Maximizing Membrane Efficiency

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.

Cleaning Parameters

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 [email protected].