WATER CHEMISTRY IMPLICATIONS OF HIGH RECOVERY MEMBRANE SYSTEMS

摘要

Reverse osmosis (RO) systems separate and simultaneously concentrate ions in solution as they de-water the feed stream. As a result, the feed solution becomes more and more concentrated as it sequentially passes through the membrane modules in series within the system. The concentrations of feed compound can rise rapidly as the water recovery increases. For example, the concentrated feed, or brine residual stream, from an RO system operating at 75% water recovery contains compounds that are approximately four times as concentrated as they were in the raw feed entering the system. Increasing the water recovery by only 5% increases the concentration of these compounds in the brine by another 25%, or to approximately five times the feed concentration. The implications of the increasing concentrations of sparingly soluble compounds and the potential interactions between species of compounds that may occur as they become more and more concentrated must be considered when designing an RO membrane system for high water recovery. Moreover, these concentration effects must be evaluated at the system operating pH, temperature and pressure, since ionic compound solubility is affected by these conditions. In addition to the effect that water recovery has on the concentrations of ionic compounds, the resultant effect of high salt concentration at the membrane surface also needs to be considered. As the feed stream is transported through the membrane barrier, water molecules travel through the membrane more quickly than the various ions in solution and these solute ions collect on the membrane surface at a higher concentration than the feed stream. This effect is termed concentration polarization in the industry. A well-designed system has sufficient cross-flow velocity within the membrane modules throughout the system array, and therefore sufficient back-diffusion from the membrane surface into the bulk feed flow within the module. However, as recovery increases, concentration polarization effects also increase and the ion solubility may be exceeded at the membrane surface even if it is not exceeded in the bulk module feed flow. As the need for fresh water supply increases, especially in semi-arid climates, there is a need to increase the recovery of both new and existing RO systems to meet the demand. The selectivity of most RO membranes today is high enough to be able to produce excellent quality from available brackish (and seawater) sources at high water recovery. Cost-effective disposal of the concentrated brine residual is one of the primary factors affecting the feasibility of RO projects at either coastal or inland sites. With limited options for RO brine disposal, the volume of brine is a factor in design decisions, and high recovery should be investigated during the membrane process development phase of a project. Higher recovery coupled with high separation capability increases the overall efficiency of an RO system.