Common operating failures and solutions of sludge thickening and dewatering systems

Centrifugal dewatering machines are commonly used for sludge dewatering in wastewater treatment plants. Their main function is to concentrate and dewater the remaining sludge from the wastewater treatment system, preparing it for transportation. This analysis examines failures encountered during normal operation of a sludge concentration and dewatering system at a municipal wastewater treatment plant and proposes feasible solutions.

The sludge concentration and dewatering system plays a crucial role in wastewater treatment plant operation. However, this system frequently encounters malfunctions that severely impact normal operation. Therefore, a detailed analysis of the failure modes of the sludge concentration and dewatering system is of significant practical importance.

Introduction to Sludge Concentration and Dewatering System

1. Brief Introduction to the Main Process Section of Wastewater Treatment Process

A municipal wastewater treatment plant in a certain city has a design treatment capacity of 8×104 mm³/d. The main wastewater treatment process employs a modified oxidation ditch. For advanced wastewater treatment, coagulation, sedimentation, filtration, and disinfection processes are primarily used. The main treatment structures of the wastewater treatment plant include mechanical mixing tanks, reaction tanks, horizontal sedimentation tanks, and V-shaped filter tanks. After treatment in the V-shaped tanks, the wastewater is directly discharged to disinfection equipment for disinfection. The disinfection standard ensures that the disinfected wastewater meets discharge standards. The advanced treatment stage of this wastewater treatment plant does not have a reaction sedimentation tank. The sludge settled at the bottom of the horizontal flow tank is collected by a scraper and suction sludge machine and directly discharged into the sump, then re-entering the wastewater treatment system for advanced treatment. Therefore, this city's wastewater treatment plant does not have a sludge return system, nor does it accumulate chemical sludge. The dewatering device only needs to treat some activated sludge from the secondary sedimentation tank sludge chamber.

2. Brief Introduction to Sludge Concentration and Dewatering System Process

Some activated sludge is produced in the secondary sedimentation tank sludge pump room of this wastewater treatment plant and transported to the sludge storage tank via the sludge pump room. The main equipment used to transport activated sludge in the sludge pump room is a screw pump. After natural sedimentation in the sludge storage tank, the upper part forms a clear liquid, which is discharged from the clear liquid outlet of the sludge storage tank to the internal sewage well; the sludge settled at the bottom of the sludge storage tank is transported by a screw pump to a centrifugal dewatering machine for solid-liquid separation. After separation, the water is directly discharged, and the resulting sludge cake is transported by a sludge cake transport vehicle to a sludge disposal plant for further processing.

Common Malfunctions of Sludge Concentration and Dewatering System

1. Common Malfunctions of Oil Mist Lubrication System

Oil mist lubrication is widely used in industrial production for lubricating friction surfaces, especially in lubricating bearings with high speeds and heavy loads, achieving good lubrication effects. This lubrication method not only saves a large amount of lubricating oil but also effectively prevents pollution. The oil mist lubrication system in the centrifugal dewatering machine of the wastewater treatment plant bears a large load between the bearings during operation; therefore, the following problems are often encountered during actual operation: First, the pressure switch often automatically disconnects; second, the red signal lights suddenly light up; third, the level switch often disconnects; fourth, the oil mist lubrication system malfunction indication often appears on the control panel.

2. Common Malfunctions of Centrifugal Dewatering Machine System

The following problems often occur during the operation of the centrifugal dewatering machine in the wastewater treatment plant: First, the separated water is relatively turbid, and the sludge cake recovery efficiency is low; second, the torque of the centrifugal dewatering machine remains low for a long time, resulting in a high water content in the separated sludge cake; third, the centrifugal dewatering machine often experiences torque abnormalities or blockages; fourth, the content of solid sludge in the sludge cake is low; fifth, the vibration generated during the operation of the centrifuge is large.

3. Common Malfunctions of Chemical Dosing System

During operation, the dewatering machine in this wastewater treatment plant requires manual addition of white, dry, powdery cationic polyacrylamide. Polyacrylamide forms an emulsion when it encounters water. The mixed agent is then pumped to the feed end, mixed with the sludge, and enters the drum for further dewatering. Common malfunctions of the chemical dosing system include: First, the system often prompts that the amount of dry powder added is low or that the liquid level alarm information in the chemical storage tank is triggered; second, the pumps in the chemical dosing system and the feed system often experience interlocking; third, the stirrer in the chemical dosing system often triggers a stirrer abnormality alarm.

