Effects Of Pre-Oxidation On The Characteristics Of Algae And The Formation Of Algae Derived Chlorinated Disfection By Products

The incidence of algal blooms assoiated with eutropication which has already affected environment, economic development and human helth dramatically has increased significatly in both the number and magitude as a result of unbalanced rapid economical development but poor nutrient control in both wastewater effluent discharge and excessive agricultural fertilization in China. With their characteristics of negetive charge, hydrophilic property, steric repulsion et al., algal cells have excellent stability, thus algae are very hard to remove from natural water. Besides, algal blooms also cause water quality problems and endanger the safty of drinking water, including obnoxious taste and odors, releasing algal toxins, generating disfection by product precusors.Chemical pre-oxidation is an effective method to aid coagulation and has already been widely applied so that it can change algal cells surface morphology, Zeta potential and cell proportion. But it is different from natural organic matters, there were few and unsystematic study related to the effects of pre-oxidation on the formation of algal derived chlorinated disfection by products, besides some results from different studies were contradictory.Based on pre-oxidation to enhance the removal of algal, this thesis studied the effects of pre-oxidation as the aid to coagulation through measuring TOC and turbidity, we also systematicly studied the impacts of pre-oxidation on algal characteristics and chlorinated disfection by products including eight kinds of C-DBPs (TCM, MCAA, DCAA, TCAA, 1,1-DCP, 1,1,1-TCP, DCA, CH) and three kinds of N-DBPs (DCAN, TCAN, TCNM). Besides, with comparison of the effects of pre-oxidation by potassium permanganate and pre-ozonation on algal derived chlorinated disfection by products, this thesis discussed the safety of pre-oxidation to enhance the removal of algal and the way to select oxidants in preoxidation properly.Through study the factors such as potassium permanganate dosages, pH of preoxidation, the presence of Ca²⁺ in the process of pre-oxidation by potassium permanganate on the formation of algal derived chlorinated disfection by products and the changes of their formation potential during pre-oxidation by potassium permanganate to aid the coagulation, this thesis concluded that pre-oxidation by potassium permanganate has less impact on the damage of algae cells, higher acidity condition could have stronger impact on the damage of algae cells, and the presnece of Ca²⁺ could induce organic matters desorption from algae cells. Besides, preoxidation by potassium permanganate could increase the formation of algal derived chlorinated disfection by products to some extent; the oxidation pH condition in pre-oxidation play an important role on disinfection by products formation with most extent of DBPs formation reduction under acidic condition and in contrary under neutral condition, alkaline condition; the presence of Ca²⁺ could restrain the formation of some DBPs (TCM, TCAA, N-DBPs) ?? and that will be enhanced by pre-oxidation by potassium permanganate. Besides, the results also indicate that appropriate dosages of potassuim permanganate in pre-oxidizing algal bearing water could enhance the removal of algal by coagulation/sedimentation without the increase of DBPs formation.Through investigating the factors such as ozone dosages, the pH condition of preoxidation, HCO₃^- concentration in pre-ozonation on the formation of algal derived chlorinated disfection by products, and their formation potential changes of pre-ozonation to aid coagulation, the thesis conclude that pre-ozonation could result in the formation of algal derived chlorinated disfection by products, especially TCM, HAs, N-DBPs, whose concentration increase dramaticaly with the increase of ozone dosages;both direct oxidation of ozone and the oxdation of HOï¹'could promote the formation of DBPs though their roles varied very much for different types of DBPs; the formation of DBPs could be affected by the alkalinity of water obviously because alkalinity could influence the disinfection and pre-ozonation process, the total DBPs and their increased extent after pre-ozonation decreased with the increase of HCO₃^- concentration; pre-ozonation could enhance coagulation to remove algal cells, turbidity to some extent, whist it also could result in the increase of DOC, and just a few mg/L dosages of ozone could promote disinfection to produce more algal derived chlorinated DBPs.Both pre-oxidation by potassium permanganate and ozone could aid coagulation to remove algal. Potassium permanganate has less impacts on the formation of algal derived chlorinated DBPs as it has little impact on the damage of algal cells during pre-oxidation; whist pre-ozonation could generate much higher extent of DBPs precursors and could result in much more chlorinated DBPs formation because it could damage algal cells apparently and cause cell membranes and cell walls lysis, releasing higher concentration of dissolved organic matters which are hard to remove, besides, many organic matters that are easily attacked by chlorine are produced by preozonation.When use pre-oxidation to treat water with high algal cells in practical application, we should employ potassium permanganate rather than ozone in order to remove algal and control the formation of DBPs.