Investigation Of Microalgae Cultivation By Membrane Technique And Attached Culture Technique In Wastewater For Algal Biomass Production And Nutrient Removal

Using wastewater as the microalgae cultivation medium can reduce the cost of the production of algal biodiesel and realize the treatment of the wastewater with nitrogen and phosphorus removal,but it is now subject to the lack of economical and efficient cultivation methods,especially when the low strength wastewater such as effluent of municipal wastewater treatment plant is used as cultivation medium.A continuous supply of wastewater can maintain higher nutrient loading for microalgae production,thus providing an opportunity for enhancing biomass productivity and the nutrient removal rate of microalgae cultivation.However,continuous wastewater supply will also lead to the loss of algal cells,which are usually suspended in the water.In this study,membrane technology and adsorption immobilized culture were used in the photobioreactor for the microalgae biomass production and nutrient removal from low strength wastewater.In this study,a novel membrane photobioreactor（MPBR）operated in batch flow mode or continuous flow mode was developed firstly to culture microalgae with the effluent of municipal wastewater treatment plant.With the filtration effect of the submerged membrane module,the hydraulic Retention Time（HRT）and biomass retention time（BRT）of the photobioreactor can be completely separated.Thus the concentration of microalgae biomass in the photobioreactor can be maintained at high level,and the nutrients in the influent can be effectively removed.After 35 days of culivation,the maximum growth biomass of microalgae in the continuous flow MPBR supplied with secondary effluent was 1.724 g L^-1,which was 1.64 times higher than that grown in a conventional photobioreactor（CPBR）with BG11 medium.Through the study of the accumulation process and fatty acid composition of the microalgae,it could be found that lipid accumulation was achieved for the microalgae cultured in MPBR.The fatty acid composition of the harvested microalgae was mainly composed of C16 and C18 fatty acids,and the content of polyunsaturated fatty acids was only 9.41%.So the algal biomass obtained in this study can be regarded as an ideal raw material for biodiesel production.The rapid growth of microalgae in the reactor also benefited the removal of contaminants,such as nutrients.Metal ions such as Cu,Zn,Fe,Al,and Mn in the secondary effluent were also efficiently removed.On the basis of the above research,the influence of HRT on the biomass production and nutrient removal performance of MPBR was studied.The results showed that when the HRT of MPBR was set to 2.0 d,higher levels of biomass productivity and nutrients removal rate were achieved in MPBR.So the HRT of the MPBR was set to 2.0,and long-term stable operation of MPBR was then achieved with continuous harvesting of algae from the reactor at a certain rate.The concentration of microalgae in the reactor was maintained at 1.035-1.524 g L^-1 during the culture interval.And the corresponding algal biomass productivity was 60.13 mg L^-1 d^-1.Efficient removal of nitrogen and phosphorus from the influent of the reactor was also achieved.In addition,the fouling characteristics of membrane module submerged in MPBR were analyzed.The results showed that the inner resistance was the main component of membrane fouling in MPBR.An efficient continuous microalgae cultivation process for biomass production and nutrients removal from aquaculture wastewater was also developed using MPBR in this study.The average volumetric biomass productivity in the MPBR operated at HRT of 1 day was 42.6 mg L^-1 d^-1,which was 5.8-fold larger than that achieved in batch cultivation in flask.Advanced nutrients removal from aquaculture wastewater was also achieved in MPBR.The average reduction in TN and TP was 86.1%and 82.7%,respectively,after stabilization.Unionized ammonia,which is usually toxic to aquatic animals,was also effectively removed in MPBR,with effluent concentration below 0.002 mg L^-1.Therefore,it can be seen that MPBR has a good application potential for the treatment of aquaculture wastewater.In order to achieve efficient harvesting of microalgae from the photobioreactor,the adsorption immobilized culture of microalgae was studied.Batch adsorption experiments were performed to choose the best solid carrier for the algal cells,and then a unique algal biofilm photobioreactor（BPBR）was developed for attached growth of the alga cells on a culture medium consisting of treated sewage.Compared to CPBR without solid carriers,the BPBR achieved a higher volumetric biomass productivity(15.93 mg L^-1 d^-1),and a higher algal lipid productivity(4.09 mg L^-1 d^-1),Furthermore,improved nutrient removal rates(1.00 and 0.20 mg L^-1 d^-1 for N and P,respectively)were also achieved in BPBR due to the higher algal biomass productivity of the reactor.Based on the above research,a novel algal biofilm membrane photobioreactor（BMPBR）equipped with solid carriers and submerged membrane module was developed for microalgae cultivation and secondary effluent treatment.With the utilization of BMPBR,the production and harvesting of algal biomass and advanced nitrogen and phosphorus removal from wastewater can be achieved simultaneously.The volumetric microalgae production achieved in BMPBR was 72.4 mg L^-1 d^-1,which was 1.44-fold larger than that in suspended growth MPBR.Furthermore,72.4%of the total produced algal biomass was immobilized as algal biofilm in BMPBR,which was beneficial for the harvesting of algal biomass.In terms of nutrient removal,BMPBR showed better nitrogen removal performance than MPBR due to its higher algal biomass productivity.The algae regrowth on surface of the harvested solid carriers resulted in a higher biomass yield than that from the initial growth on fresh surface.