The Comparative Study Of Synergistic Inactivation Of Different Bacteria In Drinking Water By Ultrasonic And Chlorination

Increasing human population and industrialization have led to a wide range of contaminants and pathogens within the water bodies.The lack of access to safe drinking water has been one of the main challenges that humanity is facing in the 21 st century.The existing traditional drinking water disinfection technology relies mainly on chlorine disinfection alone,which has high disinfection efficiency and can effectively inactivate most of the microorganisms in the water.However,with the increasing pollution of water bodies,the appearance of some chlorine-tolerant microorganisms leads to the need for higher chlorine content or longer disinfection time during water treatment.In addition,a series of harmful disinfection by-products are produced during chlorination disinfection process.Therefore,it is very necessary to find an efficient and environmentally friendly disinfection technology for drinking water.Ultrasonication(US),an clean technology producing no by-products,has gained much attention since the use of ultrasonic techniques was proposed for bacteria inactivation.Although many related experiments have been conducted,the majority of the experiments were conducted in laboratory environments with batch process.In addition,it is difficult to ensure the complete disinfection by using ultrasound treatment alone,which limits the industrial application of ultrasound.Therefore,it is of practical significance to study the inactivation of microorganisms by continuous-flow ultrasound combined with chlorination process.In this paper,Escherichia coli(E.coli),Bacillus subtilis(B.subtilis)and Staphylococcus aureus(S.aureus)were selected as indicators of water treatment effects.The following aspects listed as below were studied and several important conclusions were obtained.(1)In order to study the effect of ultrasound on the inactivation of different types of bacteria,the factors affecting the inactivation of Escherichia coli,Bacillus subtilis and Staphylococcus aureus by ultrasound were analyzed.The experimental results show that the inactivation ratio of the three bacteria increased first and then tended to be stable with the extension of the ultrasound treatment time,and the best ultrasound treatment time is 30 min.The disinfection effects increased first and then decreased with the increasing ultrasound frequency for Escherichia coli under the same conditions and the best inactivation effect was obtained at 70 kHz.However,the disinfection effects of Bacillus subtilis and Staphylococcus aureus both were reduced with the increase of frequency and the best inactivation effect were obtained at 17 kHz.The input power density of ultrasound have a remarkable effect on the inactivation efficiency.In a certain range,increasing the ultrasonic power density can effectively improve the inactivation effect,but the power density is too high,the increase of inactivation effect is not obvious,and even the phenomenon of decline is observed.(2)The effect of dual-frequency ultrasound on inactivation was investigated,and compared with single-frequency ultrasound inactivation under same experimental condition.Results indicate that higher inactivation effect was achieved at dual-frequency ultrasound compared with the corresponding single-frequency ultrasound.After the dual-frequency ultrasound treatment of 17 kHz + 33 kHz for 60 min,2.13,1.72,and 1.54 log inactivation ratio were finally achieved for Escherichia coli,Bacillus subtilis and Staphylococcus aureus,respectively,the increase was 0.38,0.28,and 0.30 compared with the best single-frequency ultrasound treatment,respectively.Although the dual-frequency ultrasound had better inactivation effects compared with single-frequency ultrasound,it could not achieve an ideal disinfection level.(3)In order to improve the inactivation effect,the effects of ultrasound pretreatment combined with chlorination disinfection process on the inactivation was studied,and compared with chlorination disinfection alone.The experimental results indicate that the inactivation process of chlorination alone can be divided into two stages: lag phase and growth phase.The lag phase is shortened with the increase of chlorine concentration.After 10 min of ultrasound pretreatment,the log inactivation ratio rapidly increased at the initial stage of chlorination disinfection,and the lag phase is obviously shortened,in which 3.85,3.52 and 3.65 log inactivation ratios were achieved when 8 mg/L NaClO disinfection lasted 10 min with 17 kHz + 33 kHz ultrasound pretreatment for Escherichia coli,Bacillus subtilis and Staphylococcus aureus,respectively.The lower frequency ultrasound is beneficial to improve the inactivation effect,but the higher frequency ultrasound does not obvious improve the inactivation effect.(4)In order to explore the optimal operation parameter combination for ultrasound pretreatment,the orthogonal experiment design was carried out by selecting three levels of three factors: ultrasonic frequency,ultrasonic power density and ultrasonic pretreatment time.The primary and secondary relationship of various factors on the effect of inactivation was studied.The results of orthogonal experiment show that the primary and secondary effects of various factors on the inactivation effect is: ultrasonic frequency > ultrasonic power density > ultrasonic pretreatment time,in which ultrasonic frequency and ultrasonic power have a significant effect on the inactivation ratio,while ultrasonic pretreatment time has no significant effect on the inactivation ratio.The optimal operation parameter of ultrasound pretreatment are 17 kHz + 33 kHz dual-frequency ultrasound,ultrasonic input power density of 20 W/L and ultrasonic pretreatment time of 5 min.(5)The effective utilization ratio of NaClO disinfectant of the continuous treatment system was analyzed.Ultrasound pretreatment could obviously reduce residual chlorine concentration and increase the effective utilization ratio of disinfectant.Compared with that using 8 mg/L NaClO disinfection alone,the effective utilization ratio of disinfectant increased 25.1%,40.1% and 35.7% for Escherichia coli,Bacillus subtilis and Staphylococcus aureus,respectively,which could satisfy the requirements of Chinese national standard GB 5749-2006 in terms of the residual chlorine concentration(below 4 mg/L).