17.1 Introduction 352

17.1.1 Definition of Terms 352

17.1.2 Objectives and Scope of the Chapter 353

17.1.3 Why Should Biomass Deflocculation under Transient

Conditions Be Studied? 353

17.2 Transient Conditions and Stresses in Engineered Biological Treatment Systems 354

17.2.1 Carbon Substrate Transients 354

17.2.2 Periodic Substrate Oscillations 356

17.2.3 Environmental Transients 357 Dissolved Oxygen (DO) Transients 357 Toxicity and pH Transients 357 Temperature Transients 358

17.3 Biomass Deflocculation under Transients or Stresses in Biological Treatment Systems 362

17.3.1 Transients Leading to Biomass Deflocculation 362

17.3.2 Mechanisms of Biomass Deflocculation 363

17.4 Mechanisms of Activated Sludge Deflocculation under Mesophilic-Thermophilic Temperature Transients 367

17.4.1 Sludge Floc Responses to the Temperature Shifts 367

17.4.2 Proposed Mechanisms of Sludge Deflocculation 372

17.5 Summary and Future Perspectives 374

References 376

©2005 by CRC Press 351

Biological systems are subject to transient conditions. For instance, natural and designed microbial systems commonly undergo variations in the environmental conditions to which microbial biomass is exposed. Engineered biological treatment systems, although generally designed to operate under steady-state or stable conditions, experience environmental transients that can compromise their performance. Loss of the microbial aggregate structure and disintegration of bioaggregates appear to be common responses of suspended biomass to transient operating conditions and environmental stresses. These changes can be related to inhibition of metabolism, to physiological stress responses, and to physico-chemical abiotic effects.

This chapter reviews common transient conditions and stresses in microbial systems engineered for the treatment of wastewater, including aerobic and anaerobic wastewater treatment processes. Emphasis is given to technologies with biomass in suspension where bioaggregate disintegration, that is, sludge deflocculation, has been identified to occur under microbial physiological stresses and transient conditions, and where biomass deflocculation poses challenges to the operation of the systems. Special attention is given to activated sludge deflocculation under mesophilic-thermophilic temperature transients, as part of recent work in our research group. Mechanisms leading to biomass deflocculation or weak bioaggregate structure are reported, and proposed for the case of mesophilic-thermophilic temperature upshifts in order to explain experimental observations.

17.1 INTRODUCTION 17.1.1 Definition of Terms

For the purpose of this chapter, transient conditions or transients refer to dynamic, variable, or shifting conditions. The term deflocculation is used to describe the process of disintegration of microbial mass aggregates. Biomass aggregates or flocs are terms used to describe the physical entities formed by the gathering together of microorganisms under physiological conditions1 in engineered systems. Therefore, for the purpose of this chapter, the term flocs encompasses other terms generally applied to similar structures: pellets, granules, clusters, agglomerates, with or without the prefix bio. Deflocculation is defined as the coming apart of the constitutive elements of a microbial aggregate that renders it smaller or disperses it into solution. The terms deflocculation, floc disintegration, and disaggregation are synonymous in this work. Nonetheless, it is important to differentiate the specific processes whereby deflocculation may occur.

The overall term of deflocculation can be understood as the process of sludge floc destruction and size reduction involving floc fragmentation or floc erosion. Floc fragmentation is the fracture of flocs into smaller pieces that lead to more exposed floc surface area, and floc erosion is the detachment of small particles or components from flocs under hydrodynamic, shear stress.2,3 The term sludge deflocculation has been more widely used in the literature on biological wastewater treatment to account for both floc fragmentation and erosion. Floc erosion appears to govern the generation of colloidal particles causing increased effluent turbidity and suspended solids under shear stress.2,4 The preference to use deflocculation in the literature may arise from the inability to separate the distinct phenomena of floc fragmentation and floc erosion, especially when studying floc stress that is not due to hydrodynamic shear.

Overall, the processes of biomass aggregation and disaggregation are considered a dynamic equilibrium. Sludge flocs appear to be under a dynamic equilibrium between flocculation (aggregation) and deflocculation (disintegration),2,4,5 and also more specifically under an equilibrium between adhesion and erosion.2,3

17.1.2 Objectives and Scope of the Chapter

Although transients are not always linked to stresses, this chapter focuses on common transients that disturb the operation of engineered microbial systems designed and intended to deliver a steady-state performance. The chapter is aimed at compiling the available literature on the effects of transients in biological wastewater treatment systems, both aerobic and anaerobic, where deleterious effects from transient conditions are commonly encountered. More specifically, the chapter identifies the different types of transient conditions that act as physico-chemical or physiological stresses impairing proper biological wastewater treatment performance due to biomass deflocculation. This chapter also focuses on deflocculation of bacterial- or microbial-community aggregates, resulting from reduced floc stability or weakened floc structure.

Hydrodynamic stress leading to biomass disaggregation is not reviewed in this work. Although some types of stresses and transients may directly lead to microbial floc disaggregation, other types may only impair floc stability or weaken bioaggregate structures. It is the hydrodynamic stress to which the bioflocs are normally exposed that erodes and fragments the weakened microbial aggregates. The impacts of hydro-dynamic stresses on deflocculation are better understood, and have been studied in nonbiological aggregates6,7 and in some microbial aggregates, such as activated sludge flocs.2-4 The effects of hydrodynamic stresses like extensional deformation, rotation, and translation on microbial flocs are not considered in this work, but remain an area where comprehensive understanding may still be required.8,9

The impact of transient conditions on the deflocculation of aggregates of filamentous fungi, cellular slime moulds, algae, or eukaryotic microbes are not covered since this has not been reported to pose a problem in the operation of engineered microbial processes. Deflocculation of yeast aggregates has been reviewed elsewhere as part of yeast flocculation reviews.1

17.1.3 Why Should Biomass Deflocculation under Transient Conditions Be Studied?

The interest in identifying and understanding the factors causing microbial disag-gregation and the mechanisms of deflocculation has arisen from the need to operate biological processes more robustly, that is, with constant outputs that comply with stringent quality requirements. Biomass deflocculation poses a complex challenge to biological treatment since it leads to the loss of biocatalyst used in pollutant degradation, to solids discharges, and to the discharge of bioactive compounds adsorbed onto biomass. A better understanding of what causes deflocculation and how different factors affect this process will permit its control. These ideas have motivated a great deal of research in the area of microbial aggregation and adhesion, for which there have been many reviews.1'10-12 Nevertheless, the process of microbial disaggregation in biological treatment processes has received little attention. The literature on microbial deflocculation in biological treatment under different stresses is relatively recent (1995 to the present), with some sporadic studies before 1995, especially in aerobic treatment. The increase in the number of more recent studies in this area responds to the increasing necessity to control reliably biological treatment systems as they are pushed to their limits.

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