Improved solubilization of activated sludge by ozonation in pressure cycles

► Ozonation in pressure cycles resulted in improved sludge solubilization. ► Solubilization is evidenced by increased soluble TN (860%), TP (640%), protein (>260%), and DNA contents. ► Enhanced outcomes are via cell rupture prompted by pressure cycles. The generation of a large volume of activated sludge (AS) from wastewater treatment has increasingly become a great burden on the environment. Anaerobic digestion is routinely practiced for excess waste sludge; however, the process retention time is long because of kinetic limitation in the hydrolysis step. We tested the feasibility of applying ozone in pressure cycles to enhance the disintegration and solubilization of AS with the goal to prepare them for digestion using reduced ozone dose and contact time. The AS was subjected to repetitive pressure cycles in a closed vessel in which an ozone gas mixture was compressed into the slurry to reach 1040kPa in the headspace to be followed by rapid venting. For a returned AS with total COD (tCOD) of 8200mgL−1, a dose of 0.01gO3g−1 total suspended solids (TSS) delivered via 20 pressure cycles within 16min resulted in a 37-fold increase of the sCOD/tCOD ratio (due to increased soluble COD, i.e. sCOD) and a 25% reduction of TSS, in comparison to a dose of 0.08gO3g−1 TSS via bubbling contact over 15min that resulted in a 15-fold increase of the sCOD/tCOD ratio and a 12% reduction of TSS. Sludge solubilization was evidenced by increased dissolved contents of total phosphorous (from 10 to 64mgL−1), total nitrogen (from 14 to 120mgL−1), and protein (from