During normal operation of an R/O water treatment system, the membrane elements will often suffer a reduction in performance due to the accumulation of small particles, colloids, microorganisms, or precipitated salts collecting on the membrane surface.
Some of these deposits can result in a catastrophic membrane failure in a relatively short period of time. Pretreatment methods, such as cartridge filtration and chemical pretreatment, are intended to reduce the accumulation of these troublesome deposits.
The deposition of these particles is referred to as scaling or fouling. Mineral compounds such as calcium carbonate and the sulfates of barium, strontium, and calcium form hard crystalline precipitates (scalants). Colloidal and suspended matter, biological growth, metal oxides and silica form softer deposits that adhere to the membrane element surface (foulants). It is important to identify which types of deposit are present in the membrane element to determine the type of cleaning chemicals to be used.
It is recommended that RO performance data be taken daily when the system is in operation. From this data one can determine when it is time to perform a membrane cleaning. The three most critical operational parameters are differential pressure, normalized permeate flow, and salt rejection. When any of these parameters changes by 10% or more it is time to consider maintenance cleaning. It is most important, however, that the instruments used to measure these parameters be in good working order and properly calibrated. Cleanings should be performed based on operational performance and not as a matter of routine.
Types of Cleaners
Typically, the two types of membrane cleaners are low pH and high pH solutions. Low pH solutions are used to dissolve membrane scales such as calcium carbonate, calcium phosphate, iron sulfide, metal silicates, and metallic oxides. High pH cleaners are formulated to clean colloids, organic suspended matter, and biological microorganisms. As can be seen, some cleaners (low pH) are good for dissolving scalants while others (high pH) are more effective for dissolving foulants. Mineral scale formation results from mineral precipitation. Scale forms when the concentration of certain sparingly soluble constituents in the concentrate stream exceed their saturation point and precipitate onto the membrane surface. Therefore, scales tend to form in the latter stages of an R/O system where the concentrate stream is most concentrated. In a two-stage R/O system the second stage will be most susceptible to scaling, which will increase the second-stage differential pressure. It is also not uncommon to see a corresponding decrease in permeate production from the affected membrane elements.
Colloidal or particulate fouling will usually manifest itself by increased differential pressure in the first stage of an R/O system. Biological fouling, however, will occur throughout the membrane system. The required changing frequency of the prefilter cartridges will indicate a fouling problem and make the appropriate cleaner selection obvious.
Once a successful cleaning procedure has been established, continue to repeat the procedure as long as good results are achieved.
Cleaning System Overview
The system is designed to allow periodic cleaning of the RO membranes in one step with the cleaning pump capable of cleaning all seven RO pressure vessels simultaneously.
The RO elements should be cleaned at the manufacturers recommended rate of 40-45 gpm per vessel, the pump should be set for a maximum of 45*X gpm to enable the required flow be achieved with X number of vessels operating in parallel.
The RO system is supplied with cleaning isolation valves and is hard-piped to the cleaning system using PVC pipe in a permanent fashion making cleaning convenient when it is required.
During the cleaning procedure the cleaning chemicals will release solid particles of dirt, scale metal oxides etc. from the membrane surface. There is a tendency for these to agglomerate into larger particles and get picked-up again particularly at the inlet end of the first membranes in the stage. In order to prevent this; a 5 micron cartridge filter is supplied which filters the feed to the RO system during cleaning circulation. If the membranes are particularly dirty there may be a rapid build-up in these filters leading to high filter head loss. The filters should be frequently checked during the cleaning operation for head loss by comparing the pressure at the cartridge filter inlet with the pressure at the RO stage inlet and calculating the difference. Upon start-up with clean filters, the differential pressure should be less than 2 psi, at no time should it exceed 10 psi. Change cartridge filters before the differential pressure reaches 10 psi.
Types of Cleaners
Cleaning can be carried-out with “generic” cleaners as recommended by the membrane manufacturers; however, Harn recommends Avista Technologies proprietary cleaning chemicals, “ROClean”, these are blends which we have found much more effective than the generic cleaners.
The intent of the following descriptive information is to help guide the operational staff through the necessary procedures and valve operations required to carry-out first and second stage membrane cleanings. Please make sure that basic laboratory facilities and items such as pH and conductivity portable instrumentation are available for solution testing.
The cleaning system should be examined prior to use to make sure the tank is clean inside and a fresh set of elements should be installed in the cleaning cartridge filter.
Provision also needs to be made to allow for transfer of spent cleaning solution and rinse water to waste for disposal.
Following the clean – all of the permeate must be diverted to waste via the permeate return valve. The permeate quality will be virtually that of drinking water, however with traces of the cleaning chemical. During the cleaning process it is advisable that the valve to the distribution system should be 100% closed to avoid the risk of accidentally allowing chemicals to enter the drinking water system. The slightly contaminated water can be sent to the waste effluent system if it has the capacity. Provision should be made to divert at the full train permeate flow rate for approximately 20 minutes.