Experimental Study On Advanced Purification Of Domestic Wastewater With Microalgae

Secondary effluent of urban wastewater treatment plants is still loaded with largeamount of nutrients (nitrogen and phosphorus). Its vast discharge results in excess nutrientsin rivers and lakes. Consequently, algae blooms and causes a serious environmental issueknown as eutrophication. Therefore, it is a prerequisite to further reduce the nutrients levelin secondary effluent in order to suppress the occurrence of eutrophication from the source.In this regard, microalgae can effectively uptake nitrogen and phosphorus, and capturecarbon dioxide to conduct their anabolism through photosynthesis. In addition, microalgaeare particularly attractive because of their short generation time, easy culturing, and biomassutilization potential. Therefore, proper algal culturing could provide an elegant solution tofuther polishing of secondary effluent, with additional benefit of biomass utilization.In this study, the performance of dominant microalgae species (mainly cyanobacteria)cultured under laboratory conditions in nutrients elimination from both artificial and realsecondary effluent was investigated. The growth characteristics of the dominant microalgaewere studied with artificial secondary effluent, and the nutrients uptake characteristics undercertain operational conditions were investigated as well. Several key operational paramaterswere analyzed and the effect of N/P ratio on the growth of the dominat species wasparticularly explored. In addition, experiments were conducted to investigateautoflocculation behaviour of Scenedesmus. The effects of metal ions (calcium andmagnesium) and the dominant microalgae and their extracellular polymeric substances (EPS)on the autoflocculation efficiency of Scenedesmus were investigated respectively. Themechanisms of high-pH induced autoflocculation and EPS induced autoflocculation werediscussed as well.The results showed that the dominant microalgae had good treatment performance innutrients reduction from both artificial and real secondary effluent, with TN removal of60%and TP removal of90%. By comparing the uptake rates of nitrogen and phosphorus inthe steady period, it was found that nitrogen and phosphorus uptake rates were higher in thefirst eight hours after feeding wastewater, and then decreased gradually. By analyzing thechange of the N/P ratio over the whole reaction process and the growth curve of thedominant microalgae, it was concluded that the dominant microalgae favored low N/P ratiosfor maximum growth and its growth rate decreased along the increasing N/P ratio. The lipidcontent of dominant microalgae was low, accounting for2.5%of the dry weight, having no potential for bio-diesel production. But the cells of the dominant microalgae contained largeamount of EPS, which exhibited good flocculation effect on Scenedesmus suspensions. Theharvesting efficiency of Scenedesmus exceeded80%with the extracted EPS when theconcentration of calcium ions was5mmol/L in water. Flocculation of Scenedesmus couldalso be achieved simply by elevating pH (High-pH iduced autoflocculation). In such case,the presence of calcium and magnesium ions was preconditions for Scenedesmusautoflocculation, because the positively charged calcium and magnesium precipitationsinduced by high pH were the main reason for Scenedesmus autoflocculation.Correspondingly, the autoflocculation efficiency could be improved with the increase ofcalcium and magnesium ions concentrations and pH.