Maximum recovery from seawater reverse osmosis plants in Kuwait

Membrane scaling is caused by the precipitation of soluble salts as the feedwater is converted to brine. The scaling potential of any feedwater is determined from the feedwater analysis, the conversion of the RO system and the solubility limit of the various salts. In brackish water, calcium carbonate and calcium sulfate are the most common salts for which pretreatment is required. In seawater, calcium carbonate is usually the salt for which pretreatment is required. Other salts for which pretreatment calculation should be performed to ensure that solubility limits are not exceeded in seawater are barium sulfate, strontium sulfate, calcium floride and silica. The most common method used to prevent scaling in RO plants is removal of deposit-forming constituents, i.e., removing the carbonate and bicarbonate ions from the feedwater by adding acid or chemicals to water to alter the physical or chemical nature or the growth mechanisms of the depositing species i.e. adding scale control inhibitors. The two methods may be used together depending on the nature of the feedwater used. In this paper, solubility product, ion product and the Stiff and Davis Stability Index are calculated for the brine based on the feed of the seawater at Doha-Kuwait and the recovery ratio of the RO plant. The optimal recovery for the RO units at which they can operate safely without adding any scale inhibitor is reported.

a natural f e e d pH o f 7.9 c a l c i um carbonate scaling will occur, so we need to use a method to prevent this scaling. - At a feed pH o f 7.2, a 22.5% recovery can be u s ed w i t h o u t d a n g e r o f m e m b r a n e scaling. This conversion can be increased to 30.6% at feed pH value of 7.1 without any antiscalant. - At a feed pH o f 7.0 operating the plant at -At 34.2% will also be safe w i t hout any inhibitor. - R e c o v e r y can be increased depending on feed water temperature, feed water composition, pH of feedwater and addition of antiscalant. - Scale assessment in a desalination plant should be carri ed out with r e s p e c t to c o n c e n t r a t e d b r i n e st ream . I f d i r e c t analyses are not available, the reject stream then can be estimated from feedwater Table 15 Acidification to feed pH value of 6.8 pHf 6.8 6.8 U~ 1.1765 1.3446 P(alk.) 2.5330 2.4783 P(CA) 1.727 1.669 K 3.426 3.438 pH ~#r 7.686 7.585 pH b 7.30 7.22 Y 0.20 0.30 S&DSI -0.386 -0.365 -0.350 6.8 1.3641 2.4724 1.663 3.435 7.570 7.22 0.31 6.8 1.3841 2.4664 1.656 3.430 7.553 7.22 0.32 -0.333 6.8 1.4048 2.4603 1.650 3.430 7.540 7.22 0.33 -0.320 6.8 1.4261 2.4541 1.643 3.424 7.522 7.22 0.34 -0.302 6.8 1.4480 2.4478 1.637 3.422 7.507 7.22 0.35 -0.287 6.8 1.4706 2.4414 1.630 3.422 7.494 7.22 0.36 -0.274 6.8 1.4940 2.4349 6.8 1.5181 2.4283 1.623 1.616 3.422 3.420 7.480 7.465 7.22 7.22 0.37 0.38 -0.260 -0.245 6.8 1.5687 2.4147 1.602 3.420 7.437 7.22 0.40 -0.217 6.8 1.5953 2.4078 1.595 3.420 7.423 7.22 0.41 -0.203 6.8 1.6228 2.4007 1.587 3.420 7.408 7.22 0.42 -0.188 6.8 1.6512 2.3935 1.580 3.410 7.383 7.22 0.43 -0.163 6.8 6.8 1.6807 1.7113 2.3861 2.3786 1.572 3.410 7.368 7.22 0.44 -0.148 1.564 3.400 7.343 7.22 0.45 -0.123 6.8 1.7430 2.3710 1.556 3.400 7.327 7.22 0.46 -0.107 6.8 1.7759 2.3632 1.548 3.400 7.311 7.22 0.47 -0.091 6.8 1.8100 2.3553 1.540 3.360 7.255 7.22 0.48 -0.035 6.8 1.8455 2.3472 1.531 3.360 7.239 7.22 0.49 -0,019 6.8 1.8824 2.3389 1.523 3.360 7.222 7.22 0.50 -0.002 6.8 1.9208 2.3305 1.514 3.360 7.205 7.22 0.51 0.015 Table 16 The critical and safety conversion at different feed pH value pHf 7.3 Critical conversion - 7.2 7.1 7.0 6.9 6.8 10% Safety conversion - 22.5 30.6 34.2 42.3 45.0 analysis and recovery ratio. This means that we will assume all ions are equally rejected by the membrane. This is not true and presents a source of slight error in the calculation. Good scaling potential evaluated during the design stage requires two elements: 1.