Ozone and ultraviolet combined disinfection method and system for tail water treatment

Abstract

The invention relates to the technical field of tail water disinfection, and particularly discloses an ozone and ultraviolet combined disinfection method and system for tail water treatment.The method comprises the steps that combined disinfection is carried out on tail water of a current disinfection batch, disinfection quality evaluation is carried out, disinfection quality evaluation comprises the steps that the total bacterium amount of treated water is detected, and disinfection batch qualification judgment is carried out; a water sample is collected in the combined disinfection process so as to judge the uniformity of the disinfection process. When the qualification of the disinfection batch is judged to be unqualified, determining correction disinfection time; and when the uniformity of the disinfection process is judged to be non-uniform, determining that the ozone output power and the ultraviolet output power used in the subsequent disinfection batch are higher than those used in the current disinfection batch. Based on the corrected disinfection time and the adjustment of the output power, disinfection operating parameters for subsequent disinfection batches are determined. According to the invention, the disinfection time and the output power are subjected to double feedback regulation, so that the self-adaptive optimization of the disinfection operation parameters is realized, and the treated water is ensured to stably reach the standard and the uniformity of the disinfection process is ensured.

Description

Technical Field

[0001] This invention belongs to the field of wastewater disinfection technology, specifically relating to an ozone-ultraviolet combined disinfection method and system for wastewater treatment. Background Technology

[0002] Wastewater treatment plants, as key infrastructure for urban and industrial wastewater purification, directly impact the quality of the regional water environment. Effluent, the final effluent after a series of treatment processes, is the ultimate indicator of the success of the entire wastewater treatment system, and its quality is crucial. Since effluent may still contain residual pathogens, effective disinfection before discharge is the last critical line of defense against water pollution and for ensuring public health and safety, holding a vital position in the field of water treatment technology.

[0003] However, in terms of effluent disinfection, while the existing ozone disinfection method has strong oxidizing power, its dosage is difficult to control precisely. Insufficient dosage will lead to incomplete disinfection, while excessive dosage will form residual ozone or its byproducts (such as bromate) in the effluent, which are difficult to remove, constituting secondary pollution and having side effects on the aquatic ecosystem. As a physical method, ultraviolet (UV) disinfection is limited in effectiveness by the turbidity, color and suspended solids content of the water. Moreover, the characteristic of light traveling in a straight line makes it easy to form "shadow areas" or disinfection blind areas in complex flow fields, causing some microorganisms to escape. Furthermore, some microorganisms exhibit "photoresurrection" and "dark repair" after being removed from UV irradiation, resulting in poor long-term disinfection effects. In addition, the control strategies of existing disinfection systems are generally quite extensive, often adopting constant power or fixed dosage operation modes. They lack the ability to perceive and adaptively adjust the dynamic changes in influent water quality (such as flow rate, organic matter concentration, turbidity, etc.). When the water quality is good, this will result in a waste of energy and chemical agents, increasing operating costs. When the influent water quality suddenly deteriorates, the effluent microbial indicators may exceed the standards due to insufficient disinfection dosage.

[0004] To address the aforementioned problems, this invention provides an ozone-ultraviolet combined disinfection system and method for treating wastewater. Summary of the Invention

[0005] The purpose of this invention is to provide an ozone-ultraviolet combined disinfection method and system for effluent treatment, in order to solve the technical problems in the existing disinfection methods, which either have excessively high disinfectant content or fail to achieve effective disinfection.

[0006] To achieve the above objectives, the technical solution adopted by the present invention includes: The ozone-ultraviolet combined disinfection method for effluent treatment includes the following steps: Perform a combined disinfection procedure on the effluent from the current disinfection batch; Perform a disinfection quality assessment to determine the disinfection effectiveness of the current batch, including: The batch compliance determination for disinfection includes: collecting the treated water generated after the joint disinfection step, testing its total bacterial count, and comparing the total bacterial count with the first disinfection threshold. The uniformity determination of the disinfection process includes: collecting multiple water samples during the joint disinfection steps, detecting the total bacterial count of the multiple water samples, and comparing the total bacterial count with a third disinfection threshold. When the disinfection quality assessment results indicate that the disinfection effect has not reached the preset target, the disinfection operation parameters for subsequent disinfection batches are determined based on the disinfection quality assessment results. When the qualification of a disinfection batch is determined to be unqualified, the disinfection operation parameters for subsequent disinfection batches include: continuing to perform the combined disinfection steps on the treated water until the total number of bacteria in the treated water is less than or equal to the second disinfection threshold, and determining the total disinfection time accumulated since the start of the disinfection batch as the corrected disinfection time. The adjusted disinfection time will be used as the basis for determining the disinfection operation parameters for subsequent disinfection batches.

