Biofilm sloughing in integrated fixed-film activated sludge (IFAS) systems. Biofilm Sloughing in IFAS system

Biofilm sloughing is an integral aspect, and a dynamic activity of biofilm development. This study focused on biofilm sloughing on carriers in integrated fixed-film activated sludge (IFAS) systems in both laboratory (LS; synthetic feed) and full-scale (FS; municipal) systems. During biofilm development in the LS system the relative hydrophobicity of flocs and biofilm increased with an increase in carrier bound biomass. There was a decline in relative hydrophobicity of the flocs and attached biomass upon sloughing. The protein to polysaccharide ratio in the extracted EPS of the suspended biomass (flocs and sloughed biomass) and biofilms decreased and increased, respectively, which corresponds to confocal imaging showing that micro-colonies with polysaccharide rich content sloughing from the biofilms. Sloughed biomass rapidly associated with flocs to form large and compact structures that led to a decrease in SVI from 60-70 ml/L to 34 ml/L. The α and β diversities determined by 16S rRNA gene sequencing (Illumina) of FS samples revealed greater diversity of the biofilm community compared to flocs. The α diversity of biofilms and suspended solids decreased and increased, respectively, during sloughing. The shift in community structure was in part due to the loss of Comamonadacae from the biofilm. At the class level, after biofilm sloughing, Alphaproteobacteria, Gammaproteobacteria, Flavobacteria, and Acidobacteria-6 dominated on the remaining biofilm. The relative abundance of Rhodocyclacae decreased in biofilms and flocs, after biofilm sloughing. Live-dead staining revealed areas of the biofilm where viability of biomass is an additional factor in sloughing.

生物膜脱落是生物膜发育过程中不可或缺的组成部分，亦是一项动态活跃的生理过程。本研究聚焦于一体化固定膜活性污泥（integrated fixed-film activated sludge, IFAS）系统内载体表面的生物膜脱落现象，研究对象涵盖实验室规模（LS；采用合成进水）与实际工程规模（FS；处理市政污水）两类系统。在LS系统的生物膜发育过程中，絮体与生物膜的相对疏水性随载体附着生物量的增加而升高；当发生脱落时，絮体与附着生物量的相对疏水性则出现下降。悬浮生物量（絮体与脱落生物量）与生物膜的提取胞外聚合物（extracellular polymeric substances, EPS）中蛋白质与多糖的比值分别出现下降与上升，这与共聚焦显微镜成像结果相符：成像显示富含多糖的微菌落从生物膜上脱落。脱落的生物量会快速与絮体结合，形成大而致密的结构体，使得污泥体积指数（sludge volume index, SVI）从60~70 mL/L降至34 mL/L。通过Illumina平台的16S rRNA基因测序对FS样本进行分析后发现，生物膜群落的α与β多样性均高于絮体群落。在脱落过程中，生物膜的α多样性出现下降，而悬浮固体的α多样性则有所上升。群落结构的转变在一定程度上源于生物膜中丛毛单胞菌科（Comamonadaceae）的流失。在纲水平上，生物膜脱落后剩余生物膜的优势类群为α-变形菌纲（Alphaproteobacteria）、γ-变形菌纲（Gammaproteobacteria）、黄杆菌纲（Flavobacteria）与酸杆菌-6纲（Acidobacteria-6）。生物膜脱落后，红环菌科（Rhodocyclaceae）在生物膜与絮体中的相对丰度均出现下降。死活染色结果显示，生物膜中存在部分区域，其中生物活性是影响脱落过程的另一项关键因素。