Biological sludge treatment systems

Biological stabilization techniques for sludge basically consist of slowing down or even stopping the putrid fermentation that leads to odour pollution.

There are two possible strategies:



  • In line with biodegradation, by consuming the rapidly biodegradable fraction of the sludge using a controlled bioprocess, either by anaerobic digestion or aerobic stabilization of the liquid sludge or by composting the paste like sludge.


  • Taking the sludge from its biodegradation state by modifying the pH (liming) or by making the water unavailable (reducing the water activity through drying). These two systems, however, do not have a permanent effect ; if the sludge returns to its initial state, fermentation will resume.



Using a digestion process to achieve biological stabilization

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When it comes to the treatment of urban sludge, mesophilic digestion of suspended- growth sludge (around 35°C) is very widespread, as is the digestion of industrial liquid effluents with high levels of biological oxygen demand (BOD) using fixed or fluidized bed techniques.


In a digester digestible, dissolved or particulate organic matter first undergoes hydrolysis into simple compounds using specific enzymes.
An acid-producing flora then produces acid compounds from these elementary molecules.


Finally, gasification is carried out by using a methane-producing flora which ensures the advanced degradation of the MV and produces biogas.


The equilibrium of the ecosystem is achieved by maintaining the pH, alkalinity and volatile fatty acids (VFA) values within a precise range of values.

The biogas


The biogas can then be used as follows:


  • incineration in a waste gas burner (this is required in any case for safety reasons).

  • use as fuel in the boiler facility which will enable the digester's temperature to be maintained.

  • use as a gas motor fuel following possible desulphurization which will lead to the recuperation of 2 to 2.2 kWh of burnt power/Nm3 and in addition ensure the digester's temperature is maintained.

Thermophilic aerobic stabilization


Aerobic stabilization can be used at an ambient temperature.

Thermophilic aerobic stabilization entails subjecting the thickened sludge to aerobic bio-oxidation at 55°C for several days. This technique is primarily used to treat urban sludge in the USA, Germany and Switzerland.

A relatively cumbersome process, it does however have the advantage of being most effective in sanitizing the sludge, but is less effective than anaerobic digestion when it comes to degrading the pollution.