The effect of organic shock loads on the stability of anaerobic granular sludge

This study investigated the effect of organic shock loads on the stability of anaerobic granular sludge (AnGS). At double and triple shock intensity (8 and 12 kg COD/m³d organic loading rate (OLR), respectively), the accumulation of volatile fatty acids resulted in the decreasing pH of the reactors. The unstable performances of AnGS also appeared in the macroscopic properties, including the decreasing contents of relative hydrophobicity (RH), granular strength and diameter, and the fluctuation of EPS production. Noteworthily, the contents of loosely bound and tightly bound polysaccharide increased obviously and were higher than that in the control under double and triple OLR shock load. However, the growing trend of protein content was only found in the double OLR shock reactor. In addition, the N-hexanoyl-homoserine lactone (C6-HSL) content increased following the shock and then decreased during the recovery. The autoinducer-2 (AI-2) content gradually decreased under OLR shock conditions, and then rapidly increased during the recovery phase. The behaviour of the diffusible signal factor (DSF) was opposite that of AI-2. The combination of C6-HSL, AI-2 and DSF had a degraded effect on the properties of AnGS. The results provide a new reference for further research into the inter-effects among multiple signal molecules in anaerobic sludge.

3-oxo-C6-HSL: N-3-oxo-hexanoyl-homoserine lactone; AHLs: acyl-homoserine lactone; AI-2: auto-inducers-2; AnGS: anaerobic granular sludge; C4-HSL: N-hydroxybutanoyl-homoserine lactone; C6-HSL: N-hexanoyl-homoserine lactone; COD: chemical oxygen demand; DSF: diffusible signal factor; EPS: extracellular polymeric substance; HPLC: High-performance liquid chromatography; HRT: hydraulic retention time; IC: integrity coefficient; LB-: loosely bound; MLSS: mixed liquor suspended solids; MLVSS: mixed liquor volatile suspended solids; OLR: organic loading rate; PN: protein; PS: polysaccharide; QS: quorum sensing; RH: Relative hydrophobicity; SS: suspended solids; TB-: tightly bound; UPLC-MS/MS: ultra-performance liquid chromatography-tandem mass spectrometry; VFAs: volatile fatty acids.