Solid Bowl Centrifuge for Sludge Dewatering

In wastewater treatment plant, dewatering process is usually the last step used for liquid and solid separation. The purpose of doing this is to get sludge cake of certain density and strength in order to allow better handling when it comes to transportation of the sludge for final disposal. This dewatering process usually follows right after clarification and there are actually different methods which one can choose such as Solid Bowl Centrifuge.

Solid Bowl Centrifuge is widely employed in sewage plants, steel and paper mills including oil refineries. There are 3 different types of solid bowl designs, which are mainly conical, cylindrical and conical-cylindrical. Conical bowl was once the most preferred setup because it allows maximum water removal but however, in terms of achieving centrate quality, it was considered to be less effective. For cylindrical bowl however, although it generally produces wet cakes but somehow it still manages to get good centrate clarity. Due to give-and-take restrictions on both designs, a combination called the conical-cylindrical design is being developed and it is largely used in today's common solid bowl centrifuge for sludge thickening process. The reason why it was chosen is because of its ability to shift and control the balance of cake dryness and centrate quality by simply adjusting pool length depending on end user requirements.

The typical design of a conical-cylindrical solid bowl centrifuge has a rotating bowl unit joined to a conveyor by a gear system, which allows both units to rotate in the same direction but at different speed. The bowl unit usually operates at higher speed between 2000 to 2500 rotation per minute while the conveyor will move at slower speed usually at 100 rpm less. There are two different types of the design depending on whether the liquid centrate and solid cake leaves the bowl at same end or opposite ends whereby the former is known as concurrent type while the second is the called the counter current design. The conveyor in the system acts to push the sludge towards the end ports while the supernatant liquid will exit over the weirs.

To get a better picture and idea on how the whole unit operates, let us look at the diagram below showing the design of a Solid Bowl Centrifuge unit.

Solid Bowl CentrifugeNew Emerging Technologies

Apart from solid bowl, there are various types of centrifuges being developed and used in operation and over the years new technologies were incorporated to increase performance and reliability. Support systems such as feed equipment, chemical dosing facility and better transfer pumps were designed and added to allow more control and ease of operation. Sludge dewatering centrifuges basically operate based on high speed separation whereby it relies on the force of spinning in order to throw the solids out from the water and cause the sludge to stick to the outer wall surface. Based on this understanding and applying the basics, normally the operator can adjust the operating parameters so that the dewatering process can be optimized. Typically if everything is well-controlled and managed, the percent solids that are present in the dewatered sludge can be anywhere in the range of 15 – 20% but it is possible to have higher concentration if steps were taken to adjust the settings.

Performance Factors

Normally, in order to achieve better centrate clarity and to get higher solid composition, chemical conditioning is applied prior to feeding the sludge to the centrifuge. Feed rate is an important control parameter because flow that is too high can result in poor centrate quality and produce wet sludge that will be difficult to handle and defeats the whole purpose of decanting. Depending on the type of sludge fed to the system, this can also affect the final percent solids whereby heat treated and raw sludge will be the most concentrated. Other factors that are often overlooked are the control of bowl speed, odor control and generation, but in all, by adjusting the feed rate, chemical dosage and operating speed, these will usually solve the problems. Equipment defects can also result in poor performance especially as in the case of excessive vibration and constant tripping, but with regular maintenance to check the gearbox, replace the wear and tear parts, these will prevent breakdown and help to lower the cost of operation.

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