[0007] Preferably, when the uniformity of the disinfection process is determined to be non-uniform, the steps for determining the disinfection operating parameters for subsequent disinfection batches include: The ozone and ultraviolet output power used in subsequent disinfection batches should be higher than those used in the current disinfection batch.

[0008] Preferably, the disinfection operation parameters for subsequent disinfection batches include a disinfection time determined based on a corrected disinfection time.

[0009] Preferably, the disinfection operation parameters for subsequent disinfection batches are determined based on the results of the disinfection quality assessment by using the corrected disinfection time and the corresponding ozone output power and ultraviolet output power as inputs, and then calculating them through linear interpolation.

[0010] Preferably, multiple water samples are collected during the combined disinfection process by taking samples at multiple fixed sampling ports along the water flow direction of the disinfection module, or by taking samples at the same location of the disinfection module at constant time intervals.

[0011] The ozone-ultraviolet combined disinfection system for effluent treatment includes the following modules: The combined disinfection execution module is configured to perform ozone injection and ultraviolet irradiation in parallel on the effluent to carry out the combined disinfection steps; The water quality monitoring module is configured to monitor the total bacterial count in the treated water generated after the combined disinfection step and in the water sample during the combined disinfection step, in order to generate disinfection quality assessment data characterizing the disinfection effect. The parameter control module is configured to determine the disinfection operation parameters for subsequent disinfection batches based on the disinfection quality assessment data generated by the water quality monitoring module when the disinfection effect does not meet the preset target, and to control the joint disinfection execution module based on the disinfection operation parameters.

[0012] Preferably, when the disinfection quality assessment data indicates that the current disinfection batch is unqualified, the control joint disinfection execution module continues to execute the joint disinfection steps until the water quality monitoring module detects that the total number of bacteria in the treated water is less than or equal to the second disinfection threshold, and the total disinfection time accumulated since the start of the disinfection batch is recorded as the corrected disinfection time, which is used as the basis for determining the disinfection operation parameters of subsequent disinfection batches.

[0013] Preferably, when the disinfection quality assessment data indicates that the disinfection process is uneven, the ozone output power and ultraviolet output power used in subsequent disinfection batches should be higher than those of the current disinfection batch.

[0014] Preferably, it further includes: The pretreatment module is configured to perform solid-liquid separation on the effluent before the joint disinfection execution module performs the joint disinfection step on the effluent.

[0015] Beneficial effects After completing the combined disinfection step, this invention determines the batch compliance of the disinfection process by detecting the total bacterial count in the treated water. If the batch is deemed unqualified (i.e., the total bacterial count is not less than the first disinfection threshold), the combined disinfection process continues until the total bacterial count is less than or equal to the second disinfection threshold. The total disinfection time of this process is then used as the correction disinfection time for subsequent batches. This ensures that the treated water consistently meets standards before discharge, improving the reliability of effluent treatment. By performing supplementary treatment on unqualified batches and dynamically correcting subsequent disinfection times, the invention adapts to water quality fluctuations, effectively preventing the discharge of substandard water and ensuring consistent disinfection results during continuous operation.

[0016] In the process of performing the combined disinfection steps, this invention collects water samples at multiple preset sampling time points to determine the uniformity of the disinfection process. When the uniformity of the disinfection process is determined to be uneven, that is, when the total bacterial count of any water sample is detected to be higher than the third disinfection threshold, the ozone output power and ultraviolet output power are increased in subsequent disinfection batches, which effectively improves the uniformity and effectiveness of the disinfection process. By monitoring the water quality status at different time points inside the disinfection module, it is possible to promptly detect and compensate for the problem of uneven disinfection caused by water flow or equipment status, overcoming the technical deficiency of relying solely on outlet water quality detection and being unable to assess the uniformity of the process, thereby achieving more comprehensive killing of bacteria in the effluent.

