Removal Of Nitrogen And Phosphorus In Sewage By Immobilized Microalgae

At present,the major water issues in our country are the lack of fresh water resources,the problem of water pollution,and low waste water recycling rate.With the development of the sewage treatment process,the use of algae in the sewage treatment gradually gained attention due to its efficiency.Microalgae immobilization technique has the advantages of fast algal cells proliferation,easy microalgae recovery,high wastewater reuse rate.Therefore,we studied Chlorella pyrenoidosa and Synechocystis sp.,which had a strong ability to absorb nitrogen and phosphorus in water.We investigated the immobilization of these two species of microalgae,optimized the immobilization conditions of the two algae and evaluated the effect of nitrogen and phosphorus removal in sewage treatment of two species of suspended and immobilized microalgae.Then C.pyrenoidosa was selected to study the effects of different factors on the removal of N and P in the artificial and actual sewage samples.The main results are summarized as follows:1.Optimization of microalgae immobilization conditionsArtificial sewage(NH4-N 15 mg/L,TP 1.5 mg/L)was prepared.Immobilization carrier(sodium alginate),immobilization solution(calcium chloride),and immobilization time were chosen and set at three levels in an orthogonal experiment.The optimum conditions found to be:3%SA(sodium alginate)solution,1%CaC12 solution,and immobilization time of 2 hours.For Chlorella,the nitrogen removal rate was between 67.4%to 94.8%,phosphorus removal rate was between 48.9%to 79.8%.These three factors had significant effects on the removal of NH4-N.In addition to calcium chloride,other factors have a significant impact on the TP removal.Synechocystis immobilization optimal conditions were:3%SA(sodium alginate)solution,2%CaC12 solution,immobilization time 3 hours.For Synechocystis,nitrogen removal rate was between 44.3%to 58.6%,phosphorus removal rate was between 93.1%to 97.7%.Immobilization time had a significant effect on the Synechocystis removal of TP in sewage,other factors had no significant effect.2.Comparison of nitrogen and phosphorus removal ability of immobilizedmicroalgae and suspended microalgaeIn the above optimum conditions,we obtained two immobilized microalgae and compared N and P removal by immobilized and suspended microalgae in synthetic wastewater.The results showed that,when grown 96h,the NH4-N removal by immobilized Chlorella in artificial sewage was 99.99%,TP removal rate was 95.71%.The suspended Chlorella ammonia removal in artificial sewage was 98.92%,and TP removal rate was 91.56%.Therefore,the ability of NH4-N removal was higher than the TP by Chlorella in sewage.Immobilized microalgae removal effect was better than the suspended.When grown 96h,the NH4-N removal by immobilized Synechocystis in artificial sewage was 91.66%,TP removal rate was 98.98%.The suspended Synechocystis NH4-N removal in artificial sewage was 74.24%,and TP removal rate was 96.94%.Therefore,the ability of TP removal was higher than the NH4-N by Synechocystis in sewage.Synechocystis growth was dependent on phosphorus,thus was more suited for sewage with high phosphorus concentration.Chlorella is not easy to survive in the case of low phosphorus,thus more suitable for sewage treatment.3.Effects of environmental factors on the nitrogen and phosphorus removal capability by immobilized ChlorellaWe used artificial sewage and set different cell numbers(3 × 105,5 × 105,7 × 105 algal cells per bead),and analyzed NH4-N and TP after growth of 1 d,2 d,3 d,4 d.More cells(in the range of the 300,000 to 700,000)in the beads resulted in higher NH4-N removal(68.72%-95.71%)and TP removal(36.51%-90.72%).Different pH(pH 6,6.5,7,7.5)had impacts.When the pH was 7.5,the removal efficiency was the best(NH4-N 99.53%,TP 92.06%).When the pH was 6.5-7,higher pH favored better nitrogen and phosphorus removal.At pH 6,the removal efficiency was not very stable,presumably due to the pH increase caused by the algae growth.Organic compounds(potassium hydrogen phthalate,50 mg/L,100 mg/L,200 mg/L)also affected the removal.When its initial concentrations was below 100 mg/L,the nitrogen and phosphorus removal increased with the organic compound concentration increased.4.Removal of nitrogen and phosphorus and COD by immobilized Chlorella in real water samplesWe collected water samples from three locations in Nanjing:Institute of Vegetable Research,Qinhuai River,and Yueya Lake.After determination of the initial concentration of N,P,and COD in water samples after mixing,we applied immobilized Chlorella in them.Analyses were carried out after 1 d,2 d,3 d,4 d of growth.,Ammonia removal was more than 80%in three water samples.The higher initial ratio of N,P and the pH in water samples,the better effect of N and P removal.The removal of COD by immobilized Chlorella in the sample from the Institute of Vegetable Research had no positive effect but promoted the COD removal in the Qinhuai River and Yueya Lake water samples.In summary,we optimized the conditions for microalgae immobilization.Under these conditions,the N and P removal by the immobilized microalgae was better than that by the suspended microalgae.The influences of cell numbers,pH,concentration of organic compounds on the N and P removal were clarified.In addition,a good performance was demonstrated in the actual water samples.