Rheological characterization of blends of primary and secondary sludge

An understanding of sludge rheology during anaerobic digestion can improve digester performance, particularly with regards to mixing. However, the "black box" nature of digesters limits the knowledge on sludge flow behaviour, once it enters the digester. A study on the flow properties (i.e. apparent viscosity and yield stress) as well as the yielding and restructuring behaviour of primary, secondary and blends of primary and secondary sludge was carried out. Primary and secondary sludge feeds to the anaerobic digester were obtained from the Mount Martha wastewater treatment plant, Mornington Peninsula (Victoria, Australia). The Herschel-Bulkley model was used to determine flow properties of these individual sludge feeds at approximately same total solids concentration (5% TS). Creep tests (below and above yield stress) and shear stress sweep tests (at various times of rest from 60 to 3600s) were conducted to study the yielding and restructuring behaviour of the individual sludge types. Primary sludge (5% TS) was then mixed with secondary sludge (5% TS) at different ratios (0 - 80%) and their flow properties analyzed at 20 oC as sludge blends, mimicking conditions within an anaerobic digester. The Herschel- Bulkley model was used to model their flow properties. It was shown that primary sludge acted as a jammed material whose rheological parameters (apparent viscosity, yield stress and shear thinning properties) were lower than secondary sludge at approximately the same total solids concentration. The viscosity, yield stress and shear thinning behaviour of blends of inlet sludge feed increased with increasing ratio of secondary sludge to primary sludge.