Impact of gas injection on physicochemical properties of waste activated sludge: A linear relationship between the change of viscoelastic properties and the change of other physiochemical properties.

Aeration process in the waste activated sludge treatment accounts for 75% of total energy consumption of the treatment plant. The main purpose of the aeration process is to enhance the biodegradation of the liquid waste. Gas bubbles, rising through the liquid, improves mixing, reduces inhomogeneities in the treatment tank and enhances biological reactions.

Comparison between classical Kelvin-Voigt and fractional derivative Kelvin-Voigt models in prediction of linear viscoelastic behaviour of waste activated sludge.

Appropriate sewage sludge rheological models are essential for computational fluid dynamic simulation of wastewater treatment processes, in particular aerobic and anaerobic digestions. The liquid-like behaviour of sludge is well documented but the solid-like behaviour remains poorly described despite its importance for dead-zone formation. In this study, classical Kelvin-Voigt model, commonly used for sludge in literature, were compared with fractional derivative Kelvin-Voigt model regarding their predictive ability for describing the solid-like behaviour.

Impact of gas injection on the apparent viscosity and viscoelastic property of waste activated sewage sludge.

Gas injection is known to play a major role on the particle size of the sludge, the oxygen transfer rate, as well as the mixing efficiency of membrane bioreactors and aeration basins in the waste water treatment plants. The rheological characteristics of sludge are closely related to the particle size of the sludge floc. However, particle size of sludge floc depends partly on the shear induced in the sludge and partly on physico-chemical nature of the sludge.

Net positive energy wastewater treatment plant via thermal pre-treatment of sludge: A theoretical case study.

In a wastewater treatment process, energy is mainly used in sludge handling and heating, while energy is recovered by biogas production in anaerobic digestion process. Thermal pre-treatment of sludge can change the energy balance in a wastewater treatment process since it reduces the viscosity and yield stress of sludge and increases the biogas production. In this study, a calculation based on a hypothetical wastewater treatment plant is provided to show the possibility of creating a net positive energy wastewater treatment plant as a result of implementing thermal pre-treatment process before the anaerobic digester.

Predicting the apparent viscosity and yield stress of mixtures of primary, secondary and anaerobically digested sewage sludge: Simulating anaerobic digesters.

Predicting the flow behaviour, most notably, the apparent viscosity and yield stress of sludge mixtures inside the anaerobic digester is essential because it helps optimize the mixing system in digesters. This paper investigates the rheology of sludge mixtures as a function of digested sludge volume fraction. Sludge mixtures exhibited non-Newtonian, shear thinning, yield stress behaviour.

Mixing characteristics of sludge simulant in a model anaerobic digester.

This study aims to investigate the mixing characteristics of a transparent sludge simulant in a mechanically agitated model digester using flow visualisation technique. Video images of the flow patterns were obtained by recording the progress of an acid-base reaction and analysed to determine the active and inactive volumes as a function of time. The doughnut-shaped inactive region formed above and below the impeller in low concentration simulant decreases in size with time and disappears finally.

Rheological characterisation of thermally-treated anaerobic digested sludge: impact of temperature and thermal history.

This study investigated the partially irreversible effect of thermal treatment on the rheology of digested sludge when it was subjected to temperature change between 20 °C and 80 °C and then cooled down to 20 °C. The yield stress, infinite viscosity and liquor viscosity of sludge were measured at 20 °C for different thermal histories and were compared to the evolution of the solubilised chemical oxygen demand (COD) of sludge liquor. The results showed that thermal history irreversibly affects sludge rheology as the yield stress of sludge which was heated to 80 °C then cooled down to 20 °C was 68% lower than the initial yield stress at 20 °C.

Rheological characterisation of municipal sludge: a review.

Sustainable sludge management is becoming a major issue for wastewater treatment plants due to increasing urban populations and tightening environmental regulations for conventional sludge disposal methods. To address this problem, a good understanding of sludge behaviour is vital to improve and optimize the current state of wastewater treatment operations. This paper provides a review of the recent experimental works in order for researchers to be able to develop a reliable characterization technique for measuring the important properties of sludge such as viscosity, yield stress, thixotropy, and viscoelasticity and to better understand the impact of solids concentrations, temperature, and water content on these properties.

Proxy model materials to simulate the elastic properties of digested municipal sludge.

The elastic rheological properties of sludge are complex and evolve with time as a result of ageing and microbial activity. Due to the peculiar nature of sludge, this makes the measurement of physical parameters difficul. The challenge is to identify a reference material that can be used as a proxy for industrial process design or optimization.

The viscoelastic behaviour of raw and anaerobic digested sludge: strong similarities with soft-glassy materials.

Over the last few decades, municipal and industrial wastewater treatment activities have been confronted with a dramatically increasing flow of sewage sludge. To improve treatment efficiency, process and material parameters are needed but engineers are dealing with vast quantities of fundamentally poorly understood and unpredictable material Thus, accurate prediction of critically important, but analytically elusive process parameters is unattainable and is a matter of grave concern. Because engineers need reliable flow properties to simulate the process, this work is an attempt to approach sludge rheological behaviour with well-known materials which have similar characteristics.

Compression dewatering of municipal activated sludge: effects of salt and pH.

Even after mechanical dewatering, activated sludge contains a large amount of water. Due to its composition and biological nature this material is usually highly compressible and known to be difficult to dewater. In the present work, two treatments (salt addition and pH modification) are proposed to highlight some aspects which could explain the poor dewaterability of activated sludge.

Clear model fluids to emulate the rheological properties of thickened digested sludge.

Optimising flow processes in wastewater treatment plants requires that designers and operators take into account the flow properties of the sludge. Moreover, due to increasingly more stringent conditions on final disposal avenues such as landfill, composting, incineration etc., practitioners need to produce safer sludge in smaller quantities.

About peak and loop in sludge rheograms.

The thixotropic behavior of sewage sludge is analysed by varying several parameters such as the time of rest, the slope of the shear rate ramp or the data sampling. We observed that only the initial stress overshoot is time-dependent but always appears, for a given material, at the same critical angle which indicates it is linked to elastic effects: during shear, the material has a short memory. The visualisation of the velocity profile within the sheared material confirms this assumption.

Practical determination of the rheological behavior of pasty biosolids.

In this paper, we demonstrate that the rheological behavior of pasty sewage sludges, regardless of origin, treatment or composition, follows a Herschel-Bulkley model. The yield stress and solid volume fraction are found to be the only two distinctive rheological characteristics of these materials. By scaling the shear rate and the shear stress with two parameters depending only on the yield stress and the solid fraction, the flow curves of 48 pasty sludges all fall along a unique dimensionless master curve.