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AmerCable Incorporated

Articles - Sludge Treatment


Source: Lenntech - Wastewater Treatment 


Aerobic stabilization can be performed simultaneously in an activated sludge plant whereby primary and secondary sludge are continuously aerated for long periods of time. In aerobic digestion the microorganisms extend into a respiration phase where materials previously stored by the cell are oxidized, resulting in a reduction of the biologically degradable organic matter. Thus, aerobic stabilization of the entire excess sludge (including primary sludge) is energy consuming. Additionally, it calls for extra reactor volume. 
Sludge digestion is carried out in the absence of free oxygen by anaerobic organisms. The facultative and anaerobic organisms break down the complex molecular structure of these solids setting free the "bound" water and obtaining oxygen and food for their growth. Anaerobic stabilization processes work at normal temperatures (< 40C) or within the range of thermopile bacteria, where 50-65C are reached alone by the heat development of the biochemical processes. The chemical stabilization of the sludge by means of wet oxidation or addition of quicklime and thermal stabilization under high temperature and pressure, are applied less often.

The anaerobic sludge digestion takes place in the highly visible digesters.


A volume reduction of approximately 30 80 % can be reached with sludge thickening before a further treatment. At smaller wastewater treatment plants, where the sludge is driven off regularly, thickening usually takes place directly in the sludge storage tank. The sludge is compressed at the tank bottom only by the force of gravity, while above the sludge a cloudy water layer is formed, which is taken off and led back into the inlet.

On larger plants separate thickening basins exist. These basins are equipped with slow rotating vertical rods, which create micro canals in the sludge for a better dewatering. Also pure machine thickening is gaining more significance with e.g. non-stabilised sludges, that could rot during the storage.


A further reduction of the sludge amount is mostly necessary after the thickening. The liquid sludge has to be dewatered and has to conform to a dry and porous form. Dewatering can be done naturally (dry beds, solar drying), however this is only possible during a long period of time. Faster and smaller, but also more cost intensive, are machine processes such as pressing (filter press) and centrifugation (centrifuge). 

For a good dewatering, size and firmness of the sludge agglomerates are important, so that these remain porous during the compression. Flocculants are often used to achieve as high as possible drying material contents at the machine dewatering and must be specifically co-ordinated with the accruing sludge.
For the choice of the correct dewatering process it is important to consider a multiplicity of further boundary conditions: Quantity, structural situation, disposal, regulations, availability, personnel etc.


A further reduction of the sludge weight is possible with the help of sludge drying, by evaporating the remaining bound water in the sludge. However a significant reduction of the sludge volume cannot be reached with that method. 

Sludge drying procedures are based particularly on contact-, convection or radiation procedures.
Large amounts of air are not necessary during the contact drying, because the warmth is supplied by the contact between the damp product and a heated wall. Only a minimum gas flow is often planned for the evacuation of steam. That has the advantage that the expenditure for exhaust air purification is small. 

Convection drying obtains its effect by treating the sludge with hot-air. In addition ambient air is heated to a high temperature with a burner or steam heat exchanger and brought in contact with the sludge in a drum or belt dryer.

The obvious characteristic of radiation drying is that the warmth, which is necessary for the drying process, is supplied through radiation to the sludge. General examples for warmth supply through radiation are solar radiation or infrared heating elements.

For the choosing the suitable drying method one should pay attention to

Adherence to security
Environmental compatibility
Flexibility of the drying method in relation to variable sludge quantities

Because of their high investment and operating costs, sludge drying is mainly used in large wastewater treatment plants.




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