Hydraulic retention time (HRT), interpreted in written language, refers to the average residence time of wastewater within a treatment structure. Simply put, it refers to the time it takes for wastewater to enter and exit a treatment facility. The usual calculation method is to divide the effective volume of the treated structure by the amount of sewage entering the structure per unit time.
For example, suppose a pool with an effective volume of 1000 cubic meters (m ³) is designed to handle 5000 cubic meters per day (m ³/d) of water. The HRT is calculated as follows: a residence time of 4.8 hours is the hydraulic retention time. So when building structures, we must fully consider the daily processing capacity, pollutant concentration, and the requirements to be achieved after treatment. The processing capacity determines the size of the pool body. The concentration of pollutants affects the time and conditions required for the reaction. High concentrations may require longer reaction times or more complex treatment processes, which require larger tank volumes or more effective stirring, aeration, and other facilities to ensure sufficient reaction. The various water quality indicators of the effluent determine the selected treatment process and the design parameters of the tank, such as hydraulic retention time, sludge load, etc. Of course, in actual production and operation, the hydraulic retention time is also influenced by many other factors. For example, the quality and quantity of incoming water, as well as seasonal temperature changes.
The effect of temperature on HRT: Temperature can affect the activity of microorganisms. At low temperatures, the metabolic rate of microorganisms decreases, possibly to below 50% of their summer activity, resulting in a necessary increase of HRT by about 1.5 times to maintain the same treatment efficiency. The impact of influent water quality on hydraulic retention time (HRT) requires longer HRT for high concentration pollutants to ensure sufficient biodegradation time. The unbalanced nutrient ratio (C: N: P) has an impact on HRT, which can inhibit the growth of microorganisms and affect the efficiency of wastewater treatment. For example, a low C: N ratio may limit the nitrification process and require compensation by increasing HRT to ensure effective nitrogen removal. The effect of pH on HRT: A suitable pH range can enhance microbial metabolic activity and reduce the required HRT. On the contrary, an inappropriate pH value will reduce microbial activity and increase HRT requirements. For example, adjusting the pH value from 7.5 to 6.5 may increase the HRT of the nitrification process by about 20%, as nitrifying bacteria are more sensitive to pH changes. The impact of toxic substances on HRT increases the demand for HRT due to the presence of heavy metals or toxic organic compounds in the influent, which can inhibit microbial activity.
So what impact will hydraulic retention time have on our process.
Firstly, let's list several possible impacts of HRT being too short: a. Poor treatment effect: Insufficient time for sewage to react with microorganisms, resulting in insufficient removal of pollutants such as organic matter, nitrogen, and phosphorus, and difficulty in meeting effluent quality standards.
b, Microbial growth restriction: Microorganisms do not have enough time to absorb nutrients and engage in metabolic activities, which may affect their growth and reproduction, thereby reducing the biological activity and stability of the treatment system. c. Difficulty in sludge domestication: It is not conducive to the domestication and adaptation of microbial communities, and it is difficult to form dominant microbial communities that can adapt to specific water quality. d. Weak impact resistance: The processing system's ability to cope with fluctuations in incoming water quality and quantity decreases, making it prone to unstable operation. e. Increasing the burden of subsequent processing: Due to insufficient pre-treatment, more pollutants enter the subsequent processing unit, increasing the difficulty and cost of subsequent processing. f. Not conducive to the formation and stability of biofilm: For the treatment process using biofilm method, too short hydraulic retention time may not allow the biofilm to fully grow and mature, affecting its treatment effect. g. Impact on chemical reactions: If there is a step of adding chemical agents for treatment, a too short hydraulic retention time may result in uneven mixing of the agents with wastewater, insufficient reaction, and reduced effectiveness of the agents. And the possible effects of too long HRT:
a, Excessive growth of microorganisms in the system, prolonged HRT may lead to sludge aging and expansion, reduce sludge settling performance, and increase effluent suspended solids (SS) concentration. The relationship between sludge age (SRT) and hydraulic retention time (HRT) indicates that SRT should be at least 2-3 times that of HRT to maintain the activity of sludge. Of course, a brief extension of hydraulic retention time usually does not cause significant negative impacts, as the treatment system has a certain buffering and adaptive capacity. b. The residence time of sewage in the aeration tank increases, and if the aeration intensity is not adjusted accordingly, it is easy to cause excessive aeration. Consuming more electricity and increasing operating costs. Loosening the floc structure of sludge is not conducive to subsequent sedimentation treatment. c. Increasing infrastructure and operating costs may require the construction of larger capacity treatment tanks to achieve excessively long hydraulic retention times, thereby increasing infrastructure investment. It also means higher energy consumption and maintenance costs.
