Summary And Future Perspectives

Engineered microbial systems frequently encounter transient or dynamic conditions. Microbial systems where biomass form aggregates, flocs, or granules are sensitive to transients, as is the case of anaerobic and aerobic biological processes treating wastewater. Weakening of microbial aggregate structure and disintegration of bioaggregates, that is, deflocculation, appear to be common responses of suspended biomass to transient operating conditions and environmental stresses.

Transient conditions leading to poor treatment performance represent a challenge in biological wastewater treatment due to poor pollutant removal efficiency, high effluent suspended solids (ESS), and variable overall performance. Some of the impacts of transients in biological wastewater treatment systems have been identified in this chapter. For example, substrate and toxic compound perturbations have been related to high ESS, poor pollutant removal efficiencies, poor sludge settling, and sludge deflocculation. Biomass deflocculation is of particular interest because it leads to the loss of biocatalyst, the discharge of suspended solids and of solids-associated bioactive compounds. Biomass deflocculation in aerobic and anaerobic processes treating wastewater has been identified to occur under shocks of toxic compounds, low dissolved oxygen concentrations, and temperature.

Although deflocculation under different transients and stresses in biological treatment systems is becoming a well-characterized phenomenon, the actual mechanisms whereby deflocculation occurs remain little understood. Some abiotic processes leading to activated sludge deflocculation have been reported in the literature, such as the removal of Ca2+ from flocs and Fe(III) reduction to Fe(II) by the presence of sulfide or iron-reducing bacteria. Many microbially mediated mechanisms have been proposed to explain biomass deflocculation both in aerobic and anaerobic processes. These mechanisms are diverse: decreased microbial metabolism, degradation of EPS, physiological stress responses, and modified floc physico-chemical properties. However, the biotic mechanisms of sludge deflocculation remain to be proven.

Recent research in our group has shown that activated sludge deflocculation occurs due to temperature shifts in the upper limit of mesophilic treatment due to temperature shifts from 30°C to 45°C. Activated sludge deflocculation was identified to take place by the solubilization of extracellular polymeric substances (EPS) and floc fragmentation, which impaired overall treatment performance. Several mechanisms that could link the occurrence of sludge deflocculation with inhibition of metabolic activity, more negatively charged sludge, and increased maintenance respiration rates were proposed. However, the responses of aggregates of microbial communities, like that of activated sludge, to transient conditions and deflocculation need to be better understood. Important areas of future research, where work has already begun, are the link of microbial physiological stress responses with biomass deflocculation and poor treatment performance, the characterization of the chemical nature and adhesive properties of EPS in microbial aggregates under transient conditions, and the characterization of the microbial community structure of flocculent and non-flocculent biomass.

A better understanding of mixed-culture and bacterial flocculation and defloc-culation can be achieved by learning from the experience on yeast flocculation. The more fundamental understanding of how yeast aggregate and the experimental approaches used to identify the governing forces in maintaining yeast cells flocculated can serve as guidelines for improving the understanding of other microbial aggregation phenomena.

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