| Aerobic granular sludge is a process of cell self-immobilization in the presence of oxygen and is a special case of biofilm.Aerobic granular sludge has such advantages as high biomass retention time,high density and high settling velocities.It can be used in the treatment of wastewater with high organic loading rates or biological removal of nitrogen,phosphorus or toxic substances.Recently a filamentous aerobic granular sludge formed in a laboratory SBR treating sorbitol wastewater in our laboratory,which showed good settling performance and effluent quality.A fungal strain THM was isolated from this granule,which could also form aerobic granular sludge with aeration.Therefore,the aim of this study was to further investigate the microscopical structure and other characteristics of aerobic granular sludge and to disclose the mechanism on the granule formation.This paper also explored the optimized engineering condition and system stability of the sorbitol wastewater treatment system by using THM aerobic granular sludge.Results showed that the single strain THM formed aerobic granular sludge in a sequencing batch reactor(H/D=2:1)within 5-7 days,which was black,in round or elliptic shape.The THM granule was 2-3 mm in diameter and had a compact structure with villiform edge.SEM observation showed that the dominant microorganisms were filamentous fungi with bacterial cells and abundant extracellular polymeric substance attached on the surface of hypha.The sedimentation velocity of THM granule was as high as 114 m/h.Other characteristics were as follows:moisture 97.98%,wet density 1.006 g/cm3,the sludge gravity at 30? 1.0299,the granule stabilit 94.09%,the organic loading rate of sludge 1.11 Kg COD/(Kg MLSS·d)and dehygrogenase activity was 25.79?g TF-mg-1·h-1.Viable counts of bacteria and fungi in THM granular sludge were 1.15×1011 cfu/gMLSS and 1.49×109 cfu/gMLSS respectively.The main species of bacteria were Rhodobacter,Acidovorax and Sphingomonas.The major components of EPS in THM aerobic granular sludge were polysaccharide and protein,whose contents were 53.33 mg/gMLSS and 25.95 mg/gMLSS respectively.However,the EPS content of pure THM granule cultivated with sterile aeration was very low,indicating that EPS in THM granular sludge was from bacteria.Therefore it could be concluded that the formation process of THM aerobic granular sludge was as follows:With the condition of water shear force,THM hypha enwinded each other and formed a small granule as nucleus,which became bigger and bigger with the hyphal growth.Meanwile a few bacteria in air and/or influent cohered on the surface of THM hypha,secreting abundant extracellular polymeric substances which facilitated to connections between bacteria and hyphae or between THM hyphae,leading to the formation of a compact aerobic granular sludge.THM aerobic granular sludge had good performance in the treatment of sorbitol wastewater,which could work stably when the volumetric loading rate was?1.2 kg COD/(m3·d).The COD and SS concentration of the effluent was lower than the limits regulated in national discharge standards.As a biological process,granular sludge could be easily influenced by various engineering conditions and environmental factors.Therefore we also studied the effects of organic loading rate,pH,temperature and Ni2+ ion on THM granular sludge system during the treatment of sorbitol wastewater.The optimized process condition was as follws:volumetric loading rate?1.2 kg COD/(m3·d),pH 6.0?8.0,temperature at 20?35?,Ni2+concentration?2.5 mg/L.The process stability of THM granular sludge system was alsao studied using the sorbitol wastewater as the influent at different shock loads.Results from the present study showed quite high process stability since it showed high tolerance to the following conditions:organic load 2.4 kg COD/(m3·d),pH range 4.0?10.0 and Ni2+40 mg/L. |
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