Gravity Table and Rotary Screen Thickener vs Centrifuge for Mixed Liquor Thickening
Need for Thickening
In waste water treatment plants, one of the most common technologies for secondary treatment is the activated sludge process. The Biosolids produced sometimes require further treatment before they are suitable for disposal, in order to reduce the volatile matter which can create odours etc. These further treatment processes are best carried out on concentrated sludges, compared to the typical solids content of around 0.3% which is used in the activated sludge process.
Originally, settling in sedimentation type thickeners was used, however these units have the disadvantage that they are limited in the solids content achievable, and the retention time can cause problems to the process. The alternative technologies are:
Straining and low pressure dewatering using Gravity Table or Rotary Screen Thickeners after flocculation, or
Gravity Tables and Rotary Screen Thickeners
The Gravity Table drainage belt thickener is a simple conveyor like device, with a permeable belt.
The Rotary screen thickener is a rotating drum with a permeable filter cloth attached, through which the sludge is strained. (This unit should not be confused with a rotary screw thickener, which has a static outside screen, and a rotating
screw. The rotary screw thickener is not suitable for low consistency sludges).
The centrifuge operates by accelerating the feed to thousands of RPM, and the increased "g" forces segregate the solids from the liquid through the small differences in specific gravity between the two. A screw conveyor, the "decanter", moves the solids at a small relative velocity to the casing, to the discharge port.
Comparison of Technologies
The Gravity Table belt thickener is an extremely simple machine, low in capital and operating costs. It is easily controlled, and does not need sophisticated electronics. The process of thickening thin sludges is carried out most efficiently by simple drainage and light pressure application. There is no shearing action on the floccs, and the capture is usually very high, (typically 98%).
A further advantage when compared to the centrifuge is that no segregation or selection of particular part of Biosolids occurs. If a centrifuge is operated without flocculant, it acts as a separation machine, and preferentially selects the lighter fraction of the Biosolids for recirculation in the centrate. The filamentous organisms in the sludge tend to be in this lighter fraction, and this selection effect can affect the process adversely. The result is that centrifuges generally are operated with polymer flocculant dosage in order to achieve the same level of capture as a Gravity Table drainage belt Thickener.
Governments are currently trying to reduce greenhouse gas emissions. A centrifuge when thickening low consistency sludges is extremely inefficient, as a large amounts of energy are required to accelerate the entrained water. The Gravity Table solution is therefore a much smaller generator of greenhouse gases.
The polymer flocculant consumption of a Centrifuge to achieve the same level of capture as a Gravity Table is roughly the same, as the centrifuge action of high shear forces during acceleration, and screw conveying, damages the floccs.
In summary, the Gravity Table is a simple, reliable, economical solution to sludge thickening which has been successfully used by many authorities, including Hunter Water, Wyong Shire, Ipswich Council, Glenorchy Shire and many other references in Australia and overseas. Its lower power consumption is favorable in greenhouse gas control.