Analysis of problem alages in surface water treatment and effect of chemical dosing on their removal
Low turbidity along with abundant algae is frequently found in raw water taken from reservoirs by water treatment plants (WTPs). Because the density of the algae is too low to be effectively removed from the water by coagulation-sedimentation, the burden on the operation of the filtration unit in the WTP is significantly increased. In addition, the algae may clog the filter and slow down or stop the filtration. While a hybrid pre-oxidation/coagulation-sedimentation process is commonly adopted to destabilize and remove the algae in raw water, it is difficult to effectively remove strip or needle algae before the sand filtration in the WTP because the setting rate of these algae is rather poor. As a result, the filter is easily clogged with these algae, which significantly impedes its operation and causes an increased frequency of backwashing. Therefore, the optimum pretreatment, such as oxidation and coagulation, is the key to improve the removal performance in algae by water treatment plants. In this study, the characteristics of dominant algae and water quality in raw water from Bao-San first reservoir were investigated in different seasons. The species of problematic algae in the sedimentation and filtration process of Bao-San WTP was also investigated. In addition, the raw water taken from Bao-San first reservoir was concentrated to the desirable algal concentration in order to carry out the experiments of pre-oxidation combined with coagulation. The effects of pre-chlorination by NaOCl and ClO2 on the characteristics of algal cell (i.e., algal structure, chlorohyll a and cell integrity) before and after pre-chlorination was studied. It is simultaneously estimated that the effect of pre-chlorination coupled with coagulation by different coagulants (i.e., Alum and Polysilicate iron (PSI) on the performance of coagulation/sedimentation and sand filtration for algae-laden water treatment. The results have shown that the dominant algae of reservoir vary with the seasons. During fall and winter seasons, Cyclotella, Navicula, Oscillatoria, Synedra sp., and Melosira are in full bloom, while Cyclotella, Tetraedron, and Euglenophyta bloom in the spring and summer seasons. The problematic algae species existing in the sand filter include Cyclotella, Pediastum, Navicula, Oscillatoria, Synedra sp., and Euglenophyta. In oxidation, algal cells can be destroyed by NaOCl and ClO2 oxidation as a result of the reduced activity of algae accompanied by the increased settleability. Algal cells have been undergone more strong damage by ClO2 than by NaOCl treatment thereby improving the settleability of algae better. When the activity of algae reduces, the concentration of chlorohyll a was decreased along with the reduced quantity in integrated algal cell and the increased quantity in impaired algal cell. Furthermore, the ratio of various dissolved organic matter (DOC) in raw water was not changed obviously after chlorination with NaOCl and ClO2, while only the increased aromatic protein along with reduced natural organic matter (i.e., humic substance-like and fulvic acid-like) was found after ClO2 oxidation of raw water due to the stronger oxidation ability of ClO2. In pre-chlorination/coagulation of algae-laden water with low turbidity, the performance of coagulation/sedmentation and filtration can be effectively improved by pre-chlorination. For alum coagulation, the improvement in the performance of coagulation/sedimentation and the filterability of supernatant by ClO2 pre-treatment is superior to that by NaOCl pre-treatment. However, for PSI coagulation, NaOCl has the similar performance to ClO2.
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