Applications using Reverse Osmosis membrane technologies are everywhere in our lives – from tiny under counter units polishing our drinking water through industrial units preparing water for processes or purifying effluents for reuse and massive seawater desalination plants producing thousands of m³ of drinking and irrigation water every day. The membranes used in this technology face many potential problems, namely scale and deposits, biofouling, physical stresses and chemical damage. For many of these problems we have developed preventive solutions, in the form of pretreatment, antiscalants, dispersants, biocides and better operating procedures. Membranes that have suffered chemical damage and the resulting increased salt passage however, were simply replaced with the associated high cost and lost production. But now there is an alternative – KURIVERTER RC technology by KURITA. This Technology allows us to rejuvenate the membrane, restore salt rejection and postpone membrane replacement until it is budgeted for and convenient. This at a fraction of the replacement cost using an easy to apply CIP process.
Conditions within reverse osmosis membranes are favorable for the growth and proliferation of bacteria. Unchecked bacterial growth will rapidly lead to biofilm formation and reduce flux through the membrane, affecting both feed and permeate flowrate. Energy consumption to maintain permeate flow will increase as the situation deteriorates and eventually lead to plant shut down due to unacceptable permeate quality.
The most common and cost effect way to date of controlling bacterial growth, has been by means of chlorine addition in either or both the intake of SWRO or in the pretreatment section of the membrane unit.
Chlorine and other oxidizers are however damaging if they come into contact with the polyamide surface layer and the membrane structure will deteriorate over time. Many membrane manufacturers rate their product stability in terms of “chlorine hours”.
Oxidation leads to the cleavage of the amide bonds within the material of construction of the membrane and in effect creates holes which allow ions to pass through the membrane to the permeate, which would normally be retained and rejected in the brine. This is evidenced by an increase in the normalized permeate conductivity and a decrease in the salt rejection.
Oxidation damage can be verified by XPS which will show the exposed carboxlate groups and the increased chlorine levels due to substitution.
This process is irreversible and leads to reduced permeate quality and eventually premature membrane replacement.