Practical tips

Storing membrane elements

Elements taken out of service are usually preserved in a sodium bisulfite solution of 1 to 1.5%. Flush and preserve again about every three months, and sooner when it is warm. Check the pH of the solution regularly. If it drops below 3, replace the solution. During storage, pressure build-up can indicate bacterial growth and gas production. The storage procedure of your membrane manufacturer governs.

Starting up new elements

Flush new membranes with a short alkaline CIP and then measure the clean water flux (CWF). The CWF is the flux on clean water at a fixed TMP. It shows the state of the membrane and mainly indicates how effective the cleaning is. The first CWF, before any production, is the zero measurement.

After the first production run and CIP, a membrane can lose up to 30% of its CWF value. This is normal. After the second run and CIP there is some further decrease. In the days after that, the CWF should stabilise.

Over time, CWF and retention slowly decrease. Set limits on performance and compare them with the actual situation: that determines when to replace membranes.

Reading a membrane data sheet

The important items are the retention and/or cut-off, the dimensions and the processing limits. The CIP resistance also matters: many NF and RO membranes cannot be cleaned with chlorine.

Units in membrane data