Joo-Hwa Tay

The essential role of hydrodynamic shear force in the formation of biofilm and granular sludge

Biofilm and granular sludge processes are promising biotechnology for wastewater treatment. The formation, structure and metabolism of immobilized microbial community are associated very closely with hydrodynamic shear force in reactors. Therefore, this paper attempts to review the essential role of shear force in the formation and performance of biofilm and granular sludge. More compact, stable and denser biofilms, aerobic and anaerobic granules form at relatively higher hydrodynamic shear force. It is clearly shown that shear force has significant influences on the structure, mass transfer, production of exopolysaccharides, metabolic/genetic behaviours of biofilms, aerobic and anaerobic granules. In an engineering sense, hydrodynamic shear force can be manipulated, as a control parameter, to enhance microbial granulation process. It can be concluded that the knowledge regarding the effects of hydrodynamic shear force on biofilms and granules is far from complete and much research is still needed to fully understand the relevant mechanisms. Some of these future research niches are therefore outlined.

Aerobic granular sludge: Recent advances

Aerobic granulation, a novel environmental biotechnological process, was increasingly drawing interest of researchers engaging in work in the area of biological wastewater treatment. Developed about one decade ago, it was exciting research work that explored beyond the limits of aerobic wastewater treatment such as treatment of high strength organic wastewaters, bioremediation of toxic aromatic pollutants including phenol, toluene, pyridine and textile dyes, removal of nitrogen, phosphate, sulphate and nuclear waste and adsorption of heavy metals. Despite this intensive research the mechanisms responsible for aerobic granulation and the strategy to expedite the formation of granular sludge, and effects of different operational and environmental factors have not yet been clearly described. This paper provides an up-to-date review on recent research development in aerobic biogranulation technology and applications in treating toxic industrial and municipal wastewaters. Factors affecting granulation, granule characterization, granulation hypotheses, effects of different operational parameters on aerobic granulation, response of aerobic granules to different environmental conditions, their applications in bioremediations, and possible future trends were delineated. The review attempts to shed light on the fundamental understanding in aerobic granulation by newly employed confocal laser scanning microscopic techniques and microscopic observations of granules.

Microscopic observation of aerobic granulation in sequential aerobic sludge blanket reactor

Aims: This paper attempts to provide visual evidence of how aerobic granulation evolves in sequential aerobic sludge blanket reactors.

High organic loading influences the physical characteristics of aerobic sludge granules

Aims:u2002The effect of high organic loading rate (OLR) on the physical characteristics of aerobic granules was studied.

The role of cellular polysaccharides in the formation and stability of aerobic granules

Aims:u2002This paper attempts to investigate the role of cellular polysaccharides in the formation and stability of aerobic granules.

Metabolic response of biofilm to shear stress in fixed-film culture.

Aims:u2002In a biofilm reactor, detachment force resulting from hydraulic shear is a major factor that determines the formation and structure of steady state biofilm. The metabolic response of biofilm to change in shear stress was therefore investigated.

Detachment forces and their influence on the structure and metabolic behaviour of biofilms

The biofilm reactor has been frequently applied in wastewater treatment. The formation, structure and metabolic activity of the biofilms are closely associated with the detachment forces in the reactors. This paper reviews the essential role of detachment forces in the biofilm process. A more compact, stable and denser biofilm can be formed at a relatively higher detachment force. The detachment force has significant influence on the structure, mass transfer, production of exopolysaccharides, metabolic and genetic properties of the biofilm. In an engineering sense, the detachment forces can be manipulated, as a control parameter, to produce a more stable and compact biofilm for use in wastewater treatment. The molecular and genetic mechanisms responsible for the detachment force-associated phenomena are not yet fully understood.

Microbial communities of aerobic granules: Granulation mechanisms

Aerobic granulation is an advanced biological wastewater treatment technology. This study for the first time identified the microbial communities of sliced samples of mature granules by polymerase chain reaction (PCR) amplification and denaturing gradient gel electrophoresis (DGGE) technique and those of whole growing granules by high-throughput sequencing technique. The sliced sample study revealed that mature granules have a spherical core with anaerobic Rhodocyclaceae covered by an outer spherical shell with both aerobic and anaerobic strains. The growing granule study showed that the flocculated flocs were first transited to young granules with increased abundances of Flavobacteriaceae, Xanthomonadaceae, Rhodobacteraceae and Microbacteriaceae, then the abundances of anaerobic strains were increased owing to the formation of anaerobic core. Since the present granules were cultivated from flocculated flocs, the microbial community data suggested that granules were formed via a deterministic rather than via a random aggregation-disintegration mechanism.

A kinetic model for energy spilling-associated product formation in substrate-sufficient continuous culture

It has been demonstrated that excess substrate can cause uncoupling between anabolism and catabolism, which leads to energy spilling. However, the Luedeking‐Piret equation for product formation does not account for the energy spilling‐associated product formation due to substrate excess. Based on the growth yield and energy uncoupling models proposed earlier, a kinetic model describing energy spilling‐associated product formation in relation to residual substrate concentration was developed for substrate‐sufficient continuous culture and was further verified with literature data. The parameters in the proposed model are well defined and have their own physical meanings. From this model, the specific productivity of unit energy spilling‐associated substrate consumption, and the maximum product yield coefficient, can be determined. Results show that the majority of energy spilling‐associated substrate consumption was converted to carbon dioxide and less than 6% was fluxed into the metabolites, while it was found that the maximum product yield coefficients varied markedly under different nutrient limitations. The results from this research can be used to develop the optimized bioprocess for maximizing valuable product formation.