Residual Aluminum Characterisation In Water Treartment And Its Control By Chitosan As An AID Coagulant

One of the current concerns for water treatment plants using Alum as acoagulant is the higher concentration of residual Aluminium in the finished water.The objective of this study is to examine the variations of residual Aluminium at theMopanshan Water Treatment Plant of Harbin city.By analyzing the data ofMopanshan Water Treatment Plant with regard to the variations of total/dissolvedAluminium levels,it is found that the residual Aluminium present in the finishedwater was due to the use of PACl as a coagulant. Before and after water treatment,PACl addition increased the residual Aluminium from15to55μg/L. It wasobserved that a removal rate of90.4%particulate Al was observed at thesedimentation tanks, and an average of92.3%of particulate Al was further removedby filtration. For much of the finished water total Aluminium were in dissolvedform（63.6%）.Based on the analysis of the data on total/dissolved Aluminium turbidity,COD_Mnpresent in the raw water and the applied PACl dosage. it is found that PACldosage and COD_Mnof raw water played a major role in controlling residualAluminium in the finished water.When changing the PACl dose from65mg/L to80mg/L, lower concentrationof Aluminium was observed when the PACl dosage was lower. If the PACl/COD_Mnratio was above22, the total Aluminum in the filtered water was increased. Thetreated water turbidity correlated very poorly with the Aluminum in the treatedwater which indicated that much of the Aluminium in treated water was in dissolvedform.Chitosan is a natural polymer prepared from crab, shrimp and lobster shells. Ithas been used as coagulant in water treatment to avoid the human health problemscaused by residual Aluminum and chemical polymers in water. In this study, thewater from Mopanshan dam characterized by low turbidity was treated usingchitosan as coagulant aid together with PACl and Aluminum sulphate. The resultshows that chitosan was not as effic ient as Alum and PACl, if it is used as primarycoagulant for treating Mopanshan raw water or turbid water（Water with desiredturbidity was prepared by mixing kaolin clay and humic acid）.However, when chitosan was applied as coagulation aid agent together withAluminum sulfate or PACl, good turbidity and Aluminum removal was achieved with a dosage of0.5¹mg/L chitosan after30minutes of sedimentation time. Theresidual organic matter in finished water caused by addition of chitosan as acoagulation aid is negligible because it is used at very small doses. Hence, chitosancould be used as a natural coagulant aid for drinking water treatment with lowestrisks of organic release.