Abstract: The MBR membrane system combines the advantages of traditional activated sludge process and membrane process, resulting in a much higher quality of filtered water than that of single technology filtered water, and its performance has been recognized. In addition, with the current improvement of sewage discharge standards, the rise of membrane processes, and the decrease in membrane cost, MBR process membrane systems have been widely used, and theoretical research on MBR process membrane systems has gradually improved. However, in urban sewage treatment systems, this process is still in its infancy. The MBR process membrane system mainly relies on the solid-liquid separation technology of the membrane, with biological treatment dominating and solid-liquid separation being the key. This article elaborates on the key technologies of MBR process membrane system design for urban sewage treatment projects, hoping to provide reference for future urban sewage treatment projects. Keywords: sewage treatment; MBR membrane process 1 MBR process membrane module design points 1.1 membrane fiber (sheet) material selection points The selection of membrane fiber (sheet) material is the foundation and key of the entire process system, and its material selection not only affects the life of the membrane, but also affects the performance of the membrane. The material of membrane fibers is divided into two parts: organic membranes and inorganic membranes, each with its own advantages and disadvantages. Each factory will choose the most suitable type of membrane according to their different needs. Organic films have the following advantages: low cost, affordable price, diverse types, mature technology, and so on. However, organic films also have certain drawbacks, such as shorter usable time and susceptibility to contamination. Inorganic membranes have the advantages of not being affected by organic solvents, corrosion resistance, erosion resistance, high temperature resistance, and strong acid and alkali resistance. Therefore, inorganic membranes are often used for the treatment of factory wastewater with severe pollution and erosion. The most commonly used membrane currently is PVDF in organic membranes, which has been improved to have stable hydrophilicity and is favored by most factories.
1.2 Key points for selecting membrane fiber (sheet) performance: When selecting membrane fiber, two aspects should be considered: chemical strength and mechanical properties. Chemical strength and mechanical properties are two important determining factors for the service life of membranes. The chemical strength of membrane fibers refers to their ability to withstand the erosion of chemical substances to the greatest extent possible, that is, they should be able to clean as many types of chemical substances as possible, with high antioxidant and erosion resistance. The mechanical properties of membrane fibers refer to their ability to withstand long-term external impacts, such as the impact of high flow water and air friction cleaning. The degree of fracture and elongation at break of membrane fibers are important indicators reflecting mechanical properties. The numerical value reflects the degree to which the membrane is subjected to external pressure, and indirectly indicates the damage rate of the membrane during use. It is an important measurement indicator for measuring the quality of a membrane. 1.3 Key points for selecting membrane fiber (sheet) pore size In MBR process membrane systems, there are two commonly used membrane pore sizes. One type has a pore size between 0.01 and 0.1 μ m, and this type of membrane is called ultrafiltration membrane, also known as UF membrane; Another type of membrane with a pore size between 0.1 and 0.4 μ m, also known as microfiltration membrane or MF. In practical applications, both types of membranes meet the process requirements and there is no significant difference between the two. The membrane pore size mentioned in it is not the accurate data measured, but refers to the filtration accuracy, which is a statistical concept of normal distribution. At present, there is no unified national standard for membrane pore size, and the quality of the membrane depends on the quality of the filtered water. The filtration function of a membrane mainly depends on three aspects: first, the filtration and screening function of the membrane pores themselves; second, the adsorption function between the membrane pores and the membrane surface; and third, the filtration and adsorption function of the sediment layer formed on the membrane surface. 2 Key points for parameter design of MBR process membrane system 2.1 Membrane flux Membrane flux is an important factor affecting the stability of MBR process membrane system, and selecting appropriate membrane flux can effectively reduce the rate of permeability degradation. But the choice of membrane flux is not necessarily better, it needs to consider practical application needs and the maximum load that the membrane can pass through the water flow rate. Membrane flux is the main part of the entire process system, which refers to the maximum water flow rate that can flow through the membrane, provided that it is per unit area per unit time. There are four modes of membrane flux: one is the average membrane flux, which refers to the ideal flow rate on the membrane during design; The second is the average instantaneous membrane flux, which refers to the flux at which the membrane actually participates in the work; The third is the maximum instantaneous membrane flux, which is influenced by factors such as the peak amount of circulating water and the offline cleaning time of the membrane. When measuring the maximum instantaneous membrane flux, the most unfavorable of the two factors should be selected as the measurement of the maximum instantaneous membrane flux; The fourth is the critical instantaneous membrane flux, which is related to the chemical cleaning cycle. When the membrane flux increases but the chemical cleaning cycle decreases significantly, the measured membrane flux is the critical instantaneous membrane flux. The critical instantaneous membrane flux is an important testing criterion in MBR process membrane systems. In membrane systems, we require that all membrane fluxes must not exceed the critical instantaneous membrane flux. When designing membrane flux, it is important to choose the appropriate membrane flux based on the actual situation. The choice of membrane flux is not necessarily better, usually within a certain range. Increasing membrane flux can reduce the number of membranes used and lower costs. But when the membrane flux reaches a certain value, the increase in membrane flux may lead to rapid aging of the membrane system, reduce the service life of the membrane, and increase the frequency and cost of membrane replacement.
