Activated sludge systems for wastewater treatment: Relation between floc morphological properties and solids retention time
The objective of this work was to study the associations between the morphological properties of the biological flocs and the performance of an activated sludge system, as a function of the solids retention time. An image analysis protocol was developed to measure in a repeatable way the morphological properties of biological flocs. A pilot-scale activated sludge system fed with a synthetic wastewater was then implemented. Three values of solids retention time were tested and the following characteristics of the sludge were measured: effluent COD (chemical oxygen demand), suspended solids, sludge volume index, COD removal efficiency, and volume distributions of size and circularity of the flocs. Although the concentration of suspended solids and the removal efficiency were not sensitive to an increase in the solids retention time, the sludge volume index did seem to be sensitive to this variable, leading to a decrease of the sludge settleability. Moreover, when the solids retention time was increased, the size and the circularity of the biological flocs increased temporarily before recovering their initial values. Under steady-state conditions, the size and circularity distributions seem to be independent of the retention time of solids.