Dewaterability of sewage sludge by ultrasonic, thermal and chemical treatments

Abstract

Sludges resulting from wastewater treatment processes have a characteristically high water content, which complicates handling operations such as pumping, transport and disposal. To enhance the dewatering of secondary sludge, the effect of ultrasound waves, thermal treatment and chemical conditioning with NaOH have been studied. Two features of treated sludges were examined: their rheological behavior and their dewaterability. The rheological tests consisted of recording shear stress when the shear rate increases and decreases continuously and linearly with time, and when it increases and decreases in steps. Steady-state viscosity and thixotropy were obtained from the rheological tests, and both decreased significantly in all cases with increased treatment intensity. Centrifugation of ultrasonicated and thermally treated sludges allowed the total solid content to be increased by approximately 16.2% and 17.6%, respectively. These dewatered sludges had a lower viscosity and thixotropy than the untreated sludge. In contrast, alkali conditioning barely allowed the sludge to be dewatered by centrifugation, despite decreasing its viscosity and thixotropy.

Appraisal of changes in sewage sludge management

“…The European Urban Wastewater Treatment Directive (91/271/EEC) has lead to a significant increase in sewage sludge production and to strong limitations of its disposal routes (Ruiz-Hernando et al 2013). Consequently, the research of efficient sludge treatments is essential to ensure proper management of sewage sludge and minimization of the costs.…”

“…This practice, which has been transferred into the commercial sector only recently, results in many technological and economical advantages (Wirth et al 2015). Faster release of easily biodegradable organic matter means savings from mass and volume reduction (Maroušek 2013) and represents new financial opportunities linked with easier dewatering (Ruiz-Hernando et al 2013), stabilization and byproduct recycling (Strong et al 2011). In particular, reuse of phosphorus (Nanzer et al 2014) is the main reason why fermentation residues from sewage sludge are still used for agronomical purposes in some developing countries.…”

“…of hydrothermal pretreatment were found-by interpolation of results obtained during numerous simulations (Xue et al 2015) and subsequent multifactor optimizations (Maroušek 2012)-in the proximity 160-180°C and 15-120 min (Donoso-Bravo et al 2010). If the processing parameters are severe enough for a sufficient time, the cell walls may be destroyed, making the proteins easily accessible to biological degradation (Ruiz-Hernando et al 2013). Reuse of waste heat (Speidel et al 2015) from combustion of biogas released in subsequent anaerobic fermentation (Reza et al 2014) makes the energy balance of the hydrothermal pretreatment positive and therefore financially attractive (Ruffino et al 2015).…”

The Changes of Dewaterability of Digested Sludge Conditioned via the Dual Chemical Method

“…Other physical conditioning techniques include the use of thermal pre-treatment [12], freeze/thaw pre-treatment [13] application of ultrasonic waves [14], or microwaves [15]. These conditioning factors affect the filterability by, among others, changing the viscosity, thixotropic behaviour and mean particle size [16].…”

Rheological properties of disintegrated sewage sludge

“…In the research on rheological properties, apart from determination of flow curves and viscosity, one should also develop rheological models which allow for approximation of flow curves [6,7]. Among common rheological models used for the description of flow curves is the Ostwald-de Waele power model:…”