| Objective:To investigate the reason and level of lake pollution on campus and residential area of a university in Chongqing and explore the possibility of purifying the polluted lake based on ecosystem method,so as to provide theoretical and experimental basis for pollution treatment of the lake for the university administration.Methods:Field investigation was applied to explore the reason of pollution;the level of pollution is assessed by sensory properties and concentration of chemicals in the lake sample;chlorella was used to remove ammonia nitrogen and total phosphorus in the polluted lake sample to investigate the possibility of purifying polluted lake water based on ecosystem method.Results:1.Field research:(1)Water body of the north part of the lake on campus was green,the lake water smelled and opaque.Suspended solids could be seen in the water.The southern entrance of the lake is a drainage pipe to drain the runoff from the ground.The water flows northward along the shore of the lake beside the rubbish dump and forms a creek after dropping from a dam of about 4 meters high,which converged with the stream from Jianxin village and then flew to the Sports Technology School.(2)The upstream of the lake in residential area A was cut off due to the highway construction,and the sewage discharged from the residential buildings flowed directly into the lake.The lake water of residential area A is connected with the water body of the southern part of lake on campus through a drainage pipe,and flowed towards the School of Sports Technology through Zimu Lake and the sports ground.2.Examination results of physical and chemical indicators of lake water:(1)North part of the lake on campus:from the upstream to the downstream along the lakeshore beside the garbage station,4 sampling sites were selected to determine the level of dissolved oxygen,anionic surfactant ammonia nitrogen,total nitrogen and total phosphorus in the lake water.It was found that in September,the anionic surfactants at 4 sampling sites exceeded the standard limits(3.56mg/L,4.87 mg/L,2.87 mg/L,2.47 mg/L,respectively);level of dissolved oxygen surpassed the standard limit in September,with the concentrations of dissolved oxygen at 4 sampling sites from upstream to downstream were 0,0,0.1,0.81mg/L,respectively;the level of ammonia nitrogen at the 4 sampling sites in September all were several times higher than the limit,which were 7.99,10.27,7.75,6.69mg/L,respectively;the total nitrogen concentration at sampling sites No.1 to No.3 exceeded the standard limit(<2mg/L)in March(7.3,15.9,3.5 mg/L,respectively)and in June(8.23,14603,2.624 mg/L,respectively);the total phosphorus concentration at all the 4 sampling sites exceeded the standard limit(≤0.4 mg/L)in September;heavy metals such as cadmium,chromium(6 valence),copper and lead did not exceed the standard limits.(2)the creek beside the sports ground:It was found that dissolved oxygen(0.26mg/L)in September was lower than the standard limit(≥2mg/L),anionic surfactant concentration(0.5mg/L)in March exceeded the standard limit(<0.3 mg/L),total nitrogen concentration in March(6.74 mg/L)and June(6.892 mg/L)exceeded the standard limit(<2.0 mg/L),total phosphorus level(3.916mg/L)in September exceeded the standard 1imit(≤0.4mg/L).(3)Residential area:it was found that the sensory properties of the lake was not good;the ammonia nitrogen level(29.6mg/L,484mg/L)in both residential area A and B exceeded the standard limit(2.0 mg/L)by more than 10 times;the total nitrogen level(3.49mg/L)in the lake water of residential area B exceeded the standard limit(2.0 mg/L);and the total phosphorus level in area A(0.56 mg/L)and area B(0.98mg/L)exceeded the standard limit(<0.4 mg/L).3.Effect of treatment of polluted lake water with chlorella:(1)The optimal inoculation density of chlorella was 1Χ10~6/L.(2)Suspended Chlorella and immobilized Chlorella with concentrations of 10%and 20%respectively were added into the polluted lake water samples for 5 consecutive days to detect the removal rate of ammonia nitrogen and total phosphorus.It was found that 10%immobilized Chlorella showed the best removal efficiency on ammonia nitrogen and total phosphorus.The difference was statistically significant compared with the control group(P<0.05).(3)Chlorella suspension and sodium alginate suspension were mixed with different ratio(1:1 and 1:2)and vaccinated to the polluted lake water for the removal of nitrogen and phosphorus.It was found that no significant difference was found between these two treatments when the removal rate of ammonia nitrogen and total phosphorus was concerned.but the difference was statistically significant compared with the control group on the 5th day respectively(P<0.05).(4)The ammonia-nitrogen and total phosphorus removal rates of 1:1 immobilized Chlorella combined with activated carbon were not significantly different compared with using 1:1fixed chlorella alone,but the difference was statistically significant when compared with the control group(P<0.05).Conclusion:The pollution of the campus lake was probably caused by broken sewer as well as abandoned outdoor toilet;while the reason of residential area lake pollution might be due to the discharge of sanitary sewage surpassing the ability of self-cleaning capacity of the lake body.Suspended and immobilized chlorella can remove ammonia nitrogen and total phosphorus efficiently,with immobilized ones had better effects.Treatment with immobilized chlorella combined with activated carbon had no better effects than using immobilized chlorella alone.Immobilized chlorella can be used to purity polluted lake on campus and residential area. |
