Sewage sludge is the solid fraction of urban and industrial wastewater. It is produced in treatment plants by mechanical, biological and chemical methods. These latter are necessary in order to return the clarified water in nature without altering the ecosystem of the receiver (sea, rivers, lakes or some cases the superficial ground). The main problem of the sludge production concerns its final fate. The strategies for its treatment and disposal have been strongly influenced by the attention of the laws to the environment, by the scientific research and the subsequent technological developments.
Sewage sludge contains inorganic and organic substances. Usually, the organics are higher and mostly bio-degradable. The bio-degradability encourages to use biological treatments. Anyway, pathogen microorganisms, such as Salmonella and Streptococcus of human dejection could be in the sludge. It is well known that they are dangerous for the human health. From this point of view, thermal treatments, such as combustion gasification and pyrolysis, are methods to sterilize the sludge and to reduce the volume of the waste. Moreover the recovery of chemical energy of the sludge seems to be possible. From a literature survey information was recovered about sewage sludge gasification.
The goal of Hengyang Industry is to investigate the potentiality of energetic recovery by sewage sludge gasification. Experimental gasification tests were carried out in a bench scale rotary kiln under simulated autothermal condition. The mass rate of sludge was in the range 170–260 g/h. The gasification temperature was varied from 750 to 850 °C, equivalence ratio was increased up to 0.24. The tests run for about 3 h. Input and output streams of the process were quantified and analyzed as to assess the cold gas efficiency. Tar content in the raw gas was determined as well. The main drawbacks occurred during the tests were highlighted to have a realistic frame of the experimented technology.