Sludge Properties

The activated sludge process is characterized by the formation of an activated mass of microorganisms capable of reducing and stabilizing the organic content of influent wastewater by metabolizing the nutrients and organic compounds found within it.



This activated mass of microorganisms forms aggregates known as sludge flocs. The AS sludge floc is typically composed of biotic components (bacteria, fungi, protozoa and some metazoa) and non-biological components (inorganic and organic particulates) that are embedded in a polymeric matrix or gel known collectively as the extracellular polymeric substances. Successful operation of any BNR-WWTW relies on the formation of a suitable microbial floc that affords solids separation to occur in the sedimentation tank or secondary clarifier. This floc formation is thus critical for rapid biomass separation from the treated wastewater as well as entrapment of colloidal solids and other constituents, thereby producing a clear final effluent.



Disturbance of floc formation affects the sludge settling properties in the final clarifier, resulting in an undesired high suspended solids (SS) concentration in the final effluent, as well as increased operational costs and a severe malfunction of the WWTW.



Floc formation is facilitated by the ‘sticking’ together of microbial cells (both floc forming and filamentous bacteria) through a complex process involving physical, chemical and biological phenomena. The floc structure can also be used as an indicator of process performance e.g. pin point flocs (occurring at long sludge ages and very low food to microbes ratios leading to a low sludge volume index and turbid effluent), diffuse flocs (seen in filamentous bulking conditions resulting in a high sludge volume index), and ideal flocs (filamentous bacteria conserved within flocs, are in balance with floc-forming organisms resulting in good settling in the final clarifier). In theory good settling depends primarily on the flocs’ structural properties and on the activated sludge microbial population.