[0017] Based on the determined correction disinfection time and output power adjustment, and according to the preset proportional relationship, the present invention comprehensively determines the disinfection operation parameters for subsequent disinfection batches, realizing closed-loop optimization and adaptive optimization of disinfection operation parameters. While ensuring the disinfection effect, it reduces operating energy consumption. By performing dual feedback adjustment on disinfection time and disinfection power, it dynamically determines the effective parameter combination that meets the current water quality treatment needs, avoiding energy waste caused by over-disinfection and improving the economy and operating efficiency of the system. Attached Figure Description

[0018] Figure 1 This is a flowchart of a method provided in an embodiment of the present invention. Detailed Implementation

[0019] Example 1 Please see Figure 1 As shown in the figure, this embodiment provides an ozone-ultraviolet combined disinfection method for effluent treatment, which dynamically optimizes disinfection efficiency and safety by adaptively adjusting disinfection operating parameters. The method specifically includes the following steps: Pretreatment steps are performed on the wastewater entering the system. Specifically, larger volume impurities and suspended solids in the water are removed by solid-liquid separation methods such as bar filtration, grit chamber, or primary clarifier to obtain wastewater to be disinfected. This is used to reduce the turbidity of the wastewater to prevent clogging or scaling of the ozone injection device and ultraviolet irradiation equipment in the subsequent disinfection module. This ensures the mass transfer efficiency of ozone in the water and the penetration depth of ultraviolet light with a wavelength of 254nm, creating stable physical conditions for the subsequent combined disinfection steps. The combined disinfection process involves introducing the wastewater to be disinfected into the disinfection module, injecting ozone into it, and simultaneously activating the ultraviolet irradiation equipment to irradiate the water. The ozone forms uniformly distributed microbubbles in the water through injection devices such as microbubble diffusers, thereby maximizing the gas-liquid contact area and reaction efficiency. At the same time, the specific wavelength of ultraviolet light emitted by the ultraviolet irradiation equipment destroys the nucleic acid structure of bacteria, viruses, and other microorganisms in the water, achieving rapid inactivation.

[0020] The combined disinfection step is performed according to an initially set disinfection baseline time. The disinfection baseline time is a reference value for adaptive adjustment by the system. It is the initial or standard treatment duration set for a single disinfection batch. After the disinfection baseline time ends, the treatment of the current disinfection batch is completed, and treated water is obtained.

[0021] The disinfection effect is verified immediately. Specifically, treated water is collected at the outlet of the disinfection module to form the first test water. This first test water is a specific water sample collected at the outlet of the disinfection module after the disinfection batch is completed, used to determine whether the batch treatment is qualified. The total bacterial count in the first test water is tested within a time window of 3 to 5 minutes after the collection. This time window is set to ensure that the test results can truly reflect the water quality status at the moment of disinfection, effectively avoiding evaluation bias caused by microbial photo-reactivation or dark-reactivation. The total bacterial count is compared with the first disinfection threshold. The first disinfection threshold is the legal upper limit of the total bacterial count for judging whether the disinfection batch is qualified, set according to the relevant national or local water quality discharge standards. It is a hard indicator of compliance. The relevant national or local water quality discharge standards are preferably the "Pollutant Discharge Standard for Urban Wastewater Treatment Plants" GB 18918-2002. If the total bacterial count is less than the first disinfection threshold, the current disinfection batch is deemed qualified; otherwise, if the total bacterial count is greater than or equal to the first disinfection threshold, it is deemed unqualified.

[0022] If the previous disinfection batch was deemed substandard, the disinfection duration adjustment procedure will be initiated immediately: It should be noted that the disinfection time adjustment procedure refers to an automated processing flow that is initiated when a disinfection batch fails to meet standards. Its core task is to extend the disinfection time of the current batch until the water quality meets the standards, and to update the time benchmark for subsequent batches based on this result. Details are as follows: The reflux device of the effluent module is controlled to return the batch of treated water to the disinfection module to continue the joint disinfection process. During this process, the total bacterial count is continuously monitored. At this time, a second disinfection threshold that is more stringent than the first disinfection threshold is used for judgment. The second disinfection threshold is a total bacterial count control limit that is lower than the first disinfection threshold used in the disinfection time correction process. Its main purpose is to establish safety redundancy and ensure that the corrected disinfection parameters have stronger robustness, that is, to ensure that the corrected disinfection time still has sufficient bactericidal ability when dealing with future water quality fluctuations. When the total number of bacteria in the treated water first drops to less than or equal to the second disinfection threshold, disinfection is immediately stopped, and the total disinfection time accumulated from the start of the disinfection batch to this moment is recorded for the unqualified disinfection batch. This time is defined as the corrected disinfection time, and this corrected disinfection time is updated as the disinfection reference time for subsequent disinfection batches, thereby completing the adaptive adjustment of the disinfection duration.

