Microbial self-aggregation, in which microbial cells are organized into dense and fast settling granules with a diameter from 0.5 to 10 mm, is extensively studied due to its practical importance in both anaerobic and aerobic biological wastewater treatment. Anaerobic and aerobic microbial granules have different properties and applications and are considered separately in this book.
Formation of anaerobic granules is discussed in Chapter 1. There are many theoretical explanations, which must be taken into account in practical performance of granular anaerobic wastewater treatment. It is favorable for the microorganisms to be very close to each other in the granule in order to achieve high substrate conversion rate. Possible advantages of microorganisms in anaerobic granule in comparison with flocculated or suspended microorganisms are as follows:
1. aggregation leads to heterogeneous community and facilitates syn-trophic relationships, especially interspecies hydrogen and formate transfer;
2. granulation protects cells from predators, such as anaerobic ciliates;
3. under unfavorable conditions for growth (e.g. extreme pH), a more favorable micro-environment can be maintained within the aggregates so that metabolism can be sustained;
4. the diffusion of substrates and fermentation products can be facilitated due to the formation of the channels in the granule.
Most valuable data for the practice are given in Chapter 2, where the effects of such factors as temperature, pH, upflow velocity, hydraulic retention time, organic loading rate, and type of substrate on anaerobic granulation are described. The real applications of anaerobic granulation are described in Chapter 3. The reader can find the description of granulation process in upflow anaerobic sludge blanket reactor (UASB), expanded granular sludge bed reactor (EGSBR), hybrid anaerobic reactor (HAR), anaerobic continuous stirred tank reactor (ACSTR), anaerobic baffled reactor (ABR), anaerobic sequencing batch reactor (ASBR), and anaerobic migrating blanket reactor (AMBR). The main problem associated with the granular sludge systems is the long start-up period required for the development of anaerobic granules. In cases where a reactor is seeded with flocculant sludge, obtained from municipal wastewater sludge digesters, it usually takes several months or even a much longer period before the system can be operated. In order to reduce the lengthy start-up of granular sludge-based systems, technologies for enhanced and rapid production of anaerobic granules are highly desirable and sought after. Another possibility of rapid start-up is the use of granular sludge from in-operating reactors as the seeds. This has the advantage of being able to achieve the desired performance within a short start-up period. However, the availability of granular seed sludge is limited, and the costs for purchase and transportation of the seeds can be high.
A major part of this book is devoted to aerobically grown microbial granules, which can be used or are used in the wastewater treatment. Advantages of aerobic wastewater treatment using microbial granules instead of conventional flocs of activated sludge are retention of granulated biomass in a reactor, diversity of physiological functions of microorganisms in the granule, and resistance of the microorganisms inside the granule to toxic substances. Aerobic granulation is a gradual process from seed sludge to compact aggregates, further to granular sludge, and finally to mature granules. To accelerate industrial application of the aerobic granulation technology, a sound understanding of the mechanisms behind aerobic granulation is highly desirable. Mechanisms of granulation and factors affecting aerobic granulation are discussed in Chapters 4 and 5. Such aspects of microbial self-immobilization as hydrophobic interactions, role of exopolysaccharides and other exopolymers in aerobic granulation, role of hydrodynamic shear force and selection pressure, substrate composition, organic loading, feast-famine regime, feeding strategy, concentration of dissolved oxygen, reactor configuration, solids retention time, cycle time, settling time, and exchange ratio are discussed in these chapters. In sequencing batch reactor, three major factors of selection pressure had been identified: the settling time, the volume exchange ratio, and the discharge time.
Aerobic granules, which are usually spheres or ellipsoids with size from 0.2 to 7 m have complex structure including radial inclusions, concentric layers, and central core. The granules are covered with filamentous, smooth, or skin-like surface, which is dominantly hydrophobic or hydrophilic. The interior of a granule is gel-like matrix, containing black matter or gas vesicule in central part of a big dense granule. There were found layers and microaggregates of specific microorganisms connected with the channels facilitating diffusion of substrates and products of metabolism. There are a layer of anaerobic bacteria and a core of lysed biomass in the central part of aerobically grown microbial granules. These structural elements of the granules together with the principles of structural optimization are described in Chapter 6.
