In recent years, the water quality of drinking water has been highly valued by the society. On the basis of achieving 106 water quality standards, more and more water companies and related scientific research and technical personnel have turned their attention to the detection and removal of disinfection by-products, antibiotics and other trace pollutants. so the application and optimization of advanced treatment process represented by ozone activated carbon process has gradually become a hot spot in water supply industry. However, in the daily operation of water plants with advanced treatment process, there is still room to improve the real fine management. This article takes ozone as an example is to put forward the idea of residual ozone and accurate ozone dosing
1.The necessity of precise ozone dosing?
In the whole process of water production, ozone is generally used in pre ozonation of high algae raw water and contact oxidation in front of biological activated carbon filter. The addition of ozone mainly acts on the natural organic matter (NOM) in the water. Through advanced oxidation, the macromolecular organic matter is decomposed into small molecule organic matter, which is convenient for the subsequent adsorption and biodegradation of biological activated carbon filter. The insufficient ozone dosage may lead to insufficient contact oxidation, which makes the biological activated carbon filter ineffective in biodegradation of organic pollutants and leads to the organic matter index exceeding the standard; On the one hand, excessive ozone will directly lead to the increase of energy consumption and operation cost of the water plant. On the other hand, Guan Chunyu and other researchers found that excessive ozone will increase the electronegativity of NOM, increase the electrostatic repulsion between NOM and activated carbon, which is not conducive to the adsorption of activated carbon. At the same time, residual ozone will destroy the microporous structure of activated carbon, change its surface chemical properties, and affect the service life of activated carbon, With the increase of ozone dosage, it is easy to cause the problem of carcinogenic disinfection by-products such as bromate, and bring harm to the public environment.
In the daily production process, due to the relatively stable characteristics of raw water quality and the purpose of pre ozone dosing, the need for accurate control of raw water ozone dosing is relatively not so important. Generally (excluding sudden water source pollution accidents) according to the corresponding design parameters, it is necessary to control the ozone dosing accurately, According to the regular change of water quality, the corresponding adjustment can meet the corresponding requirements. However, due to the complex factors in each link and the relatively frequent changes in water quality, it is relatively important to adjust the ozone dosage in front of the biological activated carbon filter. Therefore, it is very important to control the ozone dosage in front of the biological activated carbon filter and the residual ozone concentration in the biological activated carbon filter.
2.Problems in ozone dosing in water?
2.1Problems in ozone flow measurement?
The core of accurate dosing is accurate measurement, rapid response and accurate dosing. From the perspective of PLC control principle, the accurate dosage of ozone should be determined by the real-time data of influent water quality and the residual ozone concentration in the biological activated carbon filter. The former determines the theoretical dosage of ozone, while the latter feedbacks the actual effect of the dosage through practice and optimizes the dosage. More importantly, in order to ensure the effectiveness of accurate dosing, the measurement of actual dosing, that is, the source control of ozone dosing, is the core of accurate dosing, which directly determines whether the follow-up work is meaningful. However, there are some technical problems in the online measurement and online detection of the above factors.
(1) Inaccurate source control of ozone dosing
The measurement of the ozone flow and concentration in the outlet pipe of the ozone generator is the key to accurately control the ozone dosage. At present, the metal rotor flowmeter or orifice plate differential pressure flowmeter commonly used in the industry are not accurate in measuring the mixture of ozone and pure oxygen; Although the thermal mass flowmeter has good measurement accuracy, its price is high. Due to the strong corrosion of the mixture of ozone and pure oxygen, the equipment also has the problem of high failure rate in practical application. At present, the domestic measurement and detection institutions can not measure or calibrate the high-range ozone concentration detection instrument at the outlet of the ozone generator. The accuracy of ozone concentration detection can only depend on the accuracy of the ozone concentration detection instrument itself. On the market, there are many brands of high-range ozone concentration detectors with different quality, and a few products are of good quality. However, with the extension of working time, the instrument itself will produce large errors, which can not guarantee to truly reflect the ozone concentration at the outlet of the ozone generator.
In the practical application of waterworks, the measuring instruments for ozone dosage are often provided by the ozone generator manufacturer to realize the closed-loop control of the equipment working state of the manufacturer. They are only responsible for the operation of the ozone dosage and cannot be used as reliable values for subsequent accurate control. Therefore, under the premise that the source measurement of ozone dosage is out of control, It is very difficult to realize the precise dosing of ozone. The dosage of ozone is mainly calculated according to the water quantity and the operation parameters provided by the design institute.
(2) Interference of CO3's detection in water
At present, there are two kinds of methods for residual ozone concentration detection, one is online instrument detection, the other is laboratory detection. Polarographic electrode type instruments are generally used for on-line instrument detection. This kind of instrument is not sensitive enough to low concentration of residual ozone, and passivation will occur when it is used in low concentration (below 0.2 mg / L) working environment for a long time. After passivation, this kind of instrument does not respond to the change of ozone concentration, so the reliability of this type of instrument has a big problem. DPD colorimetry is generally used in laboratory test, and the results are relatively reliable. However, for the waterworks using the pre chlorination process, the DPD colorimetric method will produce large deviation, because the residual chlorine in the water will cause positive interference to the results of DPD colorimetric method, making the measured value higher than the actual value. In this case, measures must be taken to remove the interference of residual chlorine on the measurement of residual ozone. At present, the laboratory detection frequency of residual ozone in most waterworks is too low, even it is difficult to achieve the detection frequency once a week. The laboratory detection means are not valuable for accurate dosing control
2.2 The restriction of assessment index and the lack of attention of water company?
(1) Lack of evaluation index guidance
At present, the core of the assessment of water plant operation focuses on the quality and quantity of finished water, energy consumption and operation cost of the water plant. The attention to the precise dosing of ozone, especially the control of residual ozone, is relatively low and lacks the constraints of assessment indicators. Moreover, the problem of residual ozone is a relatively advanced subject of fine control, The impact on the actual operation of the water plant can be ignored for the time being, so there is a lack of internal motivation in promoting the corresponding work of refinement.
(2) Lack of water company's attention
Due to the continuous optimization and improvement of water purification technology and the guarantee of raw water quality, the conventional process of many water plants can basically meet the limit requirements of the national standard, and the indicators such as TOC and CODMn closely related to ozone dosing can basically meet the requirements, so the water plants have not paid attention to the accurate dosing of ozone. Moreover, as an operation unit, the water plant does not have enough scientific research power to carry out corresponding research and development. In the absence of effective solutions, most water plants mainly carry out operation management based on the operation parameters provided by the Design Institute, so it is difficult to put the optimization of ozone dosing on the agenda.
3.what suggestions on exactly control of ozone dosing are there for water company?
Although the control of ozone concentration and the precise control of ozone dosing are not the main focus of water plant, from a long-term perspective and the general direction of refined management, the precise control of ozone dosing is bound to be a difficult problem that water plants need to overcome in the pursuit of higher water quality and more efficient and economic operation in the future
(1) Joint ozone online detection equipment suppliers, research institutions and design institutes to carry out some forward-looking experimental research in conditional water plants to gradually solve the bottleneck problems of accurate measurement and online concentration detection in the accurate control of ozone dosing, so as to sort out the technical blocks in the preceding links for accurate CO3 control in water;
(2) Gradually form a set of effective solutions to deal with the precise control of ozone, and guide the water plant to implement the precise control of ozone dosing with mature solutions, strengthen the research on the impact between the rest ozone concentration and the operation of biological activated carbon filter