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. Results showed that the fractional Kelvin-Voigt model best fitted the experimental data obtained from creep and frequency sweep tests. Whereas, classical Kelvin-Voigt could not fit the frequency sweep data as this model is not a function of angular velocity. Also, the Kelvin-Voigt model was unable to predict the creep data at low stresses

Effect of thermally-induced protein solubilisation on rheology of activated sludge

Thermal pre-treatment of activated sludge was shown as an effective method to increase biodegradability of activated sludge for higher biogas production in digesters. The underlying reason for better digestion is solubilisation of organic matter in sludge. This study investigates sludge compositional and rheological changes due to thermal degradation of organic matter when the activated sludge is subjected to cyclic thermal treatment between 20 and 80°C. Protein was directly measured by a spectrophotometer as the major organic matter which is irreversibly solubilised with thermal treatment. The calorimetric method was also used to track the irreversible organic matter denaturation. The apparent viscosity and yield stress of thermally-treated activated sludge was less than untreated sludge. The calorimetric results confirmed the irreversible and gradual denaturation of organic matter. The protein solubilisation rate showed a similar trend to the denaturation kinetic calculated from calorimetric data

Object Security in the Internet of Things

The Internet of Things and the constrained environment that comes from the growth of constrained devices connected to the Internet brings new security challenges that cannot be solved in a satisfactory way with only transport layer security. A more flexible solution is required, both to protect sensitive data and user privacy but also to distribute policies in a secure and standardized way. The same privacy problems arise in the general web setting with processing and storage more and more moving into the cloud. One solution is to move the security higher up in the network stack and to protect objects instead of byte streams, as proposed in the IETF draft [15] evaluated in this thesis. Focusing on this solution, an implementation has been proposed, and tests and measurements have been carried out to show its overhead. The implementation, as well as the measurement results, is presented in this work.I takt med att ”Internet of things” växer och antalet begränsade enheter ökar uppstår allt fler säkerhetsutmaningar som inte kan lösas tillräckligt bra med transport layer security. Det finns ett behov av en mer flexibel lösning, dels för att skydda känslig data och användarnas integritet och dels för att distribuera policyer på ett säkert och standardiserat sätt. Samma integritetsproblem återfinns i generella webbmiljön då beräkning och lagring blir mer molnbaserat. En lösning på problemet är att flytta säkerhetsåtgärder högre upp i internetstacken och skydda objekt istället för byte-strömmar. Dessa säkerhetsåtgärder har föreslagits i utkastet från IETF [15] som utvärderas i denna avhandling. En implementation, med fokus på denna lösningen, föreslås i avhandlingen. Tester och mätningar har utförts för att visa lösningens overhead. Genomförandet av testerna och mätresultaten presenteras också i detta arbete

Impact of thermal treatment on the rheological properties and composition of waste activated sludge: COD solubilisation as a footprint of rheological changes

The rheological properties of sludge are key parameters in the design and optimisation of wastewater treatment processes. However, these properties are irreversibly altered when the sludge is exposed to heat (e.g. either passing through heat exchangers of anaerobic digesters or during thermal pre-treatment process before digesters). Literature showed that thermal pre-treatment of waste activated sludge (WAS) solubilises particulate organic matter in sludge and increases the biodegradability of WAS in anaerobic digesters. This paper reports the similarity between the kinetic of rheological enhancements due to thermal treatment and the rate of organic matter solubilisation in the WAS when it is subjected to cyclic thermal treatment between 20 and 80 °C up to 1 h. This work observed the evolution of rheological characteristics and soluble oxygen demand (sCOD) of 3.5-9.9 wt.% WAS at different temperatures and for different thermal histories (1 h thermal treatment at 50, 60, 70 and 80 °C). Results showed that, the rate of yield stress and infinite viscosity reductions due to thermal treatment (for 1 h treatment at 50, 60, 70 and 80 °C) were linearly proportional to the rate of solubilisation of organic matter. Because similar results were observed in the literature with digested sludge, these results confirmed that rheological measurement (e.g. infinite viscosity and yield stress) can be used to predict the level of solubilisation of organic matter in any type of sludge when it is exposed to thermal treatment. Vice versa, the rate of COD changes can be used as an indicator of the rate of rheological changes

Impact of temperature and duration of thermal treatment on different concentrations of anaerobic digested sludge: Kinetic similarity of organic matter solubilisation and sludge rheology

