Zero Liquid Discharge Concept – Brine Minimization Technology

ZLD concepts, brine minimization  technologies, and challenges in desalination

High recovery systems aimed at  brine minimization have been  defined in municipal desalination  as those systems achieving  recoveries higher than 92% .  ZLD is defined as a high  recovery system allowing that no  effluent leaves the ground-level  plant boundary. In a ZLD  approach, all the brine is either  recovered by a combination of  technologies to produce  desalinated water or dry salts.  Technologies commonly  recommended in ZLD processing  systems include: RO, vacuum  evaporators, crystallizers,  evaporation ponds and spray  dryers. Salinity and composition  of the brine to be processed in  the ZLD system has a substantial  influence on capital and  operating costs. Sequential and  selective removal/ recovery of  salts from concentrated brine  should follow from low to high  solubility levels. Although  technically feasible, high  recovery and ZLD systems are  currently not economically viable  in municipal desalination. As  previously stated, membrane  desalination is considered to be  the predominant technology to  be used in municipal desalination  and ZLD systems. Operating  costs are reduced by applying a  system including consecutive RO  stages for brine minimization.

By  implementing  ‘seawater  membranes’ in the second stage, Rw  increases and additional  permeate output can be  achieved  . The osmotic  pressure depends on the  concentration of dissolved salts  in solution . Due to the lower  salinity of the brackish feed  water, ‘RO line 1’ operates at a  lower pressure. The brine  generated in ‘RO line 1’ is then  fed into ‘RO line 2’, where the  salinity becomes higher. Overall,  tandem RO processes for  maximum water recovery and  RO brine minimization are  considered to be promising  alternatives in brackish water  desalination. Particular process  conditions need to be analysed  carefully on a site-by-site basis .

Electrodialysis (ED) or electrodialysis  reversal (EDR), forward osmosis  (FO) and membrane distillation  (MD) are also membrane  separation processes. ED is  another desalination technology  that employs electrical potential  difference to move ions through  ion-exchange membranes. As  shown in Table 6, ED is  considered as an alternative to  RO but it is often only  recommended for treating  brackish water with TDS level  below 10,000 mg/L . For higher  salinities, RO is more competitive  since ED cost is proportional to  the amount of salts carried  through the membrane [59].  Further research is required to  avoid scaling in ED units and to  improve selectivity and  permeability of membranes . FO  is another technology for brine  concentration with low energy  consumption. In contrast to RO,  the osmotic pressure is the  driving force for mass transport .

The main drawbacks of FO technology  are the risk of salt precipitation  on the membrane, the need to  develop more robust membranes  and a suitable draw solute to  improve the separation process.  Table summarizes the main  features of significant  concentration technologies  applicable in municipal  desalination depending on feed  water salinity.

As shown in Table , RO is by far  the most cost-effective solution in  terms of energy consumption,  capital and operating costs. MD,  FO and ED technologies have  been tested on a pilot plant scale  for RO brine minimization in  inland desalination, although it is  difficult to assess their feasibility  on an industrial scale. Martinetti  et al. tested/compared  vacuum-enhanced direct contact  membrane distillation (VEDCMD)  and FO for RO brine  minimization in two different  streams with TDS levels  averaging 7,500 and 15,000  mg/L. Rw levels in both  technologies were limited by salt  precipitation.