MechChem Africa January-February 2021

Zutari’s electro-desalination water treatment solution for industry

Zutari process engineer, Louis Koen, talks about Membrane Capacitive Deionisation (MCDI), a commercially-available electro-desalination water treatment technology that has become a successful alternative to reverse osmosis (RO) membrane treatment trains.

to be fed to the RO skids at relatively high pressure to overcome the osmotic pressure across the membranes. As such, the process is considered energy-intensive. Very good pre-treatment of the water is also required to safeguard the RO membranes against potential fouling. Conversely,MCDI technology is governed by electrochemical processes that operate at relatively lower pressures. The design is fo- cused on the electrochemical removal of sol- ute (ions) present in thewater. Thisdistinction is important when considering the scalability and appropriateness of the two technologies for feed water with various salinities. Take two feed streams flowing at the same rate, one with a low salinity (less than 2000mg/ ℓ ) andonewith a high salinity (more than 4 000 mg/ ℓ ). The difference in cost as- sociated with RO plants to treat each stream will be negligible, as the cost is influenced by the solvent quantitymore than anything else. In contrast, the design of MCDI plants for each feed stream will vary significantly owing to the different salinity levels, which in turn will impact the cost associated with each MCDI plant. The MCDI plant treating lowsalinitywaterwill be significantly cheaper relative to that treating high salinity water. Consequently, MCDI plants have become economically advantageous over traditional RO plants at lower salinity levels. At the start of Zutari’s full MCDI process train, the abstractedwater is filtered through two autonomous gravity sand filters as a pre- treatment step to remove particulatematter. The filtered water is then temporarily stored in a balancing tank, before being passed into the MCDI plant for desalination. The desalinated water is then disinfected via UV sterilisation. Prior to storage in a final treated water reservoir, sodium hypochlorite is also dosed in to provide a chlorine residual in the treated water. This simple configuration delivers a fully automated treatment works with a minimal footprint, reducing pumping requirement and chemical consumption compared to tra- ditional RO plants. Adopters of this technol- ogy can expect direct operating expenditure to be half that of the conventional approach,

How a MCDI treatment plant works Capacitive de-ionisation (CDI) uses an electrical potential difference between twoporous carbonelectrodes tode-ionise salinated water. Negatively charged ions (anions) such as chloride (Cl - ), are attracted to the posi- tive electrode (anode) and trapped there, while positively charged ions (cations) suchassodiumions(Na + )areattractedand stored in the negatively charged cathode. The process offers low investment and infrastructure cost, and does not require high pressures or temperatures and the energy cost for treatingwater is inpropor- tion to the amount of salt that is removed. ThismakesCDI an ideal solution for brack- ishwater or industrial waste streamswith low salt concentration levels. Membrane capacitive de-ionisation is a modified form of CDI that uses an ion exchange membrane to improve the ‘entrapment’ of ions during the adsorption phase, improving the salt adsorption efficiency and, compared to CDI, further reducing the required energy input. q While MCDI is already showing commer- cial advantages over RO plants, the technol- ogy is still in its infancy. “We can expect ever increasing innovationand improvement as re- searchanddevelopment in thefieldadvances. Ourin-housetechnicalexpertsarecontinually keeping abreast of commercially available technology so we canengineer solutionswith the greatest impacts for our clients, the envi- ronment andour communities,” he concludes. leading to a significant reduction in the total lifecycle cost. “Our solutions always consider the impact on the environment and communities, but we also consciously deliver outcomes that help clients stay and thrive in business. It is for this reason that our engineered solu- tions employ the use of novel technology to achieve a water-treatment performance on a par with conventional means, while reduc- ing expenditure over the full lifecycle of the asset,” highlights Koen.

W ith South Africa still expe- riencing a drought in many areas, industries that rely heavily on water for their processes will need to take a more sustain- able approach and consider alternativewater sources in their futureplanning. Thechallenge is to ensure growth while building resilience to external stresses such as long-term water scarcity. It is for this reason that Zutari is cham- pioning Membrane Capacitive Deionisation (MCDI) technology in its designs. “We can co- create solutions with our clients that utilise the latest MCDI technology in the selected process train,” says Zutari process engineer LouisKoen.MCDI is a commercially-available electro-desalination water treatment tech- nology that has become a successful alter- native to reverse osmosis (RO) membrane treatment trains. Consisting of just a two-step process, adopters of this technology need fewer pre- treatment steps, as there is a limited risk of fouling and/or scaling compared to other desalination processes. In addition, there is no need for remineralisation, typically required with RO due to exceptionally high salt rejection. RO is a pressure-driven desalination pro- cess that focuses on processing the brackish water stream via membranes that are se- lected to reject thedissolved ions in thewater. The operating cost is primarily related to the volumetric flow rate of water, which needs

16 ¦ MechChem Africa • January-February 2021

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