Rheological measurements as a tool for monitoring the performance of high pressure and high temperature treatment of sewage sludge.

Hydrothermal processing plays a significant role in sewage sludge treatment. However, the rheological behaviour of sludge during these processes is not fully understood. A better understanding of the sludge rheology under hydrothermal processing conditions can help improve process efficiency. Moreover, sludge rheology is easier to measure than chemical analyses. If a relationship could be established, it provides a possibility of using rheological measurement as a basis for monitoring the performance of hydrothermal processing. The rheological changes in thickened waste activated sludge (7 wt%) was investigated using a pressure cell-equipped rheometer during 60-min thermal hydrolysis (TH) at various temperatures (80-145 °C) and constant pressure (5 bar). Changes in the soluble chemical oxygen demand (COD) were measured using a separate reactor with a similar operating condition. The sludge behaved as a shear-thinning fluid and could be described by the Herschel-Bulkley model. At constant temperature, the yield stress and high-shear (600 s-1) viscosity of sludge decreased logarithmically over 60 min. At constant time, the yield stress and the high-shear viscosity decreased linearly with increasing TH temperature and these values was much less than corresponding properties after treatment and cooling down to 25 °C. The soluble COD of sludge also increased logarithmically over 60 min at constant temperature, and increased linearly with increasing temperature at constant time. Furthermore, the yield stress and high-shear viscosity reduction showed a linear correlation with the increase in soluble COD.

A review of wet air oxidation and Thermal Hydrolysis technologies in sludge treatment.

With rapid world population growth and strict environmental regulations, increasingly large volumes of sludge are being produced in today's wastewater treatment plants (WWTP) with limited disposal routes. Sludge treatment has become an essential process in WWTP, representing 50% of operational costs. Sludge destruction and resource recovery technologies are therefore of great ongoing interest. Hydrothermal processing uses unique characteristics of water at elevated temperatures and pressures to deconstruct organic and inorganic components of sludge. It can be broadly categorized into wet oxidation (oxidative) and thermal hydrolysis (non-oxidative). While wet air oxidation (WAO) can be used for the final sludge destruction and also potentially producing industrially useful by-products such as acetic acid, thermal hydrolysis (TH) is mainly used as a pre-treatment method to improve the efficiency of anaerobic digestion. This paper reviews current hydrothermal technologies, roles of wet air oxidation and thermal hydrolysis in sludge treatment, and challenges faced by these technologies.