Pressure-assisted ozonation and aeration of activated sludge for cell rupturing and anaerobic digestion enhancement

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. This study tested the feasibility of applying cycles of pressure-assisted ozonation (PAO) to enhance the disintegration and solubilization of AS for digestion using reduced ozone dose and contact time. The kinetics of solid reduction and biogas production of PAO-treated sludge are evaluated in this study. The ultimate goal is to introduce an effective treatment method that produces increased soluble organic contents from AS for enhanced excess sludge reduction and biogas production during conventional anaerobic digestion. Results show that for the returned AS with total chemical oxygen demand (tCOD) of 8200 mg L-1, a dose of 0.01 g O3 g-1 total suspended solids (TSS) delivered via 20 pressure cycles within 16 min 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 0.08 g O3 g-1 TSS via bubbling contact over 15 min 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 64 mg L-1), total nitrogen (from 14 to 120 mg L-1), and protein (from < 15 to 39 mg L-1) in the sludge suspension after treatment, indicating significant solubilization of AS. The subsequent anaerobic digestion tests showed solids reduction to be significantly dependant on food-to-inoculum (F/I) ratios in the range 0.5 to 2 rather than treatments when unacclimated inoculum was used. However, improvements of solids reduction and biogas production by conventional ozonation and PAO resulted when using acclimated inocula at F/I = 0.4 to 0.8. Using PAO at a smaller dose (10 mg O3 g-1 TSS) at F/I of 0.8, VSS reduction and biogas production were 1.4- and 2.6-fold higher, respectively, than those of conventional ozonation (52 mg O3 g-1 TSS). This study has demonstrated the advantage of using PAO in disrupting excess activated sludge and in terms of improved solids reduction and biogas production during subsequent anaerobic digestion.