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.
2. Current Standard Operational Procedure
Currently, in order to reduce the damage due to oxidization, most membrane units will employee either or both the addition of a reducing agent like sodium metabisulphite and or activated carbon. These processes aid the removal of active oxidizer in the final feed to the membranes and reduce the risk of oxidation damage.
However these processes are not 100% efficient and over time small amounts will reach the membrane and oxidation will occur. Sometimes due to process control failures, large amounts of oxidizer reach the membranes and severe oxidation damage can occur quickly, leaving the plant in an unforeseen situation and unable to supply the correct quality of permeate water.
In both cases, the current solution is to replace the membranes. This is a time consuming and costly process, especially if needed before planned replacements are due.
3. New State-of-the-Art Solution – KURIVERTER RC
Kurita Water Industries Ltd. has developed a solution which can extend the life of a membrane that has been damaged by oxidation – KURIVERTER RC is a rejuvenation process that chemically patches the membrane and can restore salt rejection and permeate quality to almost new conditions.
4. Product Description and RC Process
KURIVERTER RC is a three component system named KURIVERTER RC 200, RC 300 and RC 400. All three components are powders and are mixed and diluted using the standard available CIP cleaning system installed with the membrane unit. Typically we need a mixing unit on the CIP tank to ensure good homogenous dissolution before circulation. Otherwise no other equipment is necessary.
A prerequisite to starting the RC process, is that the membranes are thoroughly cleaned using either the standard CIP process for the system or in cases of severe membrane deposits or fouling, Kurita will recommend a specialized cleaning process. Scaled or fouled membrane surfaces will prevent the reaction of the RC 200 with the carboxylate groups and the rejuvenation process will be ineffective – CLEAN MEMBRANE SURFACES ARE ESSENTIAL before starting the RC process.
For each application, a customized cleaning and RC application schedule will be compiled by Kurita technical staff to ensure success based on site design and requirements. The basic procedure after cleaning the membranes is simple – each component is dissolved and circulated for the specified time in sequence. There are no flushing steps between component additions. In some cases a pH adjustment step is needed.