Effect of ultrasonication on waste activated sludge rheological properties and process economics

The present study provides an overall view of the effect of the ultrasound treatment on waste activated sludge (WAS) rheological and dewatering properties as well as its impact on the economic balance of a theoretical wastewater treatment plant. The results showed that ultrasonication at 27,000 kJ/kg TS increased the soluble protein concentration (> 100%), bound water content (∼25%), and capillary suction time (> 100%) of WAS. The molecular weight distribution of the extracellular polymeric substances (EPS) revealed that the ultrasound treatment solubilised a portion of the peptides and low-molecular-weight proteins. The thixotropic behaviour of the WAS was analysed by means of a rheological structural model that defines the time evolution of a structural parameter as a function of kinetic coefficients for the breakdown and build-up processes. The ultrasound treatment reduced the kinetic coefficients for the breakdown process and changed the fast speed of alignment of flocs because of the reduction of WAS structures. Similarly, the creep tests revealed that the ultrasound treatment at 27,000 kJ/kg TS reduced the initial elasticity (∼80%) and the zero-shear rate viscosity (∼60%), which means that the internal structure of the WAS loosened and disrupted. Finally, a techno-economic analysis showed that ultrasonication was not yet economically favourable since its implementation increased 14% the net cost for WAS treatment and disposal. However, a sensitivity analysis illustrated that increasing electricity revenue and reducing biosolids disposal costs through improvement in WAS biodegradability is important to make ultrasound implementation economically attractive.

Effect of ultrasound, low-temperature thermal and alkali pre-treatments on waste activated sludge rheology, hygienization and methane potential

Waste activated sludge is slower to biodegrade under anaerobic conditions than is primary sludge due to the glycan strands present in microbial cell walls. The use of pre-treatments may help to disrupt cell membranes and improve waste activated sludge biodegradability. In the present study, the effect of ultrasound, low-temperature thermal and alkali pre-treatments on the rheology, hygienization and biodegradability of waste activated sludge was evaluated. The optimum condition of each pre-treatment was selected based on rheological criteria (reduction of steady state viscosity) and hygienization levels (reduction of Escherichia coli, somatic coliphages and spores of sulfite-reducing clostridia). The three pre-treatments were able to reduce the viscosity of the sludge, and this reduction was greater with increasing treatment intensity. However, only the alkali and thermal conditioning allowed the hygienization of the sludge, whereas the ultrasonication did not exhibit any notorious effect on microbial indicators populations. The selected optimum conditions were as follows: 27,000 kJ/kg TS for the ultrasound, 80 °C during 15 min for the thermal and 157 g NaOH/kg TS for the alkali. Afterward, the specific methane production was evaluated through biomethane potential tests at the specified optimum conditions. The alkali pre-treatment exhibited the greatest methane production increase (34%) followed by the ultrasonication (13%), whereas the thermal pre-treatment presented a methane potential similar to the untreated sludge. Finally, an assessment of the different treatment scenarios was conducted considering the results together with an energy balance, which revealed that the ultrasound and alkali treatments entailed higher costs.