The Alkaline Flocculation Harvesting Of Dunaliella Salina And Recycling Culture With Bicarbonate

Microalgae enable to make more efficient agricultural production partially instead of terrestrial plants and use them as raw materials for processing into various products because of the characteristics of rapid growth and high yield per unit area.Current high costs of commercial-scale algal production prevent the widespread use of this renewable source,which mainly including the cost of culturing and harvesting.The most commonly used methods are centrifuge,organic and inorganic flocculants and dissolved air flotation.However,centrifuges are very energy-intensive,while organic polymers such as chitosan are expensive.What's more,there is a large amount of residual nutrients cannot be reused and discharged in the growth medium,which increases the input of materials,water and contamination.The goal of this study is to investigate harvesting and recycling culture of Dunaliella salina.Alkaline flocculation as the method to harvest microalgae can reduce energy consumption without extra pollution and the supernatant can be recycled.The growth medium can be treated with physical and chemical methods to remove inhibitors to achieve recycling culture,which is necessary for the economical and sustainable production.Firstly,it is simulated in bicarbonate culture system of artificial seawater for flocculation precipitation model,where Calcite and ferric hydroxide mainly exist as the precipitation form of Ca2+ and Fe3+.On the basis,the effect of flocculation by p H,Ca2+,Mg2+ and Fe3+ was investigated in artificial seawater bicarbonate cultivation system.It was found that the harvesting efficiency of Dunaliella salina was increased as the increase of p H and the concentration of Ca2+,Mg2+ and Fe3+.And there is synergy between these four factors.The most suitable conditions of flocculation for Dunaliella salina were p H=10.5,c(Ca2+)=8 mmol/L,c(Mg2+)=3 mmol/L,c(Fe3+)=160 ?mol/L.The residual ratios of Ca2+,Mg2+ and Fe3+ respectively were 4.1%,64.3%,and 0,and the concentration of ions in the supernatant was lower than that in the culture medium,so that it could be used repeatedly to culture Dunaliella salina to achieve the purpose of recycling the medium.The growth stage of Dunaliella salina was not the main influencing factor of flocculation,but the culture medium gradually deteriorated with the culture time,which hindered the flocculation process.The growth medium with optimal flocculation condition can be flocculated in 24 hours,and harvesting efficiency can reach 80%,which is significantly higher than the growth medium without treatment.The growth medium of Dunaliella salina cannot be cultured repeatedly without any treatment.Three physical and chemical methods were used to treat the growth medium.The growth medium treated with powder activated carbon(PAC)could be recycled for 3 times,which make Dunaliella salina grow normally and promote the accumulation of ?-carotene.The growth medium treated with 0.5 mmol/L NaClO was able to complete 3 cycles and promoted the accumulation of ?-carotene of Dunaliella salina.But toxic substances caused by NaClO appeared and accumulated in recycled medium,which inhibits the growth of Dunaliella salina.However,NaClO can reduce the concentration of total organic carbon(TOC)in recycled medium,which is lower than the concentration in fresh medium.The growth medium filtered by 0.22 ?m membrane could be recycled for 5 times.The performance of Dunaliella salina was the same in recycled medium and fresh medium.It is successful to combine recycling culture with 0.22 ?m filter membrane and alkaline flocculation in optimal flocculation condition,which can achieve more than 80% harvest efficiency,and at the same time,Dunaliella salina can normally grow in recycled medium.The study successfully explored harvesting method for Dunaliella salina by alkaline flocculation to achieve low-cost,low-energy,high-yield and non-polluting,even make it possible to reuse the growth medium.Meanwhile,physical and chemical treatments of the growth medium can remove inhibitors,so that the effect of recycling culture in recycled and fresh medium is identical,which definitely reduce the cost of cultivation.They are successfully matched for alkaline flocculation and recycling culture and an integral set of culture process for Dunaliella salina was finished.The technology has important significance for industrial production of Dunaliella salina.