A uniaxial cyclic compression method for characterizing the rheological and textural behaviors of mechanically dewatered sewage sludge

Abstract

The mechanically dewatered sewage sludge with total solid content around 20% on a weight basis is very similar to yield stress fluid, its complex transition between solid and fluid states is not perfectly reversible and especially challenging in terms of pumping, land spreading and drying. To characterize the rheological and textural properties of highly concentrated sludge, a specific methodology based on uniaxial single and cyclic compression tests is developed. Three types of sludge samples (fresh original, fresh premixed and aged original ones) are extruded into cylinders and pressed between two parallel plates using a material testing machine. In single compression, the bioyield point beyond which the sludge fractures is around 7.3 kPa with true strain equal to 0.21. The cyclic compression tests reveal that the sludge behaves as a viscoelastic body when the true strain is smaller than 0.05 and as a visco-elasto-plastic once exceeding the yield stress. The elastic module is around 78 kPa; the viscosity is deduced, in the order of magnitude 10-10 Pa·s and the yield stress is estimated about 4 kPa. In the unloading phase, the sludge behaves again as a viscoelastic body with clear hysteresis. With the increase of compression speed, the viscosity declines, which confirms that the sludge is a shear-thinning material. The yield stress and the bioyield increase with compression speed, but it does not induce extra internal damage in the samples since the resilience and the cohesiveness are unaltered. The reliability and sensitivity of the method is justified by highlighting the changes of sludge behavior due to aging and premixing effects: both decrease the strain energy density, but do aggravate the adhesiveness of the sludge; the aging makes the sludge less cohesive, while the premixing does not modify its cohesiveness. In spite of changes in test conditions, the elastic module of sludge samples remains unchanged.

Modelling of the rheological behavior of mechanically dewatered sewage sludge in uniaxial cyclic compression

“…The mechanically dewatered sewage sludge was sampled at the wastewater treatment plant (WWTP) of Albi (France). It is produced from extendedly aerated, thickened and digested municipal wastewater (Liang et al, 2017). The sludge was collected at the outlet of the centrifuge and then transported immediately to the laboratory within 1 h for testing.…”

“…Its initial total solid (TS, standard EN 12880:2000) content was ranged from 18.5 to 21 wt % and the volatile solid (VS, standard EN 12879:2000) content is of about 63% (of dry weight). It was shaped into cylinders using a manual extruder according to a procedure previously described (Liang et al, 2017). Finally, all the extruded sludge samples have a density of 1060 kg/m 3 and the same dimensions, with a radius r 0 ¼ 17 mm and a height h 0 ¼ 51 mm, corresponding to the aspect ratio h 0 /2r 0 ¼ 1.5.…”

“…The sewage sludge in form of a liquid becomes more and more viscous with increasing total solid content (TS), resulting in the decrease of flowability. When the TS reaches 18e25% after these dewatering processes, the sludge exceeding its liquid limit can no longer flow under the effect of gravity, but maintains its shape as a solid (Liang et al, 2017). Only if submitted to a high enough stress, it could continuously deform as yield stress fluids.…”

“…As the solidlike sludge is highly resistant to flow, it makes the flow control a real challenge in pumping, land spreading and drying (Baudez et al, 1998;Lotito and Lotito, 2014). Furthermore, the aging of organic matter inside sludge may also modify its rheological and textural properties (Liang et al, 2017). Consequently, the quantitative characterization of rheological properties (e.g.…”

“…In a previous paper, a uniaxial compression method was developed to characterize the rheological behaviors of sewage sludge with TS between the liquid and plastic limits (Liang et al, 2017). The protocols from sludge sampling to testing machine settings of the mechanical tests and the methods of sludge rheological behavior identification, based on the experimental true stress-true strain curves, were detailed, and the primary estimations of elastic modulus, viscosity and yield stress orders of magnitude of tested samples were illustrated and verified.…”