The nature and rate of contamination are related to the water supply conditions; contamination accumulates gradually. If measures are not taken early on, contamination can relatively quickly degrade the performance of membrane elements. Different contaminants can cause varying degrees of damage to the membrane elements. Whether membrane elements are contaminated can be determined by regularly monitoring the overall performance of the system.
Suspended Solids
Turbidity is commonly found in surface water and wastewater, with particle sizes greater than 1μm (colloids may be smaller than 1μm). In unmixed solutions, these particles can settle out as suspended solids (colloids remain suspended). After pretreatment, the indicators should be reduced to turbidity less than 0.5 NTU and SDI15 less than 5.
Membrane Pollution Symptoms: Increased inlet pressure, increased inter-pass pressure, increased membrane weight, reduced water production, and decreased desalination rate.
Remedial Actions: Inspect the sealing condition of the front-end precision filter or replace the filter element (or filter bag).
Colloidal Pollutants
Colloidal pollutants are widely present in surface water or wastewater, predominantly located at the front end of reverse osmosis systems. These particles, measuring less than 1μm, remain suspended in non-stirred solutions and can be single or composite compounds made up of organic or inorganic components. Inorganic components may include silicic acid, iron, aluminum, and sulfur, while organic components may consist of tannic acid, lignin, and humus. Post-preprocessing, the indicators should be reduced to turbidity less than 0.5 NTU and SDI15 less than 5.
Membrane Contamination Symptoms: Increased feedwater pressure, increased pressure drop between segments, and reduced water production.
Action Plan: Inspect the sealing condition of the front-end precision filter or replace the filter element (or bag), assess the rationality of the dosing of scale inhibitors and other chemicals, and check for incompatibility between chemicals.
③ Organic Pollutants
Organic pollutants are primarily found in the front end of the reverse osmosis system, commonly present in surface water or wastewater. They typically adhere to the membrane surface, being natural humic organic matter derived from decomposing plant material and often charged. There is no specific TOC (total organic carbon) content specification for the feedwater of reverse osmosis membrane elements, but a TOC content of 2x10^-5 in the feedwater should be noted.
Membrane Contamination Symptoms: Decreased water production, increased feedwater pressure, increased inter-pass pressure, increased membrane weight, decreased desalination rate, and off-odor.
Remedial Actions: Alkaline chemical cleaning, adjusting operational parameters of biological treatment processes (if a biological treatment process is in place at the front end).
④ Biological Pollutants
Biological pollutants are commonly found in surface water or wastewater, characterized by their tendency to initially form at the front end of reverse osmosis systems, then expand throughout the entire system. The pollutants typically include bacteria, biofilms, algae, and fungi, with a warning concentration of 10,000 cfu (colony-forming units) per milliliter. Therefore, it is essential to control biological activity.
Membrane Contamination Symptoms: Increased inter-pass pressure, reduced product water yield, increased feedwater pressure, decreased desalination rate, increased membrane weight, and off-odors.
Remedial Measures: Alkaline chemical cleaning, addition of non-oxidizing disinfectants (either regular shock dosing or stable continuous dosing).
