The Role of Floc Morphology and Composition on Susceptibility of Biomass to Shock Loads of Chemical Toxins

The structure and composition of activated sludge flocs has been extensively studied and characterized, leading to a better understanding of its influence on bioflocculation, settling and dewatering properties of biosolids. Microscopic observations have shown that a floc is a multilevel porous structure in which macroflocs are formed by smaller aggregates or microcolonies, believed to result from the division of microorganisms. The cohesion of this multilevel structure is dictated by many factors, such as internal hydrophobicity, extracellular polymeric substances (EPS) composition, and multivalent cation bridging. It is believed that EPS, which is mainly composed of protein and carbohydrate polymers, forms a gel-like matrix in which microcolonies are embedded through both electrostatic and hydrophobic interactions.Previous work in our laboratory has studied the process effects of shock loads of chemical toxins on the activated sludge process, using laboratory-scale sequencing batch reactors (SBRs). One of the main process upsets observed for at least five of the chemicals tested – cadmium, 1-chloro-2,4-dinitrobenzene, 2,4-dinitrophenol (DNP), octanol and pH 11 shock – was an increase in effluent total suspended solids (TSS) as compared to a control. This effect occurred very rapidly, as it was noticed for all the toxins starting on the first cycle immediately after the shock load. Two different solids retention times (SRTs) were used in these studies and showed that the sensitivity of different biomasses to the toxins was distinct, both in the degree of TSS increase relative to a control and in the level of toxin-induced respiration inhibition.We believe that the complex composition and structure of activated sludge flocs is one of the factors that may impact the tolerance of activated sludge to stress and that the presence of EPS may protect the bacterial cells against toxins. Other factors such as floc size and surface area may also play a significant role in defining the susceptibility of activated sludge to toxic shock.Activated sludge from a full-scale facility was exposed to intense shear (10 and 30 seconds) to break up the floc particles, thereby altering the Particle Size Distribution (PSD) of the sludge. The inhibition induced by cadmium, DNP and N-ethylmaleimide (NEM) for the two sheared and non-sheared sludge was evaluated using Specific Oxygen Uptake Rate (SOUR) assays. PSD analysis showed that the PSD did not change with time of shearing (median floc diameters were about 30-40 μm for both 10 and 30 second shear); however, soluble protein and polysaccharide analysis of both sheared activated sludges indicated that a significant amount of EPS material was released when longer shearing periods were applied. The respiration inhibition data suggest that EPS may play a role in the protection of flocs against some chemicals, as seen by a moderate to slight increase in inhibition for cadmium and DNP, respectively, for the 30 second-sheared sludge relative to the 10 second and non-sheared sludges. The characteristics of the toxin, such as solubility, octanol-water partition coefficient (Kow), tendency to become floc-associated and mode of action also seemed to influence the inhibitory response and the effects at the floc level.