Study On Nitrogen And Phosphorus Removal And Flocculation Characteristics Of A Self-flocculating Microalga

Microalgae can effectively remove pollutants such as nitrogen and phosphorus from wastewater.Microalgal biomass can be used as raw materials for biodiesel,aquatic feed and high value-added products.However,the microalgae cells have a small volume and are uniformly suspended in wastewater,which brings great difficulty to the discharge of the treated wastewater.Therefore,the key to the application of microalgae-based wastewater treatment technology is to integrate wastewater treatment and the effective separation of microalgal cells from wastewater at the same time.In the present study,a self-flocculating microalga was screened from soil in Shanxi province.The taxonomic status of the microalga was identified.Then the performance on pollutants removal and flocculation of the microalga cultivated with different nitrogen sources was investigated.Further,the performance on pollutants removal and flocculation of the microalga in municipal wastewater was evaluated.The main findings were as follows:A self-flocculating microalga was isolated from soil in Shanxi province.Microalgal cells cultivated in BG-11 medium mostly aggregated together and large amounts of secretions existed outside cells via optical observations.According to the phylogenetic analysis,it was reasonable to identify the microalga as Scenedesmus rubescens SX.There are some differences in the growth,pollutants removal and flocculation of S.rubescens SX in synthetic wastewater with different nitrogen sources.When S.rubescens SX was cultivated in ammonium-containing wastewater,its biomass reached a maximum of 422.7 mg/L on the third day.The final COD,ammonium and total phosphorus removal efficiency were 66.2%,73.87% and 55.57%,respectively.In the nitrate-containing wastewater,the maximum biomass of S.rubescens SX reached 596 mg/L.The removal efficiency of COD,nitrate and total phosphorus were 70.21%,58.27%,and 77.86%,respectively.In the nitrate-containing wastewater,the flocculation efficiency of S.rubescens SX was better than that in ammonium-containing wastewater.When S.rubescens SX was cultivated in nitrate-containing wastewater,more extracellular polymeric substances(EPS)with rich hydroxyl and carboxyl groups as binding sites for flocculation were produced.Moreover,positively charged precipitates of magnesium and calcium salts were formed via high p H induction.These precipitates neutralized negative charge groups from EPS,and S.rubescens SX exhibited the highest flocculating efficiency in nitrate-containing wastewater.For S.rubescens SX cultivated in ammonium-containing wastewater,the p H of the wastewater drastically reduced to 3.15-3.87 during the process of ammonium removal.Acidic p H of wastewater inhibited the growth and pollutants removal of the microalga.By adjusting the p H of the ammonium-containing wastewater,the maximum biomass of S.rubescens SX reached 939.3 mg/L.The removal efficiency of COD,ammonium and total phosphorus were 67.34%,97.69% and 99.77%,respectively.It indicated that a suitable p H was essential for both wastewater treatment and biomass accumulation.At the end of the cultivation,the lipid content of S.rubescens SX cultured in wastewater ranged from22.44% to 25.80%.Both linoleic acid and linolenic acid were main components of lipid and had great application prospects in the food industry and animal feed.S.rubescens SX was also cultivated in municipal wastewater.The biomass productivity of S.rubescens SX was 61.25 mg/(L·d)throughout the cultivation period.The maximum biomass concentration reached 690 mg/L.The removal efficiency of COD,ammonium and total phosphorus were 57.91%,96.38 and 93.68%,respectively.S.rubescens SX maintained high flocculation efficiency throughout the cultivation period.Especially the flocculation efficiency was up to 94.5% at the end of the cultivation.It was speculated that high concentration of iron,magnesium and calcium promoted the flocculation of the microalga.Generally,S.rubescens SX could be used for municipal wastewater treatment,which effectively combined pollutants removal and separation of microalgal biomass from wastewater,simultaneously.