APPLICATION SPECIFIC MEMBRANE CONSIDERATIONS
Seawater Desalination for Drinking Water - Along with large amounts of sodium chloride, seawater also contains smaller amounts of other ionic contaminants including boron. Boron is damaging to the health of plants and animals for the concentrations it which it is found in most seawater feedwaters. Although most seawater RO membranes reject boron to some degree, the porosity and surface charge of the membrane can make a difference in meeting specific treatment goals. Most seawater membranes list boron rejection under test conditions that can help narrow down specific membrane models to be used in system projections. The species of boron present in water is pH dependent and so pH is an important factor in Boron rejection.
Brackish Water for Industrial Commercial Applications - Brackish water is typically defined as those waters with TDS in the range of 5,000 to 15,000 PPM TDS. Brackish water RO systems typically operate in the 200 to 600PSI range depending on exact feed water TDS values. Along with general TDS rejection, there are specific ions of interest for industrial applications depending on downstream equipment. Most important for industrial applications is silica. Silica is weakly charged in water and so is more difficult to reject. Also because silica is weekly charged, it is weakly heldy by any downstream ion exchange systems and is the first ion to breakthrough (come off of) an anion resin bed. Breakthrough of silica past the anion bed of an ion exchange system typically triggers ion exchange regeneration so the less silica to pass through an RO membrane, the less often the ion change system needs to be regenerated. Regeneration of ion exchange systems requires system down time and chemical expense so significant savings can be made by maximizing upstream membrane silica rejection.
Commercial/Residential Applications - With some exceptions, most commercial and residential RO applications such as car washes and municipal drinking water point of use (inside a home) applications are bulk dissolved solid removal type applications where price and energy consumption are the most important factors. For these types of applications, the TDS of the feedwater is typically in the 250-1000PPM range and so extra low energy membranes with pressures as low as 100PSI are typically used.
RO AND NF MEMBRANE FOULING CONSIDERATIONS
In addition to dissolved solid rejection, another area of interest is membrane durability and fouling. Fouling of membranes usually occurs in three different ways- biological fouling of the membrane surface and/or feed spacer, inorganic fouling of the membrane surface and plugging of the feed channel with particulate matter. Feed water from surface water sources or other biologically active waters can cause biofouling (biological buildup or growth on the membrane surface or feed channel spacer). Biofouling is most effectively removed by high pH chemical treatments however high pH can damage some membranes causing reduction of rejection. Refer to manufacturer's datasheets for the allowable cleaning range for any membrane and remember that pH is a logarithmic scale and a pH of 12 is 10 times more basic than a pH of 11. Fouling in the feed spacer layer of RO and NF membranes can be reduced or prevented. Inorganic membrane surface fouling is ususally caused by exceeding the soluability of one or more ions in solution. Running accurate system projections can help identify feed waters that antiscalants are recommended for reducing scaling or membrane surface fouling by inorganics. Also, running an RO system at higher fluxes can cause scaling at the membrane surface through a process called concentration polarization. Concentration polarization describes a temporary buildup of ionic concentration at the membrane surface that is typically caused by operating at too high of a flux rate for influent water conditions and/or insufficient cross flow (waste water flow) across the membrane surface. Reducing recovery percentage (percent of product water vs feed water) is the easiest way to prevent or reduce concentration polarization. Reject recirculation can also be used to limit concentration polarization without sacrificing recovery rate (but does reduce product water quality and decrease overall solubility limits). Finally particulate matter plugging of the feed channel is common where particulate pretreament is insufficient. RO systems should have a minimum of high efficiency 5 micron filtration for the influent (1 micron preferred). A silt density index test can help identify if the pretreatment is sufficient. Also, choosing a membrane element with a wide feed spacer (34mil or larger) can help to reduce particulate plugging.
