Reverse Osmosis Desalination membranes troubleshooting and cleaning

Reverse Osmosis membranes must be rinsed and chemically cleaned periodically, when stopped or when performances decrease by 10-15%.

- Post service rinse
- Chemical cleaning
- Troubleshooting guide

Post service rinse

Seawater flows tangentially along the membrane, creating an salt concentration gradient along the membranes length, the last element having the most concentrated brine bulk.

When the RO is stopped or in stand-by, natural osmosis will happen between the permeate side and the concentrate side containing high salinity brine. This can damage the feed spacers by creating a vacuum in the permeate line, as water will naturally flow back to the concentrate side, driven by osmotic pressure.

To avoid this natural damaging osmosis to happen, seawater and brine are flushed off the membranes after service by permeate water taken from the permeate tank (before chlorination) and pushed in the membrane by a low pressure pump (i.e., feed pump, distribution pump or specific cleaning pump)

Membrane Chemical cleaning

Seawater flows tangentially along the membrane, creating a boundary layer on the membrane surface.

Membranes have to cleaned typically when:
- Normalized Permeate flow varies by 10-15%
- Normalized Feed pressure varies by 10-15%
- Normalized Permeate conductivity varies by 10-15%
- Pressure drop between feed and concentrate varies by 10-15%

Normalized values take into account temperature and salinity variations in feed water.

In order to ease chronicle cleaning, our systems can be equipped with cleaning In Place (CIP) station, readily connected to the membranes rack:

The CIP station, depending on the plant size includes a chemical tank with mechanical or manual stirrer, a CIP pump and a fine filter to avoid debris to enter the membranes. The chemical tank depends on the number of membranes to be cleaned at the same time.

Alkaline and Acid Cleaning solutions are recirculated around the membranes for at least 30 minutes.

We determine routine cleaning chemicals and procedures upon desalination plant layout and fouling identification:

RO Troubleshooting table: Analyzing the problem:

Permeate salinity	Permeate flow	Pressure drop	Possible cause
rapid increase	rapid decrease	rapid increase	Metal oxide fouling
marked increase	gradual decrease	gradual increase	Mineral scaling
slight increase	gradual decrease	gradual increase	Colloidal fouling
normal to increased	decreased	normal to increased	Polymerized silica
decreased	marked decrease	marked increased	Biological fouling
decreased	decreased	normal to increase	Organic fouling
increased	increased	increased	Chlorine damage
increased	increased	normal to decrease	Abrasion damages
increased	normal to increase	normal to decrease	O-ring leaks at inter-connectors or adapters
increased	normal to increase	normal to decrease	Glue line leaks due to permeate back pressure