined with the appropriate chemistry, such that the procedure does not significantly decrease membrane life.
For tubular and spiral wound modules, this is easily achieved at flow velocities somewhat lower than the design production conditions. An exception to this would be spiral wound elements processing very high viscosity streams where the cleaning flow required is higher than the process flow, e.g. the last stage of a whey protein isolate (WPI) system. In these cases, it is often expedient to design a split-stage cleaning arrangement that allows one-half of the stage to be cleaned at a time to accommodate pump and pipe sizing.
For hollow fiber systems that generally operate at lower velocities, cleaning flows are equal to or higher than the process parameters in order to provide sufficient cleaning turbulence. Unfortunately, ideal cleaning conditions are often unavailable. This is one more reason why optimizing the chemical effectiveness plays such an important role.
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.
Some membrane polymers do not tolerate chlorine and many locales have restrictions on the use or discharge of chlorine. In these instances, a peracetic acid solution is the best alternative (peracetic acid is an equilibrium compound formed in the presence of acetic acid and hydrogen peroxide). It provides better cleaning and disinfection than peroxide alone. Peracetic acid is also known as peroxyacetic acid.
When a membrane system is idle for extended periods, a preservative solution should be used to stabilize the storage solution. Long-term preservatives can be a blend of mineral and organic acids. Sodium metabisulfite is also a good choice with the added benefit of removing residual chlorine. Peracetic acid may be used in reduced dosages to store the system during short downtimes (less than 24 hours). A preservative may even be recommended when the downtime is as brief as four hours, as this is sufficient time for traces of chlorine to have a detrimental effect on the membrane polymer.
As a general rule, individual cleaning steps are 20 to 30 minutes long, although enzyme cycles may also involve a four-hour soak. The rinses between cycles require 10 to 15 minutes, and the entire CIP sequence is usually complete in 2 to 4 hours. The alkaline cycle is performed at a pH of 10.5 to 11.5, acid at 1.8 to 2.5, chlorine at 10.0 to 10.5 and storage at 3.5 to 4.0. Solution temperatures range from 40 to 55ºC for food and dairy processes with 50ºC being the norm. In general, the biggest bottleneck in the cleaning procedure is the availability of hot, soft water to fill the CIP tank between steps.
During the first weeks of operation, it is normal for the clean water flux of UF membranes to decline below the initial value by 5 to 10 percent. This is a natural consequence of membrane compaction plus a minimal degree of irreversible fouling that cannot be avoided. This tendency may be more pronounced with MF membranes due to their open pore structure. RO and NF membranes are subject to a drop in water flux by as much as 20 to 30 percent as exposure to high pressures and temperatures change the characteristic of both the composite membrane and backing.
After a one to two-week break-in period, this trend should be complete. Water fluxes should be repeatable within a ± 15% range. Operators should not be concerned about minor day-to-day variations as changes in feed stream foulants, water quality, and chemical strength will provide fluctuations in the actual clean water permeation rate. Also, many CIP regimes have steps that are only performed intermittently (e.g. once per week). This may be true for acid steps in juice plants and enzymes in dairy RO and NF systems. The result may be a noticeable difference in the water flux following the extended procedure.
As noted, many factors influence the cleaning effectiveness for a particular system. Industry experts apply general knowledge from laboratory research and field experience, but verifying a specific cleaning routine requires diligent record-keeping of the CIP performance. Optimization of the cleaning cycle has a high return on investment for food, dairy and beverage producers because it can significantly extend the life of the membranes as well as reduce chemical usage and labor costs.
Brad Milne is a processes technology leader for Koch Membrane Systems 978-694-7131 or firstname.lastname@example.org.