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As the most fundamental component among all the facilities in a wastewater treatment plant, the proper operation of the water pumps has a direct impact on the economic efficiency of the plant. Given the variety of wastewater types, such as industrial wastewater and domestic sewage, and the different compositions within industrial wastewater, the requirements for pumps in wastewater treatment processes also vary. However, the overall goals are to avoid blockages, minimize wear and tear, ensure a relatively long service life, provide corrosion resistance, be energy-efficient and environmentally friendly, as well as achieve high efficiency. At present, the pumps produced by domestic pump manufacturers still fall short of those made abroad in terms of efficiency and lifespan. However, foreign pumps are expensive, have long delivery times, high maintenance costs, and their spare parts are costly and difficult to obtain, which limits the operational stability of sewage treatment plants to a certain extent. In contrast, domestic pumps are already quite mature in terms of technology. Today, I’ll discuss with everyone the selection of pumps, their operation and maintenance, as well as how to address problems that arise when pump failures occur. The outline is as follows; you can read the sections that are relevant to you: Types of pumps commonly used in sewage treatment plants; Selection of sewage pumps (calculation of performance parameters); Operation and management of sewage pumps (submersible pumps, screw pumps); Common faults of sewage pumps and their solutions. 1. Types of pumps commonly used in sewage treatment plants: Submersible sewage pumps (WQ series). The WQ series of submersible sewage pumps belong to the centrifugal pump category. Their working principle involves a combination of the pump and a submersible motor that operates underwater – the motor is located at the upper part while the pump body is at the lower part. The rotation of the motor directly drives the blades to function. These pumps have a simple structure, and they are connected to the outlet pipe in a self-coupling manner. Remote control is possible through distribution cabinets and control panels. The performance curve and no-overload power curve of the WQ spiral centrifugal pump are shown in the figure. In municipal sewage treatment plants, it is mainly used for lifting and transporting sewage in various process stages. It is commonly used in intake pump rooms, discharge pump rooms, reclaimed water systems, backflow systems, and well and pit drainage. Some of the projects are used for the transport and return of sludge. It features simple operation, easy monitoring, automatic frequency conversion to save labor, and is eco-friendly and energy-efficient. For metering pumps, mechanical diaphragm pumps are commonly used to add chemicals for wastewater treatment, such as polyaluminum chloride, sodium acetate, and sodium hypochlorite. These pumps offer precise metering, with the stroke and speed being in a strict direct proportion to the flow rate, which facilitates the production department’s accurate control of the amount of chemical used. The single-screw pump transports the sludge flowing in from the sludge tank to the centrifuge. Screw pumps have low losses, excellent economic performance, uniform flow, high speed, and high and stable pressure; they can be connected directly to the prime mover. The technology of single-screw pumps is currently relatively mature. Foreign-brand single-screw pumps use better materials and processing techniques for their critical components (those subject to wear), which is why they have a longer service life than domestic brands. Meanwhile, domestic brands offer better cost-performance ratios compared to foreign brands. Other types of wastewater treatment plants also require peristaltic pumps (for sampling), jet pumps, vacuum pumps, multi-stage centrifugal pumps, deep well pumps, and pipeline pumps, etc.; the specific model to be chosen depends on the process requirements of the wastewater treatment plant. 2 Selection of sewage pumps In sewage treatment plants, pumps are mainly used for lifting inflowing water, raising water for tertiary treatment, sludge recirculation, and the discharge of excess sludge. Depending on the characteristics of the medium, specialized sewage pumps are generally used, such as submersible sewage pumps and sludge screw pumps. The following principles should be followed in the specific selection: The pump must meet the technical requirements regarding head and flow rate, and its operating point should fall within the high-efficiency range. During long-term operation, water pumps maintain high efficiency at the average head over many years, resulting in lower operating and maintenance costs. When checked at the maximum head, the pump can operate properly and efficiently. Based on the above principles, the selection of water pumps can be approached from the following aspects: 1) Determination of the pump’s stable operating condition point. When a pump is installed