[0023] To further diagnose the root cause of poor disinfection effectiveness, this embodiment also includes a step to verify the uniformity of the disinfection process, specifically: During the joint disinfection process, water samples are collected from multiple spatially fixed sampling ports along the water flow path of the disinfection module at multiple preset sampling time points to form a second test water set, specifically used to evaluate the uniformity of the disinfection process. The collected water samples must first undergo validity screening to eliminate interference from fluctuations in the physical properties of the water body on the bacterial detection results. This refers to a data screening logic that compares the differences between the physical parameters of the water sample (such as color, temperature, and conductivity) and the influent baseline values ​​to exclude samples whose bacterial detection results may be distorted due to abnormal fluctuations in the physical properties of the water body. The specific screening logic is as follows: The measured values ​​of color, temperature, and conductivity of each water sample are compared with the corresponding parameter values ​​of the effluent to be disinfected before entering the disinfection module. A water sample is considered a valid sample and included in the subsequent disinfection process uniformity determination only if the absolute value of the difference between the measured values ​​of at least two parameters of a water sample falls within the preset error range.

[0024] The verification step of the disinfection process uniformity refers to a specific diagnostic process that involves sampling and detecting the total amount of bacteria at multiple points and times within the disinfection module to diagnose whether there are weak areas in the spatiotemporal distribution of the disinfectant. The uniformity of the disinfection process refers to the performance index used to measure whether the distribution of disinfectants (ozone and ultraviolet light) is balanced across the entire spatial and temporal dimensions of the disinfection module. Non-uniformity means that there are weak areas in the disinfection process.

[0025] Perform a uniformity determination of the disinfection process; specifically, set a total bacterial count limit for internal process control as a third disinfection threshold. During uniformity verification, if the total bacterial count at any internal sampling point exceeds the third disinfection threshold, it indicates the existence of a weak disinfection area. The total bacterial count of each valid sample in the second set of test water samples is compared with the third disinfection threshold. If, during this process, the total bacterial count of any valid sample is found to be higher than the third disinfection threshold, the current disinfection process is determined to be uneven, and the testing of the remaining samples is stopped to improve decision-making efficiency. The non-uniformity determination results indicate that, under the current disinfection power combination, there are weak areas in the spatiotemporal distribution of disinfectant within the disinfection module. Based on this determination, in subsequent disinfection batches, the ozone output power and ultraviolet output power will be increased by a preset margin (5% to 10%) above the baseline power determined in the following steps. Conversely, if the total bacterial count of all the effective samples is less than or equal to the third disinfection threshold, the disinfection process is determined to be uniform, and subsequent disinfection batches will directly adopt the disinfection operation parameters calculated in the following steps.

[0026] Based on the results of the aforementioned steps, complete disinfection operation parameters for subsequent disinfection batches are determined. Specifically, the step of determining the disinfection operation parameters for subsequent disinfection batches based on the results of the disinfection quality assessment is achieved by using the corrected disinfection time and the corresponding ozone output power and ultraviolet output power as inputs, and then performing linear interpolation calculations.

[0027] The system's control module maintains a set of data points containing historical successful disinfection parameters. Each data point is a valid record containing parameters such as disinfection baseline time, ozone output power, etc. When determining the operating conditions for subsequent disinfection batches, the ternary set of ultraviolet output power is used to obtain the latest disinfection reference time updated by the above steps. From the set of historical successful disinfection parameter data points, two historical data points whose disinfection reference time values ​​are closest and located on either side of the new reference time are retrieved. Linear interpolation is performed between these two data points to calculate theoretical values ​​for ozone output power and ultraviolet output power that match the new disinfection reference time. These two calculated power values, combined with any possible additional ozone and ultraviolet output power increases from the above steps, constitute the final disinfection operating parameters for subsequent disinfection batches. This process transforms the linkage between disinfection time and disinfection power from empirical judgment into a data-supported, repeatable calculation process, ensuring economical and efficient operation at different disinfection times. It also ensures that the distribution of disinfectant in the water meets the uniformity requirements of the disinfection process, thereby effectively killing microorganisms while avoiding the risk of increased disinfection byproducts due to excessively high local disinfectant concentrations.

