TDS Reduction - Membrane Separation UF, NF, RO:
Technical Profile
The objective of this process is the removal of dissolved minerals (salinity) and particles from water. Salinity can basically be removed in two ways; by electro dialysis (ED) and by Reverse Osmosis. It should be recognized that both technologies only concentrate TDS in a smaller reject stream that still must be disposed of.
ED is appropriate for TDS below 10000 mg/l. Above this level, electrical consumption grows exponentially. ED can reduce TDS to around 200 mg/l before a rapid increase in electrical demand occurs. It can remove 80-90% of dissolved minerals and some low molecular weight organics. It operates at low pressure (less than 2 bar, 25 psig). Recovery can range from 70 to 95%. ED is commonly used to reduce scale and dissolved solids prior to RO or ion exchange. Some pretreatment may be required for proper operation.
During ED, ionized salts are attracted to positive and negative electrodes through selective membranes. Membranes that reject cations are placed alternating with membranes that reject anions between the cathode and anode of the cell. In this manner water is separated into desalted water and concentrated brine in alternating cells.
For TDS greater than 10000 mg/l, Reverse Osmosis (RO) is the process of choice. RO is a process that uses a membrane which is permeable to water but less permeable to dissolved species. It is operated continuously to split a water stream into a salt depleted stream called permeate and a salt rich stream called concentrate. The ratio between permeate flow and feed flow is the recovery. Recovery is always limited to a certain percentage. For water with high TDS and beneficial use requiring low TDS recovery may be as low as 50%, but generally recovery ranges between 50 and 85%. Like ED, RO is not a way of getting rid of salinity. It is only a way of reducing a problem.
Antiscalant chemicals are helpful as they allow the temporary exceedance of solubility products. They keep salts in suspension that should, based on chemistry, precipitate and form a scaling layer on the membrane. Performance is dependent on the membrane chosen, but will be around 99% for NaCl, nearly 100% for bivalent salts as sulphate, but low (80%) for boron (when not operated at high pH).
Silica solubility is a critical factor for RO performance. As silica concentrates on one side of the membrane it approaches its solubility limit. Once this limit is exceeded, silica will deposit on the membrane as a scale and plug the membrane pores. Therefore, recovery is limited by the amount of silica in the raw water to the RO. Silica solubility, however, increases with pH, which is the reason high pH RO is the most attractive process for produced water. At a pH above 10, significantly higher solubility of silica can be taken into account. This enables the RO to run at a higher recovery rate than normal.
High pH RO has other advantages for processed water treatment. Solubility of water soluble organics increases with increasing pH. It is highly dependent on the organics present, but in general a positive relation is expected as organics remain soluble and get rejected by the RO. Boron rejection increases with increasing pH as it goes to borate. At a pH of 10.5, 99% of boron is in the borate form, making reasonable boron rejections possible.
RO represents the finest level of filtration and demineralization of all membrane processes. Membranes act as a barrier to all dissolved solids and all but low molecular weight organics. RO typically removes 90 to 99% of dissolved minerals. Recovery ranges from 60 to 85% depending on influent concentration and effluent specifications. RO for desalination operates at pressures between 35 and 70 barg (500 and 1000 psig).
