| Drinking water is one of the main media for pathogen transmission.Microbial pathogens directly threaten the health of human beings,and if they are not controlled properly,it may lead to the spread of diseases.Conventional chlorine disinfection process is effective in killing most of the pathogenic microorganisms.However,with the discovery and recognition of new water-borne pathogens,among of which are microorganisms difficult to be killed,the traditional chlorine disinfection technology is facing new challenges.Therefore,it is very important and urgent to study the inactivation reaction of disinfectants to the microorganisms hard to be inactivated.In addition,disinfection of chlorine can produce harmful by-products of disinfection.Thus,it is an effective way to find a sterilization method to reduce the chlorine disinfectant and ensure the disinfection efficiency at the same time.Ultrasonication(US),a clean technology without any disinfection byproducts,has been of considerable interest since bacteria inactivation using ultrasound treatment was first published.But the relevant reports are all in intermittent research.What's more,it's difficult to achieve the ideal disinfection effect by ultrasonic US alone and it's hard to achieve industrialization.Therefore,it is of great significance to study the inactivation of microorganisms hard to be inactivated by ultrasound combined with chlorine.In this paper,Bacillus subtilis(B.subtilis),one of the microorganisms hard to be inactivated,was treated as the research object.The following aspects listed as below were studied and several important conclusions were obtained.(1)In order to study the inactivation characteristics of chlorine disinfectant to inactivate the microorganisms difficult to be inactivated,some factors affecting the inactivation of Bacillus subtilis by chlorine were analyzed.The inactivation process can be divided into two stages: delay and growth.Chlorine concentration and time all have effects on the inactivation efficiency of Bacillus subtilis by chlorine.The delay period is shortened with the increase of chlorine concentration.Increasing the concentration of chlorine or prolonging the time of chlorine is beneficial to the improvement of inactivation effect.The pH value of water is one of the most important factors that affect the inactivation of Bacillus subtilis.The inactivation ability of chlorine was much stronger under acidic condition than that of under alkaline condition.When the temperature of water is under 25?,the disinfection efficiency is significantly increased and the delay period is shortened with the increase of temperature.The initial concentration of bacteria had little effect on the disinfection efficiency.(2)In order to study the inactivation characteristics of US to inactivate the microorganisms difficult to be inactivated,the effects of ultrasonic frequency,ultrasonic power and temperature on inactivation of Bacillus subtilis were investigated.The ultrasonic frequency and power have a remarkable effect on the inactivation efficiency and with the extension of the ultrasonic time,the sterilization increased rapidly and then become steady gradually.What's more,low frequency ultrasound is helpful for inactivation of Bacillus subtilis by US and the inactivation efficiency raise with the increasing of US power,when the power is less than 105 W.However,the inactivation efficiency is not significantly increased at much higher power.When the temperature of water is less than 30?,the disinfection effect raise with the increasing of temperature of water.But the disinfection effect trend to decrease if the temperature of water increase continually leading to decline of air content resulting in energy loss when cavitation bubbles collapse.(3)In order to further investigate the inactivation characteristics of Bacillus subtilis by US,disinfection dynamic analysis was carried out.Under different US frequency and power,the inactivation kinetics of Bacillus subtilis fellow well to the first order reaction kinetics at short US time.And linear correlation is all above 0.96 showing a good linear correlation.So does the situation of the temperature.But the linear correlation varies from 0.88 to 0.95.(4)The effects of sequence of US and chlorine on disinfection efficiency by US coupled with chlorine were studied.When treated with US and chlorine simultaneously or US pretreatment,the disinfection efficiencies were rapidly increased compared to the control group,and the dosage of chlorine is decreased correspondingly.The disinfection effect was better in the process of US pretreatment than that of US and chlorine simultaneous treatment.And the former can save 31% of chlorine dosage when Bacillus subtilis are all inactivated,which is twice of the latter.(5)The effect of ultrasound time in the process of US pretreatment was observed.When the initial concentration of chlorine was 4mg/L,the log reduction of Bacillus subtilis increased from 0.712 with 1min US pretreatment to 2.25 with 5min US pretreatment,which was more than 3.5 times higher than that of 1min US pretreatment.However,when the time was prolonged to 10 min,the inactivation rate was 2.75 which increased by only 0.2 log reduction compared with that of 5min US pretreatment.Overall,the disinfection effect is better with the prolongation of pretreatment time,but it will trend to dropping because of the increase of dead bacterial inactivated by US which have enough time to attach to the surface of the activated bacterial to form newly bacterial cluster.In addition,longer pretreatment time,more energy will be consumed.Therefore,it's better to keep the US pretreatment time in 5 min.(6)The effect of single frequency ultrasound on the sterilization effect of continuous operating system was studied,among which disinfection tests by continuous chlorine were treated as control group.It found that the inactivation characteristics of continuous operation system and intermittent system was consistent,which presents that inactivation curve was divided into two stages: delay and growth.The introduction of US can significantly improve the effect of disinfection.Notably,lower frequency US(17 and 33 kHz)was in favor of the inactivation of Bacillus subtilis with US pretreatment,while higher frequency US(70 and 100 kHz)was better for inactivation of Bacillus subtilis with US and chlorine simultaneous treatment.(7)The effect of dual frequency ultrasound on the sterilization effect of continuous operating system was studied.Under the same ultrasonic power density,dual-frequency combination of US hold a much higher logarithmic inactivation efficiency than the single frequency.However,compared with the single frequency ultrasound,the inactivation effect of higher frequency dual ultrasound(70+100kHz)was not strikingly improved,which kept almost the same at the same time of chlorine disinfection.(8)The chlorine disinfection efficiency of the continuous system was analyzed.For this continuous sterilization system,the lower dual-frequency ultrasound(17+33kHz)pretreatment had the strongest sterilization effect,and can save 1/3 of the amount of chlorine disinfectant.Compared with 8mg/L chlorine alone,the utilization efficiencies of chlorine disinfectant increased from 44.25% to 85.85%,which meets the requirements of “Standards for Drinking Water Quality”(GB 5749-2006)on the concentration of residual chlorine of the effluent.(9)Whether the energy consumption density can be used as a disinfection efficiency evaluation parameter was evaluated.Alternating US input power densities(W/L)and pretreatment time(min)to keep a constant energy consumption density of 15kJ/L resulted in constant inactivation rate of Bacillus subtilis.It showed that the energy consumption density(kJ/L)could be used as a control parameter for sonication related disinfection efficiency evaluation. |
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