[0028] Disinfection operation parameters refer to the complete set of control instructions required to execute a disinfection batch. In this scheme, they are specifically represented as a ternary set including disinfection duration, ozone output power, and ultraviolet output power.

[0029] The set of historical successful disinfection parameters mentioned above refers to a database or lookup table stored in the control module, which contains multiple sets of triplet data (disinfection reference time, ozone output power, ultraviolet output power) that have been verified as qualified for disinfection in the past. This set of data forms the basis for linear interpolation calculations.

[0030] Example 2 This embodiment discloses an ozone-ultraviolet combined disinfection system for implementing the above-mentioned disinfection method. The system realizes the automation and intelligence of the disinfection process through modular design and closed-loop control logic. It mainly includes: water inlet module, pretreatment module, combined disinfection execution module, water outlet module, water quality monitoring module and control module.

[0031] The water inlet module is responsible for receiving the wastewater to be treated, and through the built-in filter, pressure regulator and water inlet valve, it achieves preliminary impurity interception and stable control of water flow pressure, ensuring that the water flows into the downstream at a preset pressure range.

[0032] The pretreatment module is connected to the inlet module. It integrates a solid-liquid separation device based on the cyclone separation principle and an intermediate water tank. It is used to efficiently remove fine suspended solids from the wastewater to be treated and temporarily store the obtained wastewater to be disinfected in the intermediate water tank. The liquid level and downstream transportation are controlled by an ultrasonic level gauge and a solenoid valve.

[0033] The combined disinfection execution module is the core reaction unit of the system. It integrates an ozone injection device (such as a microbubble diffuser) and an adjustable-power ultraviolet irradiation device. The control module can independently adjust the ozone output power and the ultraviolet output power to execute the combined disinfection steps.

[0034] The effluent module is located downstream of the combined disinfection execution module and includes a temporary buffer tank, an effluent pump, and a key reflux device. Under normal circumstances, the treated water that has passed disinfection is discharged from the system via the effluent pump. When the control module determines that the disinfection batch is unqualified based on the batch qualification judgment logic, the reflux device will be activated to guide the treated water back to the inlet of the combined disinfection execution module for re-execution of the disinfection procedure.

[0035] The sampling probes of the water quality monitoring module are strategically distributed at the outlet of the joint disinfection execution module and at multiple preset positions within the cavity of the joint disinfection execution module, respectively, to collect and form the first test water and the second test water sets. The first test water and the second test water sets are respectively used for the disinfection batch qualification determination and the disinfection process uniformity verification steps. The water quality monitoring module integrates a rapid total bacterial count detection unit and transmits the detection results (i.e., total bacterial count data) to the control module in real time. The control module is the central processing unit of the entire system. It contains all the judgment logic and calculation procedures from the aforementioned method embodiments. Specifically: Receive total bacterial count data from the water quality monitoring module and perform batch qualification determination based on the logic of real-time verification of disinfection effect; If the previous batch of disinfection was deemed unqualified, the logic of the disinfection time correction procedure will be immediately initiated, and the procedure will calculate the corrected disinfection time. Determine the uniformity of the disinfection process; Based on the linear interpolation calculation procedure and the judgment result of the uniformity determination of the disinfection process, the disinfection reference time, ozone output power and ultraviolet output power of the subsequent disinfection batch are determined, and these parameters are sent as control commands to the joint disinfection execution module, the effluent module and other corresponding execution units.

[0036] The ozone-ultraviolet combined disinfection system described in this invention can be widely used in municipal wastewater treatment plants, industrial park wastewater treatment stations, and reclaimed water reuse scenarios. Through precise and adaptive closed-loop control, it significantly improves the reliability, economy, and safety of the disinfection process.

[0037] The above description is merely a preferred embodiment of this application and is not intended to limit this application. For those skilled in the art, this application can have various modifications and variations. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of this application should be included within the protection scope of this application.