in a specific piping system, its actual operating head and flow rate depend not only on the pump’s own performance but also on the characteristics of the piping. The intersection point of the pipeline’s He–Qe curve and the pump’s H–Q curve represents the operating condition of the pump; that is, the flow rate Qm and head Hm corresponding to point m are the optimal flow rate and head for the pump. The intersection of the pipeline’s NPSHa-Q curve and the pump’s NPSHr-Q curve represents the critical cavitation point of the delivery system; the pump must operate at a flow rate lower than Qd, otherwise cavitation will occur. The pump’s characteristic curve is obtained through actual testing of the pump, and is generally provided by the pump manufacturer. The characteristic curve of the piping system should also be determined through actual measurements, but since these measurements cannot be taken before design, empirical formulas are often used for estimation. 2) Regarding the selection of flow rate Q, in cases where the flow rate does not change much, the pump flow rate is determined directly based on the required flow rate. For inlet lift pumps, the flow rate is variable and exhibits temporal variability, meaning there are significant differences between high and low levels. In such cases, it is not appropriate to use the peak flow rate as the basis for selection, as this would result in most of the operation taking place outside the efficient range, thereby increasing operating costs and energy losses. Generally, Q can be taken as Qmax / 1.2. 3) Once the flow rate is determined, the inlet and outlet pipe diameters of the pump are fixed to complete the layout of the entire pipeline, thereby determining the head. In practical selections, the resistance coefficient of the pipeline is often set too high; moreover, a margin of head is added when choosing the pump model. This results in a higher flow rate during actual operation. As the flow rate increases, the shaft power required by the pump rises, while the efficiency of the motor remains unchanged; therefore, the power input to the motor necessarily increases. Excessive power causes the motor to operate under overload, which may lead to overheating or even damage to the motor. If the flow rate exceeds Qd, cavitation will occur, leading to vibrations that severely affect the pump’s service life. Therefore, when selecting the head of a pump, it should not be set too conservatively; it should be as close as possible to the actual head of the pipeline. Generally speaking, it should not exceed 1.1 times the actual head of the pipeline. Finally, based on the determined Q and H, select the appropriate pump model. It should be noted that in practical selection, in principle, the pump model closest to the intersection point of the pump’s performance curve and the system’s performance curve should be chosen, with the optimal operating point of the pump located as far as possible within the high-efficiency region of its efficiency curve. 3 Operation and management of sewage pumps; Operation and management of submersible pumps 1) Adjust the liquid level to keep the pump operating at its optimal condition. After the equipment is selected, the intersection point of the pump’s performance curve and that of the system constitutes the optimal operating condition. However, as the operating time of the water pump increases, there is an inevitable decline in its efficiency along with wear and tear on the components that handle the flow of water. The parameter that has the most significant impact on the pump’s performance curve is the wear of the pump’s impeller; once the impeller wears out, the H-Q curve of the pump shifts downward, resulting in a decrease in both flow rate and head pressure. In order to maintain the original water treatment capacity without increasing the number of pumps, it is possible to adjust the float in the liquid reservoir to raise its level. When the level of the reservoir increases, the He-Qe curve of the system shifts to the right. At this point, the head pressure at the intersection of the two curves decreases, while the flow rate remains unchanged. This ensures that the water pump operates at its optimal operating point, meeting the process requirements while reducing equipment wear and helping to extend the economic service life of the equipment. 