In order to relieve water stress in coastal regions, the extensive utilization of seawater results in a large amount of saline wastewater, which has become a new challenge for conventional biological nutrients removal (BNR) processes. In addition, the majority of BNR processes are threatened by the gradual decrease of COD/N ratio in domestic sewage and the low COD/N ratio wastewaters generated from industry. Therefore, a cost-effective technology for treating abovementioned emerging wastewaters with high salinity and low COD/N ratio is highly desired. Aerobic granular sludge (AGS) offers a promising alternative for the conventional activated sludge process because of its compactness and energy efficiency in the simultaneous removal of chemical oxygen demand (COD), nitrogen (N) and phosp...[ Read more ]

In order to relieve water stress in coastal regions, the extensive utilization of seawater results in a large amount of saline wastewater, which has become a new challenge for conventional biological nutrients removal (BNR) processes. In addition, the majority of BNR processes are threatened by the gradual decrease of COD/N ratio in domestic sewage and the low COD/N ratio wastewaters generated from industry. Therefore, a cost-effective technology for treating abovementioned emerging wastewaters with high salinity and low COD/N ratio is highly desired. Aerobic granular sludge (AGS) offers a promising alternative for the conventional activated sludge process because of its compactness and energy efficiency in the simultaneous removal of chemical oxygen demand (COD), nitrogen (N) and phosphorous (P) from municipal and industrial wastewaters. However, the efficient start-up and stable operation of AGS remain the critical issues. In realising such economical and sustainable system, this study extends the application of AGS for treating emerging wastewaters. This study mainly focuses on the following: (1) evaluation of the feasibility of an aerobic granular system for treating emerging or challenging wastewaters; (2) assessment of strategies for the efficient start-up and stable application of AGS in emerging wastewaters; and (3) investigation of the mechanisms of aerobic granulation in emerging wastewaters.

Aerobic granules were cultivated in lab-scale sequencing airlift batch reactors (SABRs) and operated under the COD/N ratios of 1, 2 and 4 (mg COD/mg N). The physical properties and nitrification efficiency of aerobic granular system deteriorated while the COD/N ratio decreased from 4 to 1, which could be ascribed to the sharp reduction of filamentous bacteria in the granules and the low production of polysaccharide in the extracellular polymeric substances (EPS). In these cases, a synchronous reduction of tyrosine in the EPS was detected with the decrease of COD/N ratio despite the vague role of L-tyrosine in the aerobic granulation. The functions of L-tyrosine in the development and stable operation of AGS under the COD/N ratio of 1 were further proved by accelerating the formation of aerobic granules within 11 days and maintaining the stable operation of the AGS for more than 6 months. The mechanisms behind the abovementioned phenomena were correlated with the stable secretion of EPS and enrichment of genera related to the secretion of quorum sensing auto-inducers and filamentous bacteria in the AGS.

Secondly, in order to develop the AGS for treating saline wastewater, five SABRs were employed with corresponding synthetic saline wastewaters, which featured different seawater percentages of up to 100%. The granulation and stable operation of the AGS were achieved in all reactors. Nitrification efficiencies were initially inhibited by high salinity but improved after one month of operation in all cases. This effect was attributable to the microbial acclimatization and growth of ammonia-oxidising archaea in the aerobic granules. The aerobic granulation process could be effectively accelerated by seawater mixing. The full granulation of the aerobic granules was completed in one week with pure seawater fed; this period was over 50% shorter than the fastest formation reported. Based on changes in the physiochemical and microbial characteristics of the AGS in the saline wastewater treatment, the mechanisms of aerobic granulation were further explored and summarised as a three-step process: 1) the electrostatic interaction on the cell surface was neutralized by the absorption of cation on the sludge surface; 2) the production of alginate-like exopolysaccharides improved the adhesion and aggregation process; and 3) the accumulation of inorganic precipitates in the granules further enhanced the strength and density of the AGS.

In this study, the aerobic granular system was applied for treating emerging wastewaters with low COD/N ratio (< 4) and high salinity. The changes in the physiochemical and biological characteristics of the AGS were recorded, thereby providing necessary knowledge for the full-scale application of the AGS. In addition, the novel strategies for rapid granulation suggested in this study serve as viable solutions for various issues (i.e., efficient start-up and stability) and extend the application of aerobic granules. The mechanisms of aerobic granulation suggested in this study can serve as a reference in the future studies.