Claims

1. A method for ozone-UV combined disinfection of tail water, characterized in that, The method comprises the following steps: performing a combined disinfection step on tail water of a current disinfection batch; performing a disinfection quality assessment to determine disinfection effect of the current disinfection batch, comprising: performing a disinfection batch eligibility determination, comprising: collecting treated water generated after completion of the combined disinfection step, detecting total bacteria amount of the treated water, and comparing the total bacteria amount with a first disinfection threshold value; performing a disinfection process uniformity determination, comprising: collecting multiple water samples during the combined disinfection step, detecting total bacteria amount of the multiple water samples, and comparing the total bacteria amount with a third disinfection threshold value; when the disinfection quality assessment result indicates that the disinfection effect does not reach a preset target, determining disinfection operation parameters for a subsequent disinfection batch based on the disinfection quality assessment result, wherein when the disinfection batch eligibility determination is unqualified, determining the disinfection operation parameters for the subsequent disinfection batch comprises: continuing to perform the combined disinfection step on the treated water until the total bacteria amount of the treated water is less than or equal to a second disinfection threshold value, and determining a total disinfection time accumulated since the disinfection batch as a corrected disinfection time; and using the corrected disinfection time as a basis for determining the disinfection operation parameters of the subsequent disinfection batch.

2. The ozone-UV combined disinfection method for tail water treatment according to claim 1, characterized in that, When the disinfection process uniformity determination is uneven, the step of determining the disinfection operation parameters for the subsequent disinfection batch comprises: determining that the ozone output power and the ultraviolet output power used in the subsequent disinfection batch should be higher than the ozone output power and the ultraviolet output power used in the current disinfection batch.

3. The ozone-UV combined disinfection method for tail water treatment according to claim 1, characterized in that, The disinfection operation parameters of the subsequent disinfection batch include a disinfection time determined based on the corrected disinfection time.

4. The ozone-UV combined disinfection method for effluent treatment according to claim 1, characterized in that, Determining the disinfection operation parameters for the subsequent disinfection batch based on the disinfection quality assessment result is achieved by using the corrected disinfection time and corresponding ozone output power and ultraviolet output power as inputs and performing linear interpolation calculation.

5. The ozone-UV combined disinfection method for effluent treatment according to claim 1, characterized in that, Collecting multiple water samples during the combined disinfection step is achieved by collecting at multiple spatially fixed sampling ports along the water flow direction of the disinfection module, or collecting at the same position of the disinfection module at constant time intervals.

6. An ozone-UV combined disinfection system for effluent treatment, characterized in that, The method comprises the following modules: a combined disinfection execution module configured to perform ozone injection and ultraviolet irradiation on tail water in parallel to perform a combined disinfection step; a water quality monitoring module configured to monitor total bacteria amount of treated water generated after execution of the combined disinfection step and water samples during execution of the combined disinfection step to generate disinfection quality assessment data representing disinfection effect; a parameter control module configured to determine disinfection operation parameters for a subsequent disinfection batch based on the disinfection quality assessment data generated by the water quality monitoring module when the disinfection quality assessment data indicates that the disinfection effect does not reach a preset target, and control the combined disinfection execution module based on the disinfection operation parameters.

7. An ozone-ultraviolet combined disinfection system for the treatment of tail water according to claim 6, characterized in that, When the disinfection quality assessment data indicates that the current disinfection batch is unqualified, the combined disinfection execution module is controlled to continue to perform the combined disinfection step until the water quality monitoring module monitors that the total bacteria amount of the treated water is less than or equal to a second disinfection threshold value, and a total disinfection time accumulated since the disinfection batch is recorded as a corrected disinfection time, which is used as a basis for determining disinfection operation parameters of the subsequent disinfection batch.

8. An ozone-UV combined disinfection system for the treatment of tail water according to claim 7, characterized in that, When the disinfection quality assessment data indicates that the disinfection process is not uniform, it is determined that the ozone output power and the ultraviolet output power used for a subsequent disinfection batch should be higher than that of the current disinfection batch.

9. An ozone-ultraviolet combined disinfection system for the treatment of tail water according to claim 8, characterized in that, Also included are: A pretreatment module, the pretreatment module is configured to perform solid-liquid separation on the tail water before the joint disinfection execution module performs the joint disinfection step on the tail water.

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