The Process and Water Quality Specialists
Pretreatment is the cornerstone of any desalination process. Selecting appropriate pretreatment is imperative to the success of any desalination project. In the absence of adequate pretreatment, the membrane surface becomes rapidly covered by the rejected material present in the feed water and this results in membrane fouling or a drop in membrane productivity with time, increasing operational costs. Fouling of desalination membranes can be broken into four distinct categories: fouling by particulate matter, organic fouling, biological fouling, and inorganic scaling.
Fouling with Particulate Matter and Suspended Solids. If not properly dealt with, particulate matter and fine suspended solids present in the feedwater are problematic and will reduce the water throughput of the desalination membranes with time. Although, ED membranes are thought to be traditionally more robust and can handle some influent particulate matter (i.e <2 NTU), excessive particulate matter and influent suspended solids will accumulate in the spacers for the NF/RO/ED membranes and increase the head loss through the system (no effect on transmembrane pressure). Depending on the feedwater quality, a filtration system needs to be designed to reduce the influent suspended solids and particulate matter before feeding the water to the desalting membranes.
Fouling with Organic Materials. Desalination membranes are all susceptible to organic fouling depending on the source water quality. ED anion-exchange membranes are particularly susceptible to organic fouling due to the negative charge associated with natural organic matter. This can lead to process failures. Large organic anions cannot penetrate the anion-exchange membrane and will accumulate and adsorb to the membrane surface, increasing the stack resistance. Small organic molecules can also be problematic because they penetrate the membrane, but their electromobility is low and they remain inside the membrane. Fouling of this kind can make it quite difficult to clean and restore these membranes to their original electrical resistance. NF and RO membranes are fouled by organic adsorption as the membrane rejects these materials and membrane permeate is produced. It is difficult to determine the organic fouling potentials using aggregate organic measurements (i.e. TOC) in the feedwater. Some source waters with relatively low TOC (< 1 ppm) result in dramatic organic fouling while waters with high TOC (>8 mg/L) do not.
Biological Fouling. Biofilm control is important in virtually every unit process, which sets out to accomplish mass transfer in an aqueous system. Membrane manufacturers have come a long way in reducing the biodegradability of the membranes themselves, but most modern RO and NF membranes are sensitive to oxidants. Without the use of oxidizing disinfectants, it is unlikely that biofilm control will ever be adequately achieved.
Although turbulent cross-flow is maintained in all desalination membrane systems, bacteria are still capable of adhering to the membrane surface and excreting extracellular polymeric substances (EPS) to create a strong bond to the membrane surface. Once attached to the membrane, a complex community of microorganisms is created that is held together and fixed to the membrane with EPS. These consortia are generally referred to as a biofilm. Biofilms result in decreased membrane permeability in pressure driven applications and increased electrical resistance in potential driven processes, increasing the operational and maintenance costs of desalination membrane processes. The rejection of targeted contaminants can also be adversely affected.
Fouling From the Formation of Inorganic Scale. Rejection of dissolved solids with desalination membranes segregates salts into a waste stream commonly referred to as concentrate or brine. If the concentrations of these salts exceed their solution saturation, precipitates will form a scale of inorganic salts (e.g. Fe2O3, CaCO3, CaSO4, SiO2, CaF2, BaSO4, etc.) on the membrane surface. Scaling usually develops in the final stage of the RO or EDR process at the membrane surface because this is the active point of ion separation where concentrations are highest. By adjusting the feedwater recovery, the design engineer can estimate the concentrations of the dissolved solids and specify a system that does not suffer from inorganic scaling. Inorganic scaling, like other forms of fouling, will increase the operational costs and require operator attention to clean and restore the membrane system.