[ water treatment ]
water quality . The type of pretreatment system depends on the source of water as each will come with their own challenges . Groundwater contains dissolved redox-active species such as iron and manganese , which can precipitate in and clog the polishing system . These can be effectively removed using aeration and sand filtration . For treated wastewater , the primary concern are particles , organics , and microorganisms . Here , ultrafiltration in combination with UV can be used to bring the water to a suitable quality . Seawater primarily requires removal of salts , but also of particles and dormant microorganisms . Using standardised reverse osmosis ( RO ) desalination is sufficient .
Once the raw water has been pretreated , we need to address the following issues to turn it into ultrapure quality :
• Content of ions – conductivity
• Hardness
• TOC
• Silica
• Gasses
To remove the bulk of the ionic load , RO is used . The membrane blocks ions , molecules and particles and will therefore also remove organics ( TOC ) and silica . To reach sufficiently low concentrations , it is often necessary to employ a double pass RO system , where the permeate from the first RO process is filtered again in a secondary RO system . For the RO system to operate properly , the water must first be conditioned to avoid scaling and damage to the membranes . If there is free chlorine in the water , this must be removed using active carbon , to avoid oxidation of the selective layer of the membrane . Hardness due to ions such as Ca and Mg can cause scaling and limit the recovery rate . This can be handled either by using a softener that will exchange multivalent ions with Na or by dosing in an antiscalant that will stop the scaling process . RO membranes do not stop dissolved
Fig . 4 . The energy required to produce ultrapure water from different raw water sources
26 Hydrogen Tech World | Issue 6 | October 2022