Algal treatment of membrane rejects: a unique approach towards zero liquid discharge

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Abstract
A major concern in membrane-based water purification system is generation of huge concentrate stream and wastage of water. A typical Reverse osmosis (RO) or Nanofiltration (NF) system generates 20–25% reject containing high amount of dissolved salts and other contaminants. Contrary to popular belief, this reject water cannot be used without removing the contaminants or cannot be discharged anywhere. Main goal of this project is to find a cheapest and green way for treatment of RO/NF reject. Algal evaporation technique was explored in laboratory scale, to find its suitability for treatment of chloride-rich membrane reject in actual scenario and based on the results obtained, a pilot plant of 48KL was established on Hooghly Met Coke division (HMC), Tata Steel. Particular species of microalgae was selected, to take up minerals from reject water. There are several types of bacteria and symbiotic algae associated with selected micro algae survive in high TDS. A unique slope roof system, connected with algae growth tank, helps in efficient evaporation of water ensuring a Zero discharge. A markedly improved performance was achieved when algal evaporation followed solar evaporation. A total evaporation of 11 L/m2/day was observed, which was almost five times faster than Solar evaporation. Graphical abstract

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