Performance and mechanisms of wastewater sludge conditioning with slag-based hydrotalcite-like minerals (Ca/Mg/Al-LDH)

Extracellular polymeric substances (EPS) in wastewater sludge form a network structure that is highly hydrophilic and compressible. Thus chemical conditioning is always required to improve sludge dewaterability by changing the gelatinous structure of sludge flocs. Layered double hydroxides (LDH) are generally characterized by large surface area and high anion exchange capacity, so we prepared three types of hydrotalcite-like compounds (Ca/Mg/Al-LDHs) from a typical solid waste, blast furnace slag, using NaOH precipitation (giving LDHa), a hydrothermal method (LDHb), and NaOH-Na2CO3 precipitation (LDHc). The physicochemical properties of the three LDH were comprehensively characterized, and their effectiveness as sludge conditioners was evaluated. The results showed that LDH conditioning was able to promote sludge dewaterability, and conditioning efficiency was strongly dependent on LDH structural properties. LDH neutralized the negative charges onto sludge particles and interacted with EPS to increase floc strength. LDH also formed a skeletal structure that reduced sludge compressibility. In addition, there were interactions between the LDH surfaces and the OC-OH in EPS proteins, which altered the secondary structure of protein molecules, consequently increasing sludge dewaterability. The biomolecules of low-molecular-weight fractions (such as peptides and humic acids) in soluble EPS intercalated LDH. Both the surface complexation of organic matter containing carboxyl groups and the intercalation of small molecules in soluble EPS were responsible for EPS-LDH interactions. The combination of skeleton formation, electrostatic interaction, and EPS-LDH interactions resulted in compression of gel-like structure and improved sludge dewatering performance. We finally suggested a novel sludge treatment process that increases sludge dewaterability using slag-derived Ca/Mg/Al-LDH to condition the sludge, and it could be combined with pyrolysis to prepare multi-functional materials or bio-oil.