A Study On The Formation And Growth Pattern Of Aerobic Granules Under Different Sodium Chloride Concentrations

Aerobic granules, the compact structure of microbial aggregates, could provide aerobic, anoxic and anaerobic conditions for different bacteria and thus accomplish COD, nitrogen and phosphorous removal in a single granule. Outstanding settleability enables granular sludge to be configured as a compact wastewater treatment system, where a small or even no settling tank is need. The promising technology of aerobic granules has found many applications in domestic, phenol, and brewery wastewater treatment. However, little is known about the characteristics of aerobic granules for when applied for the treatment of high sodium chloride, which is ubiquitous in food process industry, for example. The objective of this work was to evaluate the formation and growth pattern of aerobic granules under different salinities.Aerobic granules were developed in three identical column type reactor using seed sludge from a wastewater treatment. After the granules occurred in the three reactors, sodium chloride was added in the substrates in order to evaluate the effects of salinity on the stability of aerobic granules. The granulation course and the behaviors of granules under different salinities were investigated, and the achievements in this work can be divided into two parts: (i) the mechanisms of granulation under different salinities were proposed and (ii) the growth patterns of aerobic granules and theirs effects on the stability of aerobic granular sludge were discussed.Result showed that: (i) Granules in R1, where the salinity remained at 1% exhibited the inner holes of granules. (ii) In R2 and R3, when the salinity increased from 1% to 2.5%, disintegration of granules was observed. (ii) However, in R3, when the salinity increased from 2.5% to 5%, the disintegration was stopped, and the granules became more compact. The disintegration of granules under the salinity of 2.5% may be due to the abominable conditions both for halo-tolerant and non-halo-tolerant bacteria. In all reactors, the size of granules increased and the diversity of size distribution became wider during the granulation period, and the size of mature granules distributed mainly in the range of 0.3-0.5 mm. Granules under the salinity of 1% showed high porosity and narrow size distribution and extracellular polymeric substances (EPS) linked the particles in the granules. In contrast, those under the salinity of 5% showed low porosity and wide size distribution and entanglement of filaments formed the frame work of granules. Disintegration of aerobic granules was found under the salinity of 2.5%, and the size distribution was found became wider, which may due to the unbalance growth between floc-forming and filamentous microorganisms. Therefore, it is reasonable to conclude that size distribution could effectively describe the stability of aerobic granules, namely, narrow size distribution indicates stability of aerobic granules. Furthermore, granules under low salinity were predominated by the floc-forming bacteria and thus EPS-linking was the main mechanisms of granulation, on the other hand, those under high salinity were predominated by filamentous microorganisms and thus the entanglements of filaments was the main mechanisms of granulation. Two models have been proposed for the growth of aerobic granules. One is homogenous growth of which the granule size distributes in a narrow spectrum and the granules growing with this model are stable because the conditions of growth and substrate utility are similar for all granules. Another is heterogeneous growth of which the granule size distributes in a wide spectrum and granules with this model are unstable.In summary, this work proposed two different mechanisms of aerobic granulation under different salinities, i.e., EPS-driven aggregation under a low salinity and filamentous enmeshment under a high salinity. Two models have been proposed for the growth of aerobic granules. Homogenous growth of which the granule size distributes in a narrow spectrum could be considered as a stable growth pattern of aerobic granules.