Considering the importance of water and nutrients in microalgae cultivation, reusing media is an outstanding step towards economic viability of algal biomass production. However, biomass harvesting method and post-harvesting conditions greatly influence reusability of culture. In the pH adjustment-sedimentation method, an efficient way of harvesting microalgae, the salinity of the medium increases after biomass separation. In this study, the effect of increasing salinity on cell growth in reused culture medium was investigated. The results indicated cell growth was increased by 24% in fresh medium with higher salinity; but, in subsequent cultures (the second and the third), cell growth decreased due to the increased salinity caused by pH-adjustment harvesting. In high salinity cultures cell density dropped more than low salinity cultures. In the medium with low salinity (3.3% NaCl), the optical density (OD) changed from 2.75 in the first use (fresh) to 2.65, and 2.08 in the second and third uses; respectively, while in the medium with higher salinity (6.8% NaCl) the OD changed from 3.41 in the first (fresh) use to 2.05, and 0.88 in the second and third uses, respectively. The findings reveal that re-nutrient is necessary to maintain reasonable cell growth capacity when the culture medium is reused.

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