HIGH-FREQUENCY ULTRASOUNDS FOR THE DECONTAMINATION AND INTENSIVE STABILIZATION OF SEWAGE SLUDGE

HIGH-FREQUENCY ULTRASOUNDS FOR THE DECONTAMINATION AND INTENSIVE STABILIZATION OF SEWAGE SLUDGE Sewage treatment plants generate quite a large amount of sludge. Because of increased environmental awareness and stringent environmental standards governing the disposal of sewage sludge, its utilization in agricultural production has been gaining increasing interest and attention in recent years. Although rich in nutrients, organic substance and microelements, in some countries sewage sludge is not accepted as agricultural fertilizer due to the awareness regarding potential health risks derived from the accumulation of heavy metals, organic compounds, and pathogens,. To be used as agricultural fertilizer, secondary sludge must be decontaminated by organic micropollutants and stabilized, aerobically or anaerobically. The use of a unique ultrasonic treatment, at 200 kHz, was studied in order to obtain both decontamination and stabilization of sewage sludge. Sonolysis experiments directly on sludge samples were carried out to degrade the linear alkylbenzene sulphonates (LAS), which is one of the most abundant classes of organic micropollutants in sludge. LAS degradation of about 40% was obtained in less than 1 hour. However, in a complex matrix like sludge, surfactants degradation is affected by different parameters: the specific energy delivered to the samples and the sludge total solids seemed the most important. On the other hand, the effects of the ultrasound irradiation at 200 kHz on sludge floc structure was investigated and compared with the “classical” sludge pretreatment at 20 kHz. Although the ultrasound treatment at 20 kHz was found to be more effective in sludge disintegration, in terms of solubilization of organic matter and flocs destruction, the treatment with 200 kHz gave also satisfactory and encouraging disintegration result. Afterwards, batch anaerobic digestion tests were performed with 20 and 200 kHz pretreated sludge to evaluate the efficacy of these two types of sludge pretreatment on the anaerobic digestion performances. Also the effect of food/inoculum ratio on the digestion performances was assessed. The treatment at higher frequency permitted to obtain satisfactory anaerobic digestion performances, as regards solubilisation and removal of organic substances and biogas production.