4. Common Malfunctions of Sludge Storage Tank

Sludge often floats in the sludge storage tank, resulting in excessive water content in the sludge after dewatering.

Solutions to Malfunctions of Sludge Concentration and Dewatering System

1. Solutions to Common Malfunctions of Oil Mist Lubrication System

The main reasons for the frequent occurrence of several malfunctions in the oil mist lubrication system of the centrifugal dewatering machine in the sludge dewatering system are:

(1) The main reasons for the frequent disconnection of the pressure switch are: ① The compressed air in the system cannot be transported stably; ② The air filter in the centrifugal dewatering machine has not been cleaned for a long time, resulting in excessive dust and blockage; ③ The pressure regulator has changed; ④ The pressure switch has changed during operation; ⑤ The solenoid valve switch has malfunctioned during operation; The main solutions are: Increase the amount of compressed air transported; replace the air filter; regularly check and set the pressure switch and other components, and restart the solenoid valve.

(2) The reason for the frequent lighting of the red indicator light is that the system oil pressure is low. Therefore, first carefully check the oil mist lubrication system and replace some units of the lubrication system.

(3) The frequent disconnection of the level switch is mainly because the lubricating oil level in the system is lower than the standard low level. Therefore, adding lubricating oil to the system can solve the problem.

Air compressor malfunctions in the system, blockage of compressor valves, and air cylinder leaks can all cause oil mist lubrication system failures. For such failures, a thorough system check can be performed to identify the specific cause of the failure. After identifying the cause, coordinate with relevant departments for repair and guide the alarm to be cleared.

2. Common Troubleshooting Measures for Centrifugal Dewatering Machine Systems

2.1 Common Causes of Centrifugal Dewatering Machine System Failures

(1) The main reason for the appearance of turbid supernatant in the centrifugal pump is that the amount of mud entering the pump exceeds the standard requirement, the pump speed difference is too large, or the centrifugal pump is overloaded.

(2) The main reasons for the high water content of the mud cake processed by the centrifugal pump are as follows: ① The concentration of sludge entering the pump is too low; ② The amount of medicine added during sludge treatment is too small; ③ The drum speed in the centrifugal pump system is too low; ④ The centrifugal pump speed difference exceeds the standard requirement.

(3) The main reasons for the low solid content of the mud cake are as follows: ① Sand or scum mixed into the sludge when entering the centrifugal pump, causing the centrifugal pump to be blocked; ② The sludge input is too large; ③ The centrifugal pump gearbox has malfunctioned; ④ The centrifugal pump speed difference is too small.

(4) The reasons for the vibration of the centrifugal pump are mainly: ① The centrifugal pump's own lubrication system has malfunctioned; ② Scum appears in the centrifuge, causing the centrifuge spiral to be entangled; ③ The base of the centrifugal pump becomes loose during operation.

2.2 Troubleshooting Strategies

(1) The turbid supernatant can be solved by reducing the sludge input, reducing the centrifugal pump speed difference, replacing the new screw conveyor, or increasing the drum speed.

(2) For the phenomenon of excessive centrifugal pump torque, the supernatant flow rate of the sludge storage tank can be increased by appropriately increasing the pump flow rate, thereby reducing the water content of the sludge after treatment. In addition, increasing the amount of medicine added and increasing the drum speed can also solve the problems caused by the high centrifugal pump speed.

(3) The problem of low mud cake solid content can be improved by appropriately adjusting the centrifugal pump speed difference, increasing the drum speed, or increasing the amount of medicine added [5].

3. Troubleshooting Measures for Common Failures in the Chemical Dosing System

If the low chemical dosage alarm can be ruled out as a false alarm, it can be solved by appropriately increasing the chemical dosage; the main reason for the interlock phenomenon between the chemical dosing system and the feed system pump is that the feed pump is not turned on in time when the chemical dosing pump is turned on. The feed pump can be turned on within half a minute of the chemical dosing pump being turned on to solve this problem.

4. Troubleshooting Strategies for Common Failures in Sludge Storage Tanks

Sludge floating generally occurs in winter, mainly because the temperature is lower in winter or the oxygen content in the remaining sludge is too low. Therefore, the sludge treatment process can be adjusted appropriately to effectively increase the oxygen content in the sludge; in addition, the aeration valve of the sludge storage tank can be opened and aerated in the air for about 10 minutes, and then the aeration amount can be reduced after the floating sludge is removed, which can solve the problem of sludge floating.