Several researchers observed the essential of proton translocation concept that (i) the hydrophobic interaction of a considerable extent was closely related to the initiation of bacterial adhesion; (ii) the proton conductance across a bacterial surface could induce surface dehydration; and (iii) the proton translocating activity could induce the protonation of bacterial cell surfaces. Based on these observations and a consideration of the proton translocating activity on bacterial membrane surfaces, a proton translocation-dehydration theory for molecular mechanism of sludge granulation was proposed and proved by experiments (Teo et al., 2000; Tay et al., 2000a). The theory suggests that the overall sludge granulation process in a typical anaerobic wastewater treatment system is initiated by the bacterial proton translocating activity at bacterial surfaces.
During the start-up, the substrate is fed into an anaerobic reactor which has been inoculated with seed sludge. The fermentative bacteria secrete extracellular enzymes into the medium to catalyze the hydrolysis/ acidification of the organic compounds. The compounds are degraded into volatile fatty acids coupling with the electron transport. Simultaneously, the proton pumps on the membranes of these bacteria are activated. The proton translocating activity can establish a proton gradient across the bacterial cell surface and subsequently cause surface protonation. The energized bacterial surfaces result in the breaking of hydrogen bonds between negatively charged groups and water molecules as well as partial neutralization of the negative charges on their surfaces. This in turn induces the dehydration of the bacterial surfaces.
The fermentation of complex organic compounds supplies the substrates to acetogens and methanogens and accelerates their growth and duplication. Similarly, coupling with the electron transport on their respiration chains, the acetogenic and methanogenic bacterial surfaces are dehydrated due to the presence of high-energy protons. By the action of external hydraulic forces, these relatively neutral and hydrophobic acidogens, ace-togens, and methanogens may adhere to each other to form embryonic granules due to the weaker hydration repulsion. These initial aggregates are strengthened by further dehydration of the bacterial surfaces, which results from the effective metabolites transference. Only those embryonic granules that are able to obtain energy and nutrients from the environment are selected. Moreover, this new physiological environment begins to induce the excretion of extracellular polymers (ECPs) to the embryonic granule surfaces.
Within each embryonic granule, there is an on-going methanogenic series metabolism. Distribution of each group of bacteria in the granules depends on the orientation of intermediate metabolites transference, which is believed to be the most efficient way for anaerobes to transfer their intermediates. Formation of well-organized bacterial consortia as mature granules is thus possible. Embryonic granules may also adhere to and integrate other dispersed bacteria while the original bacterial colonies
(or consortia) continue to grow and multiply. Granule maturation resists and blocks the unrestricted multiplication of bacterial cells because of space restriction for them to grow and to dispose off metabolites waste products. This space restriction and the continuous supply of substrates facilitates the production of ECP in large quantities.
The bacterial proton translocating activity in mature granules keeps the bacterial surfaces at a relatively hydrophobic state. Maintenance of the structure of mature granules is governed by the mechanism of proton translocation-dehydration. On the other hand, an ECP outer layer causes the hydration of the granule surface, which protects the granule against attachment to gas bubbles and shear stress existing in the UASB reactor.
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