Abstract EN:

Reduction of excess sludge production is one of the major stakes of the 21st century; excess sludge treatment accounting up to 60 % of total cost of wastewater treatment. Low frequencies ultrasounds, by there mechanical actions on flocs, are a potential way of treatment for reduces this sludge flow. The treatment tests have been performed on a lab-scale pilot made of two parallel activated sludge systems. Low frequencies ultrasonic treatment (20 kHz sonotrode, SODEVA) is placed at level of the recycling of sludge of one of the two activated sludge systems and it is possible to treat only a fraction of recycling flow. Treatment tests realized on lab-scale pilot allowed reducing sludge production of 30 %, with an increase of sludge mineralization on the treated line; the fraction of volatile solids is reduce from 75 % to 60 %. Moreover no degradation of the quality of effluent is observed. The organic matter created, have been oxidized in the pilot. In addition a decrease of flocs' diameter is observed, just as it make soluble a parts of compounds captive in flocs. Many hypotheses can be done on the effects of ultrasonic treatment -The treatment make soluble a parts of organic matter, as consequence to recycling a parts of sludge into substrates, the conversion yield of substrates into sludge isn't of 100 %, so there is a reduction of sludge production -The treatment change the physicals characteristics of sludge by a mechanical effect : flocs size is reduce, so exchange surface is increased and sludge activity supported. Now modelling allows checking hypotheses and establishes that the second hypothesis is more realistic, unlike to the first, which cannot only explain sludge production reduction. However many other hypotheses on the ultrasonic treatment effects cannot currently be isolated. So it is necessary to continue study. Moreover, the promising results make it possible to plan tests on a semi-industrial scale pilot in order to optimise the system