Municipal digested sludge is a dispersion of agglomerated particles or flocs in a liquid medium which exhibits yield stress, shear thinning behaviour and high irreversible temperature dependence. In this study, the rate of solubilisation of organic matter in digested sludge was investigated when sludge (at different solid concentrations) was exposed to thermal treatment between 50 and 80. °C for 1 to 60. min. The organic matter solubilisation was measured using chemical oxygen demand (COD) analysis on liquor of the thermally treated sludge. The effects of the abovementioned temperature range and heating duration on yield stress and the apparent viscosity of sludge at different solid concentrations have also been investigated. The results showed that the irreversible effect of temperature was much higher at higher concentrations, higher temperatures and longer duration of treatment. It was also observed that for any sludge concentration the kinetic of COD solubilisation, yield stress and apparent viscosity reductions followed a logarithmic correlation with duration of thermal treatment. This similarity was also pronounced in the linear relationship between solubilised COD and yield stress and apparent viscosity reductions in all tested concentrations and the durations of heat treatment. Modified form of Herschel-Bulkley was used to obtain a master curve, independent of temperature, concentration and duration of thermal treatment. Furthermore, a new model was proposed for the impact of temperature and the duration of thermal treatment on yield stress and apparent viscosity reduction

Effect of thermal history on municipal digested sludge rheology: Experimental and modeling

The rheological characteristics of digested sludge are severely temperature dependent. An effect of temperature on rheological properties of digested sludge is partially irreversible. In this paper, the effects of temperature change and thermal history on viscosity of digested sludge were studied using a stress-controlled rheometer which was connected to a temperature controlled water bath. Digested municipal sludge were collected from one of Melbourne wastewater treatment plants and subjected to change of temperature from 20-80°C. In addition, the effect of temperature on the sludge composition was studied by measuring solubilized chemical oxygen demand (COD) in sludge liquor. The Herschel-Bulkley equation was used to model the data. The yield stress and the apparent viscosity were measured and compared at different temperatures. The results showed that by changing the temperature, the sludge becomes more fluid; however, the rheological characteristics of sludge including yield stress and apparent viscosity do not come back to their original values after cooling the heated sludge. The solubilized COD showed a composition change after cooling the heated sludge which confirmed the irreversible change in the rheological characteristics of digested sludge

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. This decrease was due to the irreversible solubilisation of solid matter during heating as underlined by soluble COD data which did not reach its original level after thermal treatment. Measured soluble COD of sludge which was heated and cooled down was much higher than the soluble COD of initial sludge. We found a proportionality of the increase of soluble COD with the decrease of the yield stress as well as increase of infinite viscosity

An investigation 39 on rheological behaviour of thermally treated digested sludge: dependency on solid concentration

This study investigated the effect of temperature and duration of heat treatment on rheological properties of different concentration of digested sludge. The flow curve and the yield stress of 2%, 3% and 3.5% digested sludge have been measured before and during heat treatment at different temperatures between 20 and 80°C for different heating duration of 1, 15 and 30min. The effect of thermal history on rheology of sludge was then studied after cooling back sludge to 20°C (thermal history at 50, 60, 70 and 80°C). In addition, the effect of temperature and thermal history on solubilized chemical oxygen demand (COD) for sludge with different solid concentration was also investigated to prove the composition change of sludge during the heat treatment. The result showed that the irreversible effect of temperature was more evident at higher temperatures and concentrations. At higher concentrations (3% and 3.5%), by increasing the temperature, the yield stress decreased after an initial increase while for 2% sludge yield stress consistently decreased with increasing temperature and showed a linear relationship with decreasing particulate COD. The yield stress followed a great agreement with an exponential decreasing function of temperature and the temperature of thermal history

Role of regression analysis and variation of rheological data in calculation of pressure drop for sludge pipelines

Sludge pumps in wastewater treatment plants are often oversized due to uncertainty in calculation of pressure drop. This issue costs millions of dollars for industry to purchase and operate the oversized pumps. Besides costs, higher electricity consumption is associated with extra CO2 emission which creates huge environmental impacts. Calculation of pressure drop via current pipe flow theory requires model estimation of flow curve data which depends on regression analysis and also varies with natural variation of rheological data. This study investigates impact of variation of rheological data and regression analysis on variation of pressure drop calculated via current pipe flow theories. Results compare the variation of calculated pressure drop between different models and regression methods and suggest on the suitability of each method. © 2018 Elsevier Lt