Effects of alkalinity on interaction between EPS and hydroxy-aluminum with different speciation in wastewater sludge conditioning with aluminum based inorganic polymer flocculant

Extracellular polymeric substances (EPS) form a stable gel -like structure to combine with water molecules through steric hindrance, making the mechanical dewatering of wastewater sludge considerably difficult. Coagulation/flocculation has been widely applied in improving the sludge dewatering performance, while sludge properties (organic fraction and solution chemistry conditions) are highly changeable and have important effects on sludge flocculation process. In this work, the alkalinity effects on sludge conditioning with hydroxy-aluminum were comprehensively investigated, and the interaction mechanisms between EPS and hydroxy-aluminum with different speciation were unraveled. The results showed that the effectiveness of hydroxy-aluminum conditioning gradually deteriorated with increase in alkalinity. Meanwhile, the polymeric hydroxy-aluminum (Al-13) and highly polymerized hydroxy-aluminum (Alm) were hydrolysed and converted into amorphous aluminum hydroxide (Al(OH)(3)), which changed the flocculation mechanism from charge neutralization and complexing adsorption to hydrogen bond interaction. Additionally, both Al-13 and Al-30 showed higher binding capacity for proteins and polysaccharides in EPS than monomeric aluminum and Al(OH)(3). Al-13 and Al-30 coagulation changed the secondary structure of proteins in EPS, which caused a gelation reaction to increase molecular hydrophobicity of proteins and consequently sludge dewaterability. This study provided a guidance for optimizing the hydroxy-aluminum flocculation conditioning of sludge with high solution alkalinity. (C) 2020 The Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences. Published by Elsevier B.V.

Automated summary

This study comprehensively investigated the effects of alkalinity on sludge conditioning with hydroxy-aluminum, showing that the effectiveness of hydroxy-aluminum conditioning deteriorated with increasing alkalinity, leading to a change in the flocculation mechanism. Polymeric hydroxy-aluminum showed higher binding capacity for proteins and polysaccharides in EPS compared to monomeric aluminum, affecting the secondary structure of proteins and ultimately improving sludge dewaterability.