Fundamentals of Sludge Dewatering
Sludge dewatering is an important part of wastewater treatment because it reduces the amount of water contained in excess sludge and makes the sludge easier and less expensive to handle and dispose of.
Fresh excess sludge from a biological wastewater treatment process normally contains about 99.2%–99.8% water. At this stage, the sludge is highly fluid and cannot normally be transported or disposed of directly.
After mechanical dewatering, the moisture content of the sludge cake can generally be reduced to around 80%–85%. The resulting sludge cake is much easier to collect, transport, and dispose of.
Good dewatering performance, however, does not depend on the dewatering machine alone. Sludge condition, chemical dosing, equipment operation, and cleaning all have a direct effect on the final result.
Checking the Sludge Before Dewatering
Before starting the dewatering equipment, it is worth checking the condition of the sludge first.
Sludge color and odor
Normal biological sludge is generally yellowish-brown or earthy yellow in color. If the sludge becomes black and develops a strong foul odor, anaerobic decomposition may be occurring.
A slight earthy odor is usually normal. A strong hydrogen sulfide odor, however, should not be ignored. In this situation, it is better to check the sludge storage and discharge cycle before sending the sludge directly to the dewatering machine.
Sludge concentration
SV30 and MLSS can be used to understand the concentration and settling characteristics of the sludge.
If the sludge concentration is too low, the dewatering equipment may have difficulty forming a stable sludge cake. In practice, this can result in poor solids capture and a wetter cake.
PAM Preparation and Dosing
Most mechanical sludge dewatering processes use PAM (polyacrylamide) to improve floc formation.
PAM helps small sludge particles combine into larger flocs, making it easier to separate the solids from the water.
For typical applications:
Cationic PAM is commonly used for excess biological sludge.
Anionic PAM is mainly used for inorganic sludge and some primary sedimentation sludge.
The exact polymer type and dosage should still be determined through testing because sludge characteristics vary from one wastewater treatment plant to another.
Preparing PAM solution
A typical preparation procedure is:
Add water to the chemical mixing tank and start the mixer.
Slowly add the PAM while maintaining mixing.
Continue mixing for approximately 30–60 minutes to allow the polymer to dissolve completely.
It is generally better to prepare PAM solution according to actual consumption rather than storing large quantities for a long period. Prepared solution should normally not be stored for more than 24 hours.
Starting the Dewatering Equipment
Before starting the dewatering machine, inspect the complete system.
Check the main dewatering unit, sludge feed pump, chemical dosing pump, wash water pump, piping, and valves for blockages, leaks, or other abnormalities.
The wash water system is particularly important for belt filter presses and screw presses. Insufficient wash water pressure can result in poor cleaning and gradually lead to clogging of the filter cloth or other components.
The sludge storage tank should also have sufficient sludge available for continuous feeding. If a mixer is installed in the tank, it should be started to prevent sludge from settling and forming large deposits.
Flocculation: Finding the Right PAM Dosage
After the sludge feed pump and chemical dosing pump are started, the sludge and PAM solution should be thoroughly mixed before entering the dewatering unit.
The appearance of the flocs can provide useful information about whether the chemical dosage is appropriate.
If the flocs are very small and the filtrate remains cloudy, the PAM dosage may be too low.
If the flocs are very large but loose and the separation between solids and water is still poor, the dosage may be too high.
These observations are useful for adjusting the process, but visual inspection alone should not replace a proper jar test or polymer trial when optimizing the dosage.
Monitoring the Dewatering Process
Once the flocculated sludge enters the dewatering machine, the solids and water are gradually separated.
Depending on the application, equipment may include a belt filter press, screw press, or plate-and-frame filter press.
During operation, operators should pay particular attention to four things:
1. Sludge cake thickness
The cake should form evenly across the working area. Uneven cake formation can indicate problems with sludge distribution, feeding, or flocculation.
2. Filtrate quality
The filtrate should become relatively clear after proper flocculation. Excessive sludge in the filtrate usually indicates a problem with floc formation, chemical dosage, or equipment operation.
3. Equipment current and pressure
Sudden changes in motor current or operating pressure may indicate overloading, blockage, excessive sludge feeding, or another mechanical problem.
4. Spray nozzles
For equipment that relies on spray water for cleaning, blocked nozzles can reduce cleaning efficiency and eventually affect dewatering performance.
Regular observation during operation can help operators identify problems before they develop into major equipment failures.
Proper Shutdown and Cleaning
One of the easiest mistakes to make is to stop the dewatering machine immediately after the sludge feed is finished and leave the equipment without cleaning it.
A better shutdown sequence is:
Stop the sludge feed pump and chemical dosing pump.
Keep the dewatering equipment running while thoroughly washing the filter cloth, screw shaft, and relevant piping with high-pressure water.
Remove as much residual sludge as possible.
After cleaning is complete, stop the main dewatering unit and wash water pump.
Close the relevant valves.
Residual sludge that remains inside the equipment can dry and harden. During the next startup, this dried sludge may cause blockages or mechanical problems and increase maintenance costs.