Formation, Stability And Biodiversity Of Aerobic Granules In Sequencing Batch Reactors Treating Landfill Leachate

The technology of aerobic granular sludge has the advantages of high biomass content, high biomass retention, strogent ability to withstand shockloadings, and capacity of simultaneous nitrogent and phosphorus removal; it is a promising option for efficient wastewater treatment. Information on the aerobic granulation with landfill leachate is very limited. Lab-scale SBRs, fed with landfill leachate, were used in this study to investigate the formation, salt-resistance performance, stability of granules, as well as nitrogen removal performance under different influent NH4+-N concentrations. Moreover, the variations of aerobic granule properties and microbial population structure were paid more attention.Conventional activated sludge was inoculated in the SBR treating landfill leachate. Through step-rising of volumentic exchange ration from 30% to 50%, granules were cultivated successively in 50 days, and showed excellent settleability, high microbial activity and salt-resistance ability. In addition, the aerobic granulation process was accompanied with increments of extracellular protein content (PN) and surface relative hydrophobicity (SRH), which revealed the close relationships between PN and SRH with aerobic granulation.After a while of stable operation period, due to the disorder of balance between hydraulic and substrate selection pressures, a great quantity of aerobic granules began to break down into flocculent sludge. PAC was used to regulate the hydraulic selection pressure and strengthen the stability of aerobic granules. Experimental results showed that PAC addition could enhance granule strength and specific gravity (wet density of mature granules with PAC addition was1.070g/cm3), slow down the sludge growth rate and yield rate (observed yield coefficient Yobs of mature granules was only 0.103mgVSS/mgCOD), moderate the sludge concentration and size (average diameter was 0.36~0.60 mm), and reduce the dispersed tread of size distribution, so as to avoid internal segmentation of granule.High concentration of influent NH4+-N led to high concentration of free ammonia (FA) in the GSBR, which could inhibit the activity of functional bacteria, especially nitrite oxidizing bacteria (NOB). It finally resulted in the accumulation of NO2-. When the influent NH4+-N was around 788mg/L, partial accumulated NO2- was transformed to N2. Considering the partial nitrification and denitrification (PND), as well as the anaerobic ammonium oxidation (ANAMMOX) process during feeding time, TN removal efficiency in the whole cycle was 58.1%. When the influent NH4+-N was 1165mg/L, high FA concentration inhibited the activities of both ammonium oxidizing bacteria (AOB) and NOB, TN and NH4+-N removal efficiencies were only 35.0 and 39.3%, repectively. Magnesium ammonium phosphate (MAP) sedimentation process was used as pre-treatment for ammonia-nitrogen removal, and influent TN and NH4+-N of GSBR was greatly reduced. When the influent TN and NH4+-N were 399 and 366mg/L, nitrogen was mainly removed in means of complete nitrification. Coupled with other nitrogen removal pathways, like partial nitrification and denitrification, the TN and NH4+-N removal efficiencies were 75.4% and 95.6%, respectively.During the stable period of GSBR treating landfill leachate, aerobic granules were stable with dense structure, SVI5min of 38mL/g, average Zone Settling Velocity (ZSV) of 24m/h, biomass density of 28gVSS/L, sludge strgenth (in terms of integrity coefficient) of 96.3%, and water content of 97%. The granules were mainly composed of bacillus, as well as some protozoa and metazoan, like vorticella. Granules with the size of 0.20~0.60mm were the main part of the sludge. Multi-valence Ca, Fe, and Cu were the main metal elements of granules. Especially, Ca in the form of CaCO3 largely accumulated in the core of granules. While Na, K, and Mg were the main cations in the extracellular polymer substance (EPS), in comparison with the low content of Ca.(Nested) PCR-DGGE technologies were used to investigate the microbial community structure of GSBR during granulation process and other operation periods under different influent nigtrogen. The results showed that the biodiversities of total bacteria, AOB and NOB were low during the start-up period, when dinitrfiers were not discovered. With the improvement of sludge settleability and formation of aerobic granules, dinitrfiers appeared, and the biodiversities of each sort of bacteria steadily rose up. Especially the increment of AOB biodiversity and reproduction of AOB represented by Band f in the DGGE profile had significant positive effects on NH4+-N removal. When influent NH4+-N concentration increased, corresponding high FA content in the GSBR inhibited the activity of most functional bacteria, especially NOB. This situation was connected with"partial nitrification"performance. After pre-treatment by MAP, influent NH4+-N concentration decreased, bacteria biodiversity in the GSBR was stable, contributing to the efficient and steady pollutants removal. Most predominant species in the system were uncultured bacteria, the homology of these bacteria were more than 91% compared with Arcobacter butzleri, phenol-degrading bacterium, bacterium response to aromatic contamination and comamonadaceae as determinants of activated sludge settling performance. The main AOB species were Nitrosomonas, Beta-proteobacterium. Most predominant NOB were uncultured bacterium and Rhizobium sp., the homology of these bacteria were more than 98% compared with the halotolerant nitrite oxidizing bacterium, commercial nitrifying inoculum, and bacterium response to simultaneous phosphorus and nitrogen removal. Two isolated denitrifier strains were Brachymonas denitrificans.