Water in which the hardness is too high causes scale buildup in household appliances, and is known to decrease the efficiency of soap. This study investigates the effectiveness of biological treatment for water softening as opposed to the conventional method with chemical reagents. The water softening effect of biological treatment, which was prepared by circulating well water in a plastic container containing water tank pebbles for 3 months, was evaluated by introducing sample water containing a high hardness under various light irradiation conditions (continuous light irradiation, continuous shading, and intermittent light irradiation). Water hardness was decreased by 82% following 72 h of light exposure. Additionally, when exposed to intermittent light irradiation, water hardness was decreased markedly only during the light irradiation. The grown algae in the biological treatment tank were identified as Aphanochaete magnum, which inhabit widespread freshwater areas, by using 18S rRNA sequencing. Our data demonstrate that raw water can be softened using a biological treatment under various lightning conditions. It shows that the photosynthesis of naturally occurring algae plays a major role in the water softening effect. Our data suggest that biological treatment could be a novel method to soften water rather than the conventional chemical treatment.

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