Microbial diversity of aerobic granules, described in Chapter 7, was studied using cloning-sequencing method, amplified ribosomal DNA restriction analysis (ARDRA), and fluorescence in situ hybridization (FISH) with specific oligonucleotide probes. The analysis of the micro-bial community residing in the aerobically grown granule can provide information on the microorganisms responsible for granule formation, maintenance, and activity. This knowledge can be used to better the control of aerobic granulation. Data on physiological diversity, first of all, on the presence of aerobic, facultative-anaerobic and anaerobic microorganisms in the granules, were derived from identification of major microbial components of the granules. The important aspects of microbiology of microbial granules are presence of pathogens, determining biosafety of the wastewater treatment, and gliding bacteria, which are probably important microorganisms for the formation and stability of the granules.
One of the main problems of environmental engineering is removal of phosphate and ammonia/nitrate from the wastewater. Aerobically grown microbial granules are able to remove nitrogen and phosphorus from the wastewater as shown in Chapter 8. The problems encountered in the suspended growth nutrient-removal system, such as sludge bulking, large treatment plant space, washout of nitrifying biomass, secondary P release in a clarifier, higher production of waste sludge, would be overcome by developing N-removing and P-accumulating granules. A more compact and efficient granule-based biotechnology would be expected for high-efficiency N and P removal.
Together with the removal of nutrients, aerobically grown microbial granules can be applied for the biodegradation of toxic organic compounds. Advantages of microbial granules in the treatment of industrial toxic wastewater, containing phenol, are discussed in Chapter 9. Structure of these granules, their microbial content, and its response to the load of phenol are discussed aiming to find optimal strategy for the treatment of toxic wastewater with microbial granules.
One potential disadvantage of aerobic granulation is the long start-up period of granule formation from the flocs of activated sludge. Another potential disadvantage is the risk of accumulation of pathogenic microorganisms in the granule because of two reasons: 1) cells are aggregated mainly due to hydrophobic interactions and there may be accumulation of strains with high cell hydrophobicity in the granule; 2) bacterial strains with high cell surface hydrophobicity are often pathogenic ones. Addition into the reactor safe microbial cultures selected for fast formation of the granules can be used to solve these problems. Chapter 10 is devoted to the selection and use of microbial seeds (inoculum) to start-up safe granulation process. Different principles can be used in selection: strong self-aggregation of cells of one species; coaggregation of cells of different species; enrichment culture of fast-settling cells, or cells with high cell surface hydrophobicity. As shown in this chapter, application of microbial seeds for granulation can reduce start-up period from 14-21 to 2-7 days.
The conventional methods for heavy metal removal from aqueous solution include precipitation with lime or other chemicals, chemical oxidation and reduction, ion-exchange, filtration, electro-chemical treatment, reverse osmosis filtration, evaporative recovery, and solvent extraction. However, when the heavy metal concentrations in the wastewater are low, these processes would have some problems of incomplete heavy metal removal, high reagent or energy consumption, generation of toxic sludge or other wastes. Aerobic granules with strong and compact micro-bial structure would be a novel biosorbent for metal ion removal from a liquid solution. Biosorption of soluble heavy metals by aerobic granules is described in Chapter 11.
Mechanisms of aerobic granulation are finally not known. Physiological and biological diversity of the granules must be studied in more detail to understand the formation and functions of the granules. Such importance for the practical application property as granules stability was not explained yet in terms of mathematical model and reliable prediction. Microbial inoculum of fast-aggregating cells can be used for the facilitation granulation but biosafety, activity of pure cultures, and their domination in the granules must be studied in practical applications.
The book is covering almost all aspect of formation and use of micro-bial granules in the wastewater treatment. The data on aerobic microbial granulation are related mostly to laboratory systems because there are just few pilot systems in the world using aerobic microbial granules and there is no one constructed industrial facility using aerobic microbial granulation yet. However, by the analogy with anaerobic granulation which is used now worldwide, it would be possible to predict wide applications of aerobic granulation. The authors hope that this book will help researchers and engineers to develop these new biotechnologies of wastewater treatment based on aerobic granulation.
Joo-Hwa Tay Stephen Tiong-Lee Tay Yu Liu Kuan-Yeow Show Volodymyr Ivanov
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