Water Quality Characteristics And Advanced Treatment By Coagulation Of Secondary Effluent From Typical Sewage Treatment Plant

Currently, contradiction between water shortages and water demand havebecome increasingly prominent and seriously, restricting economic and socialdevelopment of China. Water reuse has become one of the most efficient solutions tosolve the current water crisis. Municipal secondary effluent as a renewable source ofwater has received widespread attention because of its good economic andenvironmental benefits.There still contains a variety of organic pollutants, pathogenic microorganismsand plant nutrients (such as N and P, etc.) after secondary treatment of urban sewageeffluent. The existence of these substances would cause a potential threat to the safeuse of reclaimed water. Therefore, it is particularly essential to use appropriatetreatment technology and reduce pollutants content of water for improving the reuseof reclaimed water’s ecological safety. To solve the above problems, thecharacteristics of organic matter in secondary effluent and after advanced treatmentfrom municipal wastewater treatment plant were studied. The aims of this study wereto understand the change of the physical and chemical characteristics after advancedtreatment process, to provide a theoretical guidance for optimization of the sewagetreatment process, and to probe into the improvement of reclaimed water quality. Theresearch mainly includes as follows:(Ⅰ) The effect of Quality characteristics insecondary effluent and the change of biological toxicity of the effluent quality at allstages was analysis under different water quality and different treatment processes.(Ⅱ) The effect of depth teratment to the remove of organic matter was studied tosecondary effluent from typical sewage treatment plant with ferric chloride andPolyaluminium Chloride as coagulant. The study investigated the effects ofcoagulation at low levels of PAM dosing and pre-oxidation of chlorine dioxidedosing, disinfection role of chlorine dioxide to secondary effluent. On this basis, thechange of hydrophilic and hydrophobic characteristics, molecular weight distributionof DOM were characterized by XAD resin separation technology and ultrafiltration membrane method before and after coagulation.(ⅰ) Characterization of organic pollutants and biological toxicity analysis ofdifferent stages effluent from typical municipal wastewater treatment plant inQingdao’s results are as follows:The total DOC removal rate of all three wastewater treatment’s primarysedimentation tank effluent could reach up to more than80%after biologicaltreatment, it indicated that the primary sedimentation tank effluent of threewastewater treatment plant are all highly biodegradable. Every molecular weightrange of the organic matter, hydrophilic and hydrophobic organic matter, acid andnon-acid organic matter in the proportion of the total DOC content of primarysedimentation tank effluent were similar when water quality of “B” and “C” werebased on domestic sewage. Compared with the primary settling tank effluent, thehydrophilic organic matter in the proportion of the total DOC content of secondaryeffluent of three wastewater treatment plants was significantly reduced under differentbiological treatment processes. It showed that the biological treatment removalefficiency to the hydrophilic organic matter are more higher. After treated bycoagulation, the organic matters of MW between the3k~10kDa and30k~100kDawere increased, and the Hydrophobic organic matters and acid organic matter in theproportion of the total DOC content were reduce. And it showed that effect ofcoagulation process on hydrophobic matter removal is better than that on thehydrophilic ones. The primary settling tank effluent of three wastewater treatmentplants were all highly biological toxicity and biological treatment could reduce thetoxicity of sewage. The primary sedimentation tank effluent and secondary effluent of“A” were the most highiy biological toxicity. The biological toxicity of effluentbecame larger after treatment of coagulation. The DOC content of effluent and thesize of biological toxicity had a certain direction certain relationship.(ⅱ) The study of coagulation process optimization’s results are as follows:(1) When the dosage of FeCl3and PAC was30mg/L and60mg/L respectively,the results showed that was investigated.The effluent quality could meet the firstclass level “A” standard of “Discharge standard of pollutants for municipal wastewater treatment plant”. Coagulation-aid effect of PAM was not obvious, whenTP, color, COD and other water quality index of the secondary effluent wererelatively low. Dosing ClO2before coagulation could enhance the removal of color,while TP, COD, turbidity removal efficiency was not affected significantly. Whiledosing ClO2in this manner, the content of fecal coliform in the final effluent wouldexceed the standard without subsequent disinfection.Adding ClO2after coagulationcould reduce the fecal coliform content effectively.At ClO2dosage of3.5mg/L, theremoval rate of fecal coliform could reach up to92.3%.(2) Removal efficiency of unsaturated C-C double bonds and aromatic organicsby FeCl3was better than that by PAC. Comparatively FeCl3was more efficient onsmall molecular organics removal, while the removal rate of the big molecularorganics of>100kDa by PAC was higher. Effect of coagulation process onhydrophobic matter removal was better than that on the hydrophilic ones. Besidesremoval rate of acid organic matter by coagulation was higher than that of non-acidorganic matter.In summary, concerning the removal of the conventional water quality indexesand organic matters, the trihalomethane formation potential and the biologicalstability of the treated water, FeCl3was a more suitable coagulant in the advancedtreatment of the secondary effluent compared with PAC.