Effects of low-dosage ozone pre-treatment on the anaerobic digestion of secondary and mixed sludge

The present study investigated the effects of ozonation pre-treatment at low-ozone dosage (below 100 mgO3/gTS0) with respect to previous studies, on the anaerobic digestion of waste-activated sludge alone and a mixture of activated sludge and primary sludge. Methane production and volatile suspended solids reduction efficiency were determined for different specific ozone dosages and compared with the values obtained in the absence of pre-treatment. Among the dosages tested in the study (from 4.8 to 73.2 mgO3/gTS0 for mixed sludge and from 3.5 to 53.6 mgO3/gTS0 for waste-activated sludge), the best results were obtained at the lowest ones: 4.8 and 3.5 mgO3/gTS0 for mixed sludge and waste-activated sludge, respectively. Indeed, at this dosage, an additional methane production of about 6% and 30% was achieved for mixed and waste-activated sludge, respectively; furthermore, the maximum CH4 production rate increased of about 21% and 33% for mixed and waste-activated sludge, respectively. With respect to the Gompertz model, the modified logistic model provided the best agreement to the experimental data of the specific methane yield production. The present study demonstrated the importance of investigating the application of low dosages when ozonation is being evaluated as a pre-treatment to enhance anaerobic digestion performance.