2.2 The design water temperature is a major factor affecting membrane flux. Within a certain range, as the water temperature increases, the activity of chemical substances in the water increases, viscosity decreases, and membrane flux increases; If the water temperature is too low, the activity of chemical substances in the water decreases, viscosity increases, and membrane flux decreases. Because the MBR process membrane system is a constant flow system, the changes in membrane flux caused by water temperature can be adjusted by changing the transmembrane pressure difference. 3 MBR Process Membrane System Layout Design Points 3.1 Grouping of Membrane Systems In order to facilitate operation and processing, membrane systems usually adopt grouping processing methods. The grouping of membrane systems has certain requirements, and it is not allowed to randomly group or determine the size of a single group. The grouping must be determined after technical and economic comparison optimization. In terms of technology, a single membrane system should not be connected to too many membrane components to avoid problems such as water ingress, air scrubbing, and uneven suction of water. In terms of economy, if the scale of a single membrane system is too large, it will lead to an increase in the number of suction pumps, pipelines, and corresponding components, as well as a decrease in the number of equipment, detectors, etc., and a reduction in the length of each pipeline. Therefore, when grouping membrane systems, it is necessary to comprehensively compare the technical and economic advantages. 3.2 Backup of membrane system To reduce unexpected accidents, some backup systems can be set up for the membrane system, but the establishment of backup equipment requires an increase in the calculation of the membrane system. Therefore, the operator can decide whether to set up a backup membrane system on their own. If there is no funding to establish a backup system, some emergency measures can be taken. In general, the backup of membrane systems includes the following aspects: firstly, the main equipment involved in the membrane system can be used in several ways and one as backup; The second is to increase the membrane flow rate and related pipeline diameter of a single membrane system, and the number of groups should be greater than or equal to four when grouping; Thirdly, when designing the membrane system, a certain amount of space should be reserved for backup, usually 10% to 15% of the space should be reserved for backup. 4. Key points for the operation design of MBR process membrane system 4.1 Membrane working cycle design MBR process membrane system mainly adopts the operation mode of intermittent water discharge. After extracting water, the system pauses for a period of time before continuing to extract water. The extraction time is about 8-12 minutes, and the pause time is about 30-120s. Different types of systems have different pause modes, some are just ordinary pauses, while others perform backwashing during the pause time. Each extraction and pause counts as one cycle, and shortening the cycle time will reduce the rate of sludge congestion. But if the cycle time is too short, it will increase the difficulty for personnel to control the membrane system and reduce the service life of the membrane components. 4.2 Membrane operating pressure design: Whether the membrane is contaminated and the degree of contamination is affected by the operating pressure. The operating pressure of a membrane has a critical pressure value, and when the critical pressure value is greater than the operating pressure value, an increase in pressure value will cause an increase in membrane flux; When the critical pressure value is less than the operating pressure value, it will aggravate the degree of membrane fouling, and the change in pressure value at this time has almost no effect on membrane flux. In practical applications, the operating pressure value of the membrane should be minimized as much as possible. A low operating pressure value of the membrane is beneficial for improving the energy utilization efficiency of the membrane, increasing the membrane flux over a longer period of time, and extending the membrane's service life to a certain extent.
There are two types of membrane operation modes: constant pressure control and constant current control. In MBR process membrane systems, constant current control is chosen for several reasons. Firstly, it can reduce membrane fouling and effectively control membrane fouling; Secondly, it can extend the cleaning cycle of the membrane and improve its operational efficiency; Thirdly, it can increase membrane flux over a longer period of time; Fourthly, it can ensure the stable and orderly operation of the entire MBR process membrane system. Constant pressure control is mainly used in systems that measure the variation of membrane flux with running time. Conclusion: Due to the numerous advantages of MBR membrane system, it has been favored by many factories, but in urban sewage treatment projects, this technology has not yet been practically applied and promoted. Therefore, based on our own experience in constructing sewage treatment plants, this article elaborates on the key technologies for designing MBR process membrane systems in urban sewage treatment projects, including MBR process membrane component design, parameter design, layout design, and operational design points, in order to provide reference for future urban sewage treatment projects.
