Experimental Study On Treatment Of Algae-Laden Water By Preoxidation-Powder Activated Carbon Enhanced Micro-Vortex Flocculation Process

The rapid growth of our economy and accelerated urbanization have created new challenges for the safety of the aquatic environment.Large quantities of organic pollutants and nutrient salts are discharged to water bodies which result in the widespread distribution of nutrient-rich water sources.Algae blooms and excessive organic pollutants often occur in lakes and reservoirs.Conventional water supply technology is not effective in treating such water bodies,and it is difficult to ensure the safety of drinking water for residents.Therefore,the enhanced coagulation process plays an important role in water treatment to efficiently remove algae and organic matter.In this paper,the eddy current clarifier was used as the main body,and the micro-eddy current enhanced coagulation test was carried out to treat algal raw water combined with pre-oxidation and powder activated carbon adsorption.In this study,the eddy current clarifier was used as the main body,and the micro-eddy current enhanced coagulation test was carried out to treat algal raw water combined with pre-oxidation and powder activated carbon adsorption,on this basis,the response surface experiment was carried out to optimize the optimal parameters of the combination process.The main research contents and results are as follows:1.Pre-oxidation combined with activated carbon enhanced coagulation experiment（1）The optimal dosage of polyaluminum chloride in the raw water of conventional coagulation treatment is 25 mg/L.（2）Pre-oxidation of potassium permanganate,sodium hypochlorite and hydrogen peroxide significantly enhanced coagulation and improved effluent quality.The increase of the dosage and oxidation time of the pre-oxidizer will gradually improve the removal rate of each pollutant,but the trend of slight decrease will occur when the dosage is too high.The optimal dosage of the three kinds of oxidizer is 0.6 mg/L,1.5 mg/L,3.0 mg/L,and the optimal oxidation time is 10 min,15 min,15 min,respectively.（3）Combined with Zeta potential and particle number analysis,Potassium permanganate was more effective than sodium hypochlorite and hydrogen peroxide in reducing particle repulsion,removing algae,organic pollutants and total phosphorus.The removal rates of turbidity,chlorophyll a,UV₂₅₄,COD_Mnand TP reached 90.11%,92.88%,79.43%,70.12%and88.89%,which were increased by 17.92%,22.35%,24.78%,19.46%and 17.84%compared with conventional coagulation,respectively Potassium permanganate was used as the pre-oxidant for subsequent tests.（4）Adding activated carbon before the pre-oxidation stage or during the pre-oxidation stage or during the coagulation stage can improve the treatment efficiency of the pre-oxidation coagulation process,and the optimal adsorption time of the three is 15 min,8min and 10 min,respectively.The best treatment effect is achieved by adding activated carbon before the pre-oxidation stage.The activated carbon is added at this time to avoid competitive adsorption in all stages and to ensure sufficient adsorption time.The optimal dosage of activated carbon at this point is 15 mg/L.The removal rates of turbidity,chlorophyll a,UV₂₅₄and COD_Mnreached 94.77%,95.83%,83.03%and 76.79%,which were 22.58%,25.30%,28.38%and 26.13%higher than conventional coagulation,respectively.The treatment effect of the coagulation process was further improved by the addition of activated carbon.2.Experimental study on micro-vertex enhanced coagulationThe treatment effect of eddy current clarifier on micro-polluted algal water under different mixing ratio of eddy current reactor was investigated,and the optimal working condition was determined according to the removal effect of turbidity,chlorophyll a,UV₂₅₄,COD_Mnand other indexes.（1）The optimal treatment flow rate of eddy current clarifier without eddy current reactor is 3.5m³/h,and the optimal dosage of coagulant is 30 mg/L.In this case,the turbidity of precipitated effluent is higher than 3 NTU,and the overall removal effect is not good.（2）The processing capacity of eddy current clarifier can be improved by three different proportions of eddy current reactor.The optimal treatment flow rate of the vortex clarifier is about 5.0 m³/h when the mixing ratio is 1（all the first reaction chamber is filled with HJTM-1reactor）and 2（all the first reaction chamber is filled with HJTM-2 reactor）.When the mixing ratio is 3（adding 3/5 HJTM-1 vortex reactor to the lower part of the first reaction chamber and 2/5 HJTM-2 vortex reactor to the upper part of the first reaction chamber）,the treatment effect of vortex clarifying tank is the best,and the optimal treatment flow rate is increased to5.5 m³/h,Zeta potential of raw water increased from-14.21 m V to-4.93 m V,potential value increased by 9.28 m V,particle rejection decreased significantly,and particle removal effect in water was significant.For particles smaller than 10μm,the removal rate is 82.8%,and for particles larger than 10μm,the removal rate is more than 95%,indicating that the working condition is reasonable.Therefore,the follow-up test is conducted in the eddy current clarifier according to the adding ratio of 3 to the reactor.3.Response surface process optimization experimentBased on the preliminary test data,with turbidity,chlorophyll a and COD_Mnremoval rates as response values,the influences of flow rate,coagulant dosage and oxidant dosage on the effect of micro-vertex flocculation process enhanced by pre-oxidation and powder activity were explored,and the pilot process was optimized by response surface analysis.（1）In terms of turbidity removal rate,the effect of oxidizer dosage is the most significant,followed by flow rate,and the effect of coagulant dosage is the least,and flow rate has interaction with oxidizer dosage,coagulant dosage and oxidizer dosage;In terms of chlorophyll a removal rate,the flow rate and the dosage of oxidant play a decisive role,in which the dosage of oxidant has the most significant effect,the dosage of coagulant has no significant effect,and the three have no obvious interaction;In terms of COD_Mnremoval rate,the effect of dosage and flow rate of oxidant is very significant,and the interaction between dosage of coagulant and dosage of oxidant is obvious.（2）The confirmatory test was carried out according to the optimal operating condition obtained from the response surface test.The validation test results are very close to the predicted values,indicating that the model fits well and the response surface analysis is reasonable.The optimum operating parameters of the micro-eddy flocculation process enhanced by pre-oxidation and powder activity are as follows:flow rate of 6.40 m³/h,coagulant dosage of 25 mg/L and oxidant dosage of 0.57 mg/L.Under these conditions,the turbidity,chlorophyll a and COD_Mnremoval rates were:90.83%,92.11%,70.46%,and the corresponding turbidity,chlorophyll a and COD_Mnof effluent are:0.93 NTU,6.7μg/L and2.82 mg/L.（3）The total cost of unit water treatment by pre-oxidation and powder activated carbon enhanced micro-eddy flocculation process is 0.2302 RMB/m³,in which the drug cost accounts for 59.8%.The combined process is economically feasible to treat micro-polluted algal water.In conclusion,The micro-vertex flocculation process enhanced by pre-oxidation and powder activated carbon has a remarkable effect on the simultaneous removal of algae and organic pollutants,and reduces the dosage of coagulant while increasing the actual treated water.The extension of the research results will effectively improve the treatment efficiency of water supply plants for water from lakes and reservoirs and guarantee the safety of drinking water.It has certain theoretical guiding function and important practical value.