2) For the daily inspection and maintenance of submersible pumps, the duty personnel should listen, observe, feel, and record. First, check whether the operating noise of the equipment is within normal limits; second, verify whether the alarm unit emits any signals indicating leakage or overheating; third, feel to see if the vibration of the guide rods is normal; fourth, record the flow rate and current during the operation of the water pump. If any issues are found during the above checks, it is necessary to analyze the reasons promptly and report them. Maintenance personnel regularly check the tightness of the submersion cable, whether the steel wire rope or guide chain is worn out, whether it is properly secured, and whether there is any debris present ； Regularly check whether the phase-to-phase resistance of the water pump is balanced, and whether the insulation resistance to ground is below 1MΩ ； Regularly check whether the electrical protection devices are reliable, and periodically tighten the wiring terminals. Check the oil quality in the oil chamber every six months to detect any leaks, and replace the oil accordingly ； Check the wear condition of the impeller to determine whether repair or replacement is necessary. A comprehensive preventive maintenance check is carried out once a year, which includes: replacing all O-ring seals ； Check the bearing wear and apply new lubricant ； Check the wear condition of the mechanical seal to determine if replacement is necessary ； Check the wear condition of the impeller, as well as the wear condition of the sealing ring between the impeller and the pump body, to determine whether repair or replacement is necessary ； Check the insulation of the motor stator for any signs of scuffing damage ； Check whether there is sand accumulation or blockages at the bottom of the pump, and check whether there is sediment at the bottom of the tank; determine if it is necessary to shut off the water supply for sediment removal. Operation and management of screw pumps: Naimo pumps (single-screw pumps) are positive-displacement pumps that are widely used in wastewater treatment plants for transporting sludge containing solid particles as well as chemical agents. Its working principle is as follows: when the screw pump is in operation, the liquid is drawn in and enters the sealed cavity formed by the rotor and the stator. Due to the rotation of the rotor, the liquid moves along the axial direction from the suction side of the pump to the discharge side within this sealed cavity; continuous suction and discharge enable the transport of the liquid. 1) Variable-speed operation of screw pumps in sewage treatment plants: Screw pumps are generally used in conjunction with sludge dewatering machines. Whether transporting sludge that needs to be treated or flocculants, strict control is required over the flow rate of the medium being transported. The flow rate of a screw pump is independent of the delivery pressure; the delivery pressure depends only on the number of stages in the screw pump, while there is a strict direct proportionality between the flow rate and the rotational speed. Therefore, to control the flow rate, it is sufficient to adjust the motor speed using a frequency converter or by employing stepless speed control. 2) Protection during operation: Temperature sensors are generally installed on the stator to monitor its temperature. If the medium is lacking and the machine operates in a dry condition, significant wear will occur on both the stator and the rotor. Moreover, the temperature rises due to friction, which further increases wear. Therefore, all screw pump manufacturers now consider dry-run protection to be an essential feature in equipment purchases. 3) Requirements of the process pretreatment unit for the normal operation of screw pumps. The stator of a screw pump is usually made of rubber, while the rotor is made of stainless steel. If the medium being transported contains sand particles, this will cause wear on both the stator and the rotor. Due to requirements related to manufacturing processes and materials, the quality of substitute products produced in China is not yet satisfactory, and imported spare parts are expensive ； Moreover, in the subsequent sludge treatment units, the flow-through components of the centrifugal dehydrator are even more sensitive to sand particles. Therefore, the amount of sand in the conveying medium should be reduced as much as possible. In the pre-treatment unit where the screw pump operates, the sand suction pump and sand-water separator installed in the aeration grit chamber are used to remove sand particles from wastewater. It is essential to ensure their proper operation in order to reduce the amount of sand in the sludge and extend the service life of the screw pump. 4 Common Faults of Sewage Pumps and Their Solutions: Due to the variations in pump models, degree of use, and methods of operation, a wide range of faults and problems can occur. The following summarizes the most common pump faults, their causes, and specific solutions. 1) The water pump cannot be started. This may be due to issues with the power supply or problems with the pump itself. The specific handling method is as follows: First, check the power supply status to see if the connectors of the power supply are secure, whether the switch is in good condition, and if the fuses are intact. If problems such as short circuits or blown fuses occur, they should be repaired promptly. If it’s not a power supply issue, check whether there is a fault with the water pump itself. The following checks should be performed: whether the packing is too tight, resulting in debris between the impeller and the pump body. Whether the pump shaft is bent or rusted. If the above problems occur, the filler should be loosened, the water intake channel should be cleared, or the pump shaft should be removed for correction or a new pump shaft should be installed. 