c. Nutrient imbalance may lead to excessive consumption of nutrients in wastewater, resulting in an imbalance in the proportion of nutrients required for microbial growth, which is not conducive to the normal metabolism and reproduction of microorganisms. Discussion on the influence of hydraulic retention time on process
Hydraulic retention time (HRT), interpreted in written language, refers to the average residence time of wastewater within a treatment structure. Simply put, it refers to the time it takes for wastewater to enter and exit a treatment facility. The usual calculation method is to divide the effective volume of the treated structure by the amount of sewage entering the structure per unit time. For example, suppose a pool with an effective volume of 1000 cubic meters (m ³) is designed to handle 5000 cubic meters per day (m ³/d) of water. The HRT is calculated as follows: a residence time of 4.8 hours is the hydraulic retention time. So when building structures, we must fully consider the daily processing capacity, pollutant concentration, and the requirements to be achieved after treatment. The processing capacity determines the size of the pool body. The concentration of pollutants affects the time and conditions required for the reaction. High concentrations may require longer reaction times or more complex treatment processes, which require larger tank volumes or more effective stirring, aeration, and other facilities to ensure sufficient reaction. The various water quality indicators of the effluent determine the selected treatment process and the design parameters of the tank, such as hydraulic retention time, sludge load, etc. Of course, in actual production and operation, the hydraulic retention time is also influenced by many other factors. For example, the quality and quantity of incoming water, as well as seasonal temperature changes. The effect of temperature on HRT: Temperature can affect the activity of microorganisms. At low temperatures, the metabolic rate of microorganisms decreases, possibly to below 50% of their summer activity, resulting in a necessary increase of HRT by about 1.5 times to maintain the same treatment efficiency. The impact of influent water quality on hydraulic retention time (HRT) requires longer HRT for high concentration pollutants to ensure sufficient biodegradation time. The unbalanced nutrient ratio (C: N: P) has an impact on HRT, which can inhibit the growth of microorganisms and affect the efficiency of wastewater treatment. For example, a low C: N ratio may limit the nitrification process and require compensation by increasing HRT to ensure effective nitrogen removal. The effect of pH on HRT: A suitable pH range can enhance microbial metabolic activity and reduce the required HRT. On the contrary, an inappropriate pH value will reduce microbial activity and increase HRT requirements. For example, adjusting the pH value from 7.5 to 6.5 may increase the HRT of the nitrification process by about 20%, as nitrifying bacteria are more sensitive to pH changes. The impact of toxic substances on HRT increases the demand for HRT due to the presence of heavy metals or toxic organic compounds in the influent, which can inhibit microbial activity. So what impact will hydraulic retention time have on our process.
Firstly, let's list several possible impacts of HRT being too short: a. Poor treatment effect: Insufficient time for sewage to react with microorganisms, resulting in insufficient removal of pollutants such as organic matter, nitrogen, and phosphorus, and difficulty in meeting effluent quality standards.
b, Microbial growth restriction: Microorganisms do not have enough time to absorb nutrients and engage in metabolic activities, which may affect their growth and reproduction, thereby reducing the biological activity and stability of the treatment system. c. Difficulty in sludge domestication: It is not conducive to the domestication and adaptation of microbial communities, and it is difficult to form dominant microbial communities that can adapt to specific water quality. d. Weak impact resistance: The processing system's ability to cope with fluctuations in incoming water quality and quantity decreases, making it prone to unstable operation. e. Increasing the burden of subsequent processing: Due to insufficient pre-treatment, more pollutants enter the subsequent processing unit, increasing the difficulty and cost of subsequent processing. f. Not conducive to the formation and stability of biofilm: For the treatment process using biofilm method, too short hydraulic retention time may not allow the biofilm to fully grow and mature, affecting its treatment effect. g. Impact on chemical reactions: If there is a step of adding chemical agents for treatment, a too short hydraulic retention time may result in uneven mixing of the agents with wastewater, insufficient reaction, and reduced effectiveness of the agents. And the possible effects of too long HRT:
a, Excessive growth of microorganisms in the system, prolonged HRT may lead to sludge aging and expansion, reduce sludge settling performance, and increase effluent suspended solids (SS) concentration. The relationship between sludge age (SRT) and hydraulic retention time (HRT) indicates that SRT should be at least 2-3 times that of HRT to maintain the activity of sludge. Of course, a brief extension of hydraulic retention time usually does not cause significant negative impacts, as the treatment system has a certain buffering and adaptive capacity. b. The residence time of sewage in the aeration tank increases, and if the aeration intensity is not adjusted accordingly, it is easy to cause excessive aeration. Consuming more electricity and increasing operating costs. Loosening the floc structure of sludge is not conducive to subsequent sedimentation treatment.
