Water Journal : Water Journal April 2011
membranes & desalination technical features 98 APRIL 2011 water poly(ether ether ketone) and poly(ether sulphone). At constant conductivity, decreasing the charge density lowered the electrostatic attraction of the Ca2+ for the charged sites, resulting in lower Ca2+ transport. ED with monovalent- selective anion-exchange membranes has been investigated for the separation of organic compounds and nutrient ions from the salts in RO concentrates (Zhang et al., 2009). A wastewater study (Taylor & Goodman, 2007) included a comparison of two-stage EDR using monovalent and conventional cation membranes in a small stack. As Table 1 shows, the Na/Ca ratio was maintained for the monovalent membrane while the Na/Mg ratio actually decreased, in contrast with the standard system (Products 1 and 2). The result was a reduction in the sodium adsorption ratio (SAR) from 10.2 to 4.5. Comparison with RO EDR has several advantages over RO. In the reclamation of tertiary-treated wastewater it has proved to be 25% less costly than RO (Reahl, 2006). It can reliably remove selected ions such as nitrate, and its performance is not affected by silica. Water recovery is very high, at 92% versus 80%, and it can function at high brine concentrations. It is not influenced by as many water constituents as RO, as shown in Table 2. EDR has been applied to the brine concentrate from RO systems, reclaiming waters of 8000mg/L salinity to give combined water recoveries of 96-98%. It has been used for many years for concentrating sea water for salt production in Japan (Turek, 2002), notwithstanding the high energy requirement. The combination of ED and RO for the improved recovery of dissolved organic matter from seawater is reported (Koprivnjak et al., 2006). Better results are possible with pulsed ED combined with RO (Gurtler et al., 2008). Disadvantages of known ED and EDR systems include the complexity of the system designs, the amount of scaling and fouling that occurs within the system, especially the membranes, and a low electrode life due to the corrosion stemming from the reactions at the electrodes. Specifically, the chlorine generated from electrolysis of chloride ions in the salt water causes corrosion, particularly corrosion of membranes, lowering their effective life. Additionally, the gas evolution, oxygen at the anode and hydrogen at the cathode, requires the need for degassifiers, increasing the complexity and cost of desalination plants utilising ED and/or EDR technology. Operation and Maintenance In addition to pressure, flow and conductivity measurement, the ED process is also monitored electrically, stack current and voltage being easily monitored parameters. Mineral acids and anti-scalant chemicals are commonly added to the brine streams to raise solubility and, hence, product efficiency. The standard clean-in-place or CIP procedures are 5% hydrochloric acid for acid-soluble scales and 5% sodium chloride at pH 9.0-9.5 for organic fouling. ED stacks may be readily dismantled for cleaning or replacement of damaged membranes (see Figure 2). Costs For feed waters of 800-2000mg/L salinity the combination of capital and long-term operating and maintenance costs can favour EDR over RO, especially when high water yields are required (Reahl, 2006). Capital costs are competitive or slightly higher, except when RO needs additional treatment prior to the membranes. A bigger building is required for EDR for large plants of >6 ML/d capacity. Operating costs are lower, with reduced pre-treatment/post-treatment and lower membrane replacement costs. For feed waters of <1500mg/L the power consumption is less for EDR. Overall cost savings often outweigh those of RO. Table 2: EDR and RO limits (Reahl, 2006; Allison, 2005). Item EDR limit RO limit Silt density index 15 (5 min SDI) 5 or less (15 min SDI) TOC, mg/L 15 2-5 Silica Unlimited Depends on water recovery Oil & grease Upto1mg/L Zero Turbidity, NTU 2 0.1-1 Chlorine, mg/L 0.5 continuous, with 15-20 spikes 0-0.1 for aromatic polyamide (PA); 1-5 for cellulose acetate (CA) pH range 0-10 3-11 for PA; 3-8 for CA Iron, mg/L 0.3 forPA;1forCA Temperature, °C 43 43 Figure 2: View of partially dismantled two-stage EDR stack with upper electrode removed, showing ports (P), electrodes (E) and spacer (S) on top of membrane (M) stack (Taylor & Goodman, 2007).
Water Journal March 2011
Water Journal May 2011