RO membrane separates pre-treated seawater into two streams; permeate and RO concentrate (ROC) under a hydraulic pressure higher than the osmotic pressure, and therefore has a higher energy requirement (65-85%) compared to other SWRO steps (Refer Table 5).
Permeate requires further treatment before distribution to communities. ROC needs further management options before discharge. Properties of permeate and ROC depend on the performance of membrane unit. Membrane fouling, which leads to poor membrane performance, is the major factor that limits use of RO technology to treat seawater (Luo and Wang, 2001).
At present, ROC is discharge back to the sea (diffuses at a specific rate at which they get blend with seawater), land (ground infiltration, evaporation basin, discharge to beach, Zero Liquid Discharge (ZLD)) and dispose to sewer lines (Morton et al., 1997, Ahmed et al., 2001, Sadhwani et al., 2005). Evaporation ponds and ZLD (brine concentrators) are the most expensive options due to statutory groundwater regulations and energy requirements, respectively (Greenlee et al., 2009). Post treatment of ROC take up a significant percentage of the total cost of desalination. Therefore, recent research has been focused on reducing ROC volume which will reduce the operational and maintenance cost. Brine volume can be reduced by further concentrating it (Martinetti et al., 2009), applying alternative membranes for RO (Elimelech, 2007) and increasing recovery of RO unit. Currently, these options have attracted a lot of research interest and pilot scale plants have been used. ROC disposal on land has a significant adverse effect on aquifer (Mohamed et al., 2005). On the other hand by discharging
Chemicals , 1.9%
Transportation, 18.4%
Power, 1.4%
Disposal, 78.3%
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back to the sea there can be impacts on marine fauna and flora (Latorre, 2005), and algae formation near the beach (Ahmed et al., 2001). Many of the Disinfection By-products (DBPs) formed during pre-treatment and post treatment (a result from reactions between organic and inorganic matter in water with chemical disinfection agents such as bromide, ozone, Cl2 etc) will be discharged with the ROC and they could affect marine ecosystems if they are not diluted sufficiently after discharge (Agus et al., 2009). On the contrary, after monitoring four years continuously, Western Australia University’s Palmer reports that (Palmer, 2012) there is not any impact on marine fauna and flora. However, there could be an impact on the marine system as Palmer, 2012 reports only from a short period research. Therefore, implementing national/global level guidelines and standards for seawater ROC discharge (either to sea or land) would be a better initiative to control impacts on environment.
Table 5: Percentage cost and specific energy comparison at each SWRO step (Wilf and Klinko, 1998, Dreizin, 2006, Semiat, 2008, Charcosset, 2009, WaterReuseAssociation, 2011)
*e-electric , 1 (intake + raw water supply + feed booster), 2 kWh/m3 of effluent, 3 (pumps + turbine + motors + auxiliary + lighting)
2.1.2.1 Brine management
Brine has high salinity value depending on the recovery rate of the RO unit and is sent for further treatment before being discharged to a land or to a water body. Generally, the TDS of the brine will be double the value of seawater (source) however will depend on the recovery of RO. Concentrated brine has TDS values of more than 65, 200 mg/L. Figure 5 shows the process flow diagrams of two SWRO plants namely, Eni Gela plant and Fujairah plant.
SWRO step Cost/ total water
price
High pressure pumping 25.4% (energy) 2.833 65-85%
Desalting process 5.4%
Post treatment 1.8% < 2%
12 (a)
(b) (c)
Figure 5: (a) Schematic of current conventional pre-treatment of Fujairah SWRO desalination plant (Al-Sarkal and Arafat, 2013); Process flow diagram of (b) one-stage
SWRO plant in Eni Gela, Sicily and (c) two - stage SWRO plant in Fujairah, UAE.
Currently, brine from most SWRO plants is discharged back to the sea (diffused at a specific rate at which they get blended with seawater (Water-Technology.net, 2013, Ahmed et al., 2001)) or to the land (ground infiltration, discharge to beach (Ahmed et al., 2001). Solar evaporation (Greenlee et al., 2009), wind aided intensified evaporation (has been only demonstrated at laboratory scale (Katzir et al., 2010), spray irrigation (Sethi, 2006)) and disposal to sewer lines (Morton et al., 1997, Ahmed et al., 2001, Sadhwani et al., 2005), zero liquid discharge (Greenlee et al., 2009) are other options for brine management. Evaporation
416 GL/day
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ponds and zero liquid discharge (brine concentrators) are the most expensive options due to statutory groundwater regulations and energy requirements, respectively (Greenlee et al., 2009, Sethi, 2006). From a survey of 137 drinking water plants which are having capacity of greater than 98 m3/day, brine disposal methods have been divided as shown in Figure 6 (Ahmed et al., 2001).
Figure 6: Brine disposal methods from a survey (Ahmed et al., 2001).
Post treatment of brine takes up 5-33% of the total cost of desalination (Ahmed et al., 2001). Therefore, recent research focuses on reducing brine volume which will reduce the O&M cost. Brine volume can be reduced by further concentrating (Martinetti et al., 2009) (using membrane distillation or electro-dialysis, recovering commercial products (Jeppesen et al., 2009)), applying alternative membranes for RO and increasing recovery of RO unit. Water recovery of single stage RO process lies between 40-60%. As Figure 6 depicts the recovery of RO process at single stage Eni Gela plant and two-stage Fujairah plant to be 45% and 41 % respectively. Hence, increase in water recovery would undoubtedly reduce the volume of concentrate. However, when the volume is less, concentration of minerals and chemicals are higher. This can cause more negative issues since many disposal regulations are based on concentrations but not on volume (Ahmed et al., 2001). Further, SWRO plants are based near beaches and major brine disposal method is diffusing it back to the sea. Therefore, if the brine is discharged back to sea, having lower concentrations is an added advantage. Main advantage
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would be the rapid rate of diffusion/dispersion. Therefore, this study focuses on brine management while reducing the volume of sludge of the SWRO process.