c. Increasing infrastructure and operating costs may require the construction of larger capacity treatment tanks to achieve excessively long hydraulic retention times, thereby increasing infrastructure investment. It also means higher energy consumption and maintenance costs.
c. Nutrient imbalance may lead to excessive consumption of nutrients in wastewater, resulting in an imbalance in the proportion of nutrients required for microbial growth, which is not conducive to the normal metabolism and reproduction of microorganisms.
d. Anaerobic environment is disrupted: If the residence time is too long, the area that should have been in an anaerobic environment may mix with oxygen, affecting the phosphorus release process of polyphosphate accumulating bacteria and reducing the phosphorus removal efficiency. It may also lead to overgrowth of other microorganisms, competing with polyphosphate accumulating bacteria for limited nutrients and living space, affecting the growth and metabolism of polyphosphate accumulating bacteria, and thereby interfering with phosphorus removal efficiency. e. Anaerobic fermentation of sludge: Prolonged anaerobic fermentation of settled sludge may result in the production of methane and other gases, causing the sludge to float and affecting the sedimentation effect. f. Chemical agent failure: If chemical agents are used during the precipitation process, prolonged hydraulic retention time may cause the agents to decompose or react with other substances, resulting in failure and affecting the precipitation effect.
Usually, we use methods to extend hydraulic retention time. Reducing the amount of treated water: This is a more direct approach, but it may affect the efficiency and scale of the treatment. Increasing the volume of the reaction tank: By expanding or renovating the reaction tank to increase its effective volume, the hydraulic retention time can be extended. By changing the flow pattern inside the reactor, such as using a U-shaped or multi chamber design, the flow path of sewage inside the reactor can be increased, thereby extending HRT. Adjusting the inflow and outflow flow rates: Reducing the inflow or outflow flow rate can also achieve an extension of hydraulic retention time. When the hydraulic retention time is extended by controlling the outflow rate to decrease, if the inflow rate remains constant, it will cause the liquid level in the pool to rise. This requires a certain level of tank capacity. If the tank capacity is too small, an increase in liquid level may cause a series of problems, such as increasing the risk of overflow and affecting the normal operation of the treatment system. There are also methods to increase hydraulic retention time in a short period of time by adjusting the reflux ratio, but this often leads to other issues.
Extending the hydraulic retention time within a reasonable range is usually beneficial for the removal of pollutants. This can allow sewage to have more sufficient contact and reaction time with microorganisms, chemical agents, etc., which helps to improve the degradation of organic matter and the removal efficiency of nitrogen and phosphorus. For example, for some difficult to degrade organic compounds, a longer hydraulic retention time can increase their chances of decomposition; In the process of biological nitrogen and phosphorus removal, it can also provide more favorable conditions for the growth and metabolism of microorganisms, thereby improving the removal efficiency of nitrogen and phosphorus. The hydraulic retention time of the tank body is in the common range: anaerobic tank: 1-2 hours coagulation sedimentation tank 1.5-3 hours anoxic tank: 2-4 hours regulating tank: 4-24 hours
Disinfection pool: 0.5-2 hours.
Fiber filter: 0.5-1 hour aerobic tank: 4-8 hours acidification tank: 4-6 hours sedimentation tank: between 30 seconds and 2 minutes. The above data is only for rough reference. The actual HRT should be adjusted according to specific sewage characteristics, treatment objectives, and environmental conditions, and reference should be made to relevant national and local standards. When designing and operating sewage treatment plants, optimization should be based on actual conditions. It should be noted that these times are theoretical guidance values and may need to be adjusted according to actual situations in practical operations.
