Characteristics Of Mixotrophic Algal Biofilm And Its Application In Nitrogen And Phosphorus Removal In Eutrophic Wastewater

With the development of economy,the eutrophication in our country is becoming more and more serious,it is urgent to dispose the eutrophication.The algal biofilm technology has attracted wide attention in N and P removal from the eutrophication because it is economical,efficient,ecological and energy-efficient.However,the traditional algal biofilm method faces with the challenges of low yield of biomass and limited removal rates of N and P.In order to overcome the above defects,this study cultivated autotrophic,mixotrophic and heterotrophic algal biofilms,and explored theirs growth characteristics,biofilm-forming characteristics and N and P removal efficiency to select the best type of algal biofilm.Then,the performance of mixotrophic algal biofilm in the open air-lift photobioreactor under different operating conditions was evaluated in terms of biomass productivity and nutrients removal,in order to provide a basis for the application of mixotrophic algal biofilm in the treatment of the eutrophication.The main conclusions:（1）The highest algal biomass and the best biofilm-forming were obtained by mixotrophic algal biofilm among the three trophic biofilms.The volumetric microalgae biomass in mixotrophic conditions（0.215 g/（L·d））was 2.3 times,8.6 times,and 5.97 times higher than those in photoautotrophic biofilm,heterotrophic biofilm,and suspended conditions,respectively.The concentration of extracellular polysaccharide in mixotrophic algal biofilm reached 29.55 mg/L,which was 1.49 times and 5.13 times of autotrophic algal biofilm and heterotrophic algal biofilm,respectively;The activity of dehydrogenase（DHA）was 1.4 mg/（L h）in mixotrophic biofilm,which was 2.19 and 16.47 times of autotrophic and heterotrophic algal biofilm,respectively.At the same time,the best N and P removal rates（95.96%and 99.23%,respectively.）were obtained by the mixotrophic algal biofilm.（2）Mixotrophic microalgae was cultured in open airlift photobioreactor,the dominant algal population was Chlorella sp.in suspension system（SBAR）and attachment system（BSBAR）.The yield of biomass in BSBAR reached 0.67 g/L,which was 1.3 times higher than that of SBAR.The concentration of extracellular polysaccharide and DHA in BSBAR were 16.9 mg/L and 6.14 mg/L,respectively,which were higher than those in SBAR.During the operation of the BSBAR,the removal rates of NH₄⁺-N and PO₄^3--P were reached 90.4%and 97.6%,respectively.It was in accordance with the water of class V in<Environmental Quality Standard for Surface Water>（GB3838-2002）.（3）Initial concentrations of NH₄⁺-N,PO₄^3--P and NH₄⁺-N/PO₄^3--P have significant effect on the growth of mixotrophic microalgae and N and P removal.When PO₄^3--P concentration（8 mg/L）remained constant,the higher yield of biomass in algal biofilm（0.76⁰.8 g/L）was attained under the condition of NH₄⁺-N in 16³2 mg/L and N/P ratio in 2/1⁴/1.The better removal rate of NH₄⁺-N（79.8%⁹1.3%）was gained under the condition that NH₄⁺-N was lower than 32 mg/L and N/P ratio was less than 4/1.The superior removal rate of PO₄^3--P（close to 100%）was obtained under the condition that NH₄⁺-N was higher than 8 mg/L;When the concentration of NH₄⁺-N（20 mg/L）remained constant,the higher yield of biomass in algal biofilm（0.69⁰.84 g/L）was achieved under the condition of PO₄^3--P in 5²0 mg/L.The better removal rate of NH₄⁺-N（85%⁹0%）was obtained under the condition of PO₄^3--P in 10²0 mg/L.The worse removal rate of PO₄^3--P（72%）was gained under the condition that PO₄^3--P was higher than 20 mg/L and N/P ratio was less than 1/1,while the removal rate of PO₄^3--P was higher than 95%in other conditions.（4）The optimal HRT of BSBAR was 4 days.In this condition,the removal efficiency of nutrients and the growth of microalgae were better than other conditions.The yield of biomass reached 0.56 g/L when HRT was 4 d,which was 1.12 times,1.48 times and 1.6 times of HRT was 5 d,3 d and 2 d,respectively.Under the condition of 28 ^oC,8000 lx illumination,and 16 h:8 h light-dark cycle,the kinetics constants of NH₄⁺-N and PO₄^3--P removal by mixotrophic algal biofilm were:N:U_max=37.74 mg/（L·d）,K_B=43.46 mg/（L·d）;P:U_max=45.45mg/（L·d）,K_B=45.73 mg/（L·d）.In conclusion,the mixotrophic algal biofilm possesses a favorable algal growth characteristics and nitrogen and phosphorus removal efficiency.It has great potential in the treatment of eutrophication.