On the rheological characteristics of sewage sludge.

VITĚZ, T., SEVERA, L.: On the rheological characteristics of sewage sludge. Acta univ. agric. et silvic. Mendel. Brun., 2010, LVIII, No. 2, pp. 287–294 The work is focused on characterization of rheological behavior of sewage sludges sampled at different stages of waste water treatment. The main attention was focused on dynamic viscosity dependence on temperature, and shear rate. The sludge samples were examined under temperature ranging from 1 °C to 25 °C and under shear rate ranging from 0.34 s−1 to 68 s−1. Rotary digital viscometer (concentric cylinders geometry) was used to perform the reological measurements. The solids content of the sludge samples ranged from 0.43 % to 21.45 % (A and C samples, respectively) and ash free dry mass from 56.21 % to 67.80 % (A and B samples, respectively). The tested materials were found to be of non–Newtoninan nature and temperature dependent. Measured data were successfully chara cte ri zed by several mathematical models (Arrhenius, Bingham Plastic, Casson Law, Exponential, Gaussian, and IPC Paste) in MATLAB® so ware with satisfying correlations between experimental and computed results. The best match (R2 = 0.999) was received with use of Gaussian model, in both cases, shear rate and temperature dependence. The results are quite useful e.g. for the purpose of technological equipment design. sewage sludge, apparent viscosity, modeling Sewage sludge is a product of wastewater treatment and it is produced in diff erent amount, quali ty, pro per ties and diff erent steps of waste water purifi cation. The main component of sludge is water. Its amount depends on the sludge sort and the way of stabilisation (aerobic, anaerobic). The raw sludge has usually a water content of 93 % to 99 % (Werther and Ogada, 1998). Therefore a dewatering (up to approx. 25 % dry solid content) or drying (to over 88 % dry solid content) can be necessary for a further utilization. The second main component is the dry solid, which is made up of organic and inorga nic substances. Basically sludge is sort as primary, se con da ry (excess) and tertiary (chemical). Primary sludge is produced through the mechanical wastewater treatment process. It consists of unsolved wastewater contaminations. Primary sludge consists of a high portion of organic matters, as faeces, vegetables, fruits, textiles, paper etc. The material is composed of a thick fl uid with a water content ranging from 93% to 97%. The activated sludge fl ows from the biological aeration basin into the fi nal clarifi er. The activated sludge fl akes settle down to the bottom and can be separated from the cleaned wastewater. The main part of the separated sludge, which is transported back to the aeration basin, is called return activated sludge. To reach a constant sludge age the unused biomass has to be removed from the bio lo gi cal treatment system as secondary (excess) sludge. The excess sludge contains not–hydrolysable particulate materials and biomass due to me ta bo lisms. Tertiary sludge is produced through further wastewater treatment steps e.g. by adding a fl occulation agent. Sludge is processed at the wastewater treatment plant in step, which is called sludge management. Sludge management is number of operation, (thickening, dewatering, transporting, storing, pumping, etc.), which requires sludge to be previously cha racte ri zed by reliable and demonstrative procedures. Mechanical fl ow properties should be used for sewa ge sludge characterizing, but there is some limitation, especially from hydrodynamics point of view – sludge can be treated as solid as well as liquid. The results of rheological measuring can be used as an useful tool for the characterization of sewage 288 T. Vitěz, L. Severa sludge to control sludge treatment processes, such as dewatering, stabilization and advanced processes, such as value–addition (Brar et al., 2007; Giordano et al., 2007). In fact, torque rheology was demonstrated as an appropriate tool to measure sludge fl ocs’ strength in mixing conditions, adjusting polymer dose and type to control suspension strength for optimizing sludge dewatering (Abu–Orf and Ormeci, 2005). In stabilization (aerobic and anaerobic digestion) and reutilization processes, the rheograms denoting principally two correlations: (1) shear stress as a function of shear rate and; (2) viscosity as a function of shear rate and time, describe the properties of sludge fl ow. These processes are important input design parameters in transportation, storage, operation of bioreactor and pumping (Seyssiecq et al., 2008). Viscosity, the basic rheological parameter has been demonstrated to play an important role in the increase of biodegradability of sludge, strongly in fl uencing mass transfer in aerobic degradation during digestion and fermentation (Verma et al., 2007). The aim of the present work was to study the rheolo gi cal profi le of wastewater sludge in diff erent stages of its pre–treatment. The main attention was focused on viscosity shear rate dependence and temprature dependence. Changes in viscosity, and change in dry–matter content was also investigated. The study is based on the hypothesis that treatment of wastewater sludge will modify the rheological properties of wastewater sludge to enhance the dewaterability for eventual disposal as well as assimilation of nutrients by microorganisms during valueaddition. MATERIAL AND METHODS