2) The water pump does not deliver water or has insufficient flow. This could be due to leaks in the suction pipe or air leakage in the bottom valve, but there are also other possible causes: blockage at the water inlet ； The depth of the bottom valve underwater is insufficient ； The water pump is not running fast enough ； The impeller is severely worn ； The water absorption does not reach or exceeds the specified level: it may not be a problem with the pump itself, but rather an insufficient amount of water available in that area. The handling method is as follows: 1. Check whether there are any abnormalities in the current and voltage of the water pump at this time. If the current decreases significantly, for example to only half or slightly more of its normal level, it could be due to worn impellers or a clogged check valve. If there is no significant difference compared to the normal current level, it is possible that there is a leak in the pipe gasket, the pipes, or the pump body. If the current is higher than normal, the problem should be sought in the wear of the vulnerable components. 2. Inspect the suction pipe and bottom valve to check for any sources of air leakage. Remove blockages such as silt from the inlet. Check the water immersion depth of the foot valve to ensure it is at least 2 times the diameter of the inlet pipe; if not, increase the water immersion depth of the foot valve. 3. Check the voltage to see if it is too low, which prevents operation. Either the power is not matched to the rotation speed, or belt slippage occurs, resulting in an excessively low rotation speed. 3) Motor overheating. There are many reasons for motor overheating; it may be due to an excessively high supply voltage, or problems with the water pump or the motor itself, among other things. The handling methods are as follows: 1. Check the power supply voltage; if the voltage fluctuates significantly, it is due to unstable voltage, and use should be stopped immediately to prevent more serious consequences. Or, appropriate heat dissipation protection measures can be installed for the motor to provide protection. 2. Check the water pump: It is possible that the selected water pump is not compatible with the motor, causing the motor to reach excessive temperatures after operating for a long time ； It is also possible that the operating time exceeds the specified limit for the motor; in such cases, the usage time and frequency should be strictly limited, the appropriate thermal protection measures should be employed, and the motor should be used according to the specifications indicated in the manual. 3. Check the wiring of the motor itself: High temperatures may be caused by incorrect wiring; if that is the case, correct the wiring promptly. 4. Check whether there is a problem with the ventilation system: see if the fan is damaged, whether it is rotating properly, and if there are any objects blocking the ventilation openings. 5. Inspect the motor’s windings: If the windings are damp or covered with dust, sand, or other debris, this may reduce their insulation properties, and such debris should be removed immediately. 6. Check the surrounding environment: When the temperature around is above 35 degrees, it will raise the temperature of the incoming air, leading to overheating of the motor; therefore, efforts should be made to improve the working environment. For example, building shade structures to provide shelter from the sun, and so on. 4) Excessive power loss. The reasons for excessive power loss are as follows: the rotation speed of the screw rod is too high. The main shaft of the water pump is bent, or the shafts of the water pump and the motor are not aligned or parallel to each other. An inappropriate pump head was selected. There is a blockage in the water pump. The balls or bearings of the motor are damaged. The handling method is as follows: First, check the circuit voltage and reduce the speed of the water pump. Correct the main shaft of the water pump and adjust the relative position of the water pump and the motor. Try to choose a pump suitable for the required head. Remove sediment and other obstructions from within or around the water pump. Replace the ball bearings of the motor. 5) Severe vibration of the pump body: The reasons for severe vibration in a water pump are as follows: The electric rotor rotates unbalance. The bearing has worn out and bent. The rotating parts of the water pump are loose or damaged. The pipeline support is unstable and wobbles. The water pump is not installed firmly or is installed too high. The motor ball bearing is damaged. The pump shaft is bent, and the shaft is not parallel to the motor shaft. The handling method is as follows: During installation, make sure the water pump is stable; it should not be installed too high. Replace the motor shaft of the water pump. Correct the bent main shaft, and adjust the positions of the motor and the water pump so that they are parallel to each other.