Nutrients Removal And Recovery Performance Of Spirulina Platensis From Saline Secondary Effluent

With the rapid development of urbanization and industrialization,fresh water scarcity has become a leading issues.The application of seawater for toilet flushing in coastal urban areas could effectively alleviate water shortage.However,there is still a certain amount of nutrients and organic carbon in secondary effluent under the optimal removal efficiency.The directly discharge of secondary effluent has the risk of causing eutrophication and even red tides.Therefore,the secondary effluent must be further disposed before being discharged.However,due to the high salinity and low ratio of COD/N,there is no green.economical and efficient method to deal with the secondary effluent of seawater flushing wastewater.Spirulina platensis can tolerate high salinity(up to 70 g/L NaCl).and it can use organic carbon source or CO2 for autotrophic,or heterotrophic,or mixotrophic cultivation.which made it potential to treat carbon-insufficient saline secondary effluent.In this study,Spirulina platensis was inoculated into synthetic secondary effluent of seawater flushing wastewater to simultaneously achieve nutrients removal and biomass harvest.The removal mechanisms of nitrogen and phosphorus by Spirulina platensis under the conditions of high salinity and low C/N ratio were also studied.Concentration of nutrients(COD,TP,NH4+-N,TN)in synthetic secondary effluent with different salinity,COD/N and initial pH were determined to evaluate the nutrients removal and recovery performance of Spirulina platensis.Results showed that contaminations(COD,TP,NH4--N and TN)in synthetic saline secondary effluent which with different salinity(0.0-3.0%).COD/N(0.0-4.0)and initial pH(7.0-10.0)could be effectively removed by Spindina platensis.Protein and lipid content and settling efficiency of Spirulina platensis under different different salinity COD/N and initial pH were determined to evaluate the nutrients recovery performance and settling performance.Results showed that the contents of protein and lipid of microalgae(to dry weight)were ranged from 46.4%and 15.0%to 65.6%and 23.5%,respectively,indicating that Spirulina platensis has the ability to produce biodiesel.After 24 h settling,the settling efficiency of microal/gae was ranged from 65.4%to 89.6%,and it was proportional to the content of polysaccharide in EPS.Ammonium could be directly assimilated by Spirulina platensis,and other nitrogen species were converted to ammonium:nitrate was firstly reduced to nitrite,nitrite reduced to ammonium,and urea was hydrolyzed to ammonium.More than 750%of nitrogen was assimilation by microalgae.Additionally,4%-10%of nitrogen was volatilized to atmosphere as ammonia.Orthophosphate could be directly used by Spirulina platensis,and organic phosphorus needs to be converted to orthophosphate.Organic phosphorus and inorganic phosphorus were accounting for 22.4%and 77.6%,respectively.And NAIP and AP accounted for 61.1%and 15.5%of TP,respectively.It provides a theoretical basis for further treatment of saline secondary effluent and a new solution for the commercial cultivation of Spirulina platensis.