Recovery of disintegrated halophilic aerobic granular sludge through ferric ion addition: Dual roles in filamentous fungal inhibition and microbial adhesion enhancement

Abstract

The disintegration of halophilic aerobic granular sludge (AGS) is often caused by filamentous fungal overgrowth, posing a significant challenge to wastewater treatment operations. However, methods to recover disintegrated halophilic AGS remain largely unexplored. This study proposes the restoration of disintegrated halophilic AGS through the addition of ferric ions (Fe³⁺). The effectiveness and mechanism of this approach are examined in terms of treatment performance, microbial population dynamics, and the properties of the activated sludge and granules. The results exhibited the dual roles of Fe³⁺ in inhibiting filamentous fungal overgrowth and enhancing microbial adhesion. As the dosage of Fe³⁺ rose from 0 to 10 mg/L, the bacterial population size grew from 5.23 × 10⁶ ± 2.01 × 10⁵ to 1.28 × 10⁷ ± 5.26 × 10⁵ copies/ng DNA, while the fungal population size decreased from 1.01 × 10⁶ ± 7.25 × 10⁴ to 5.37 × 10⁴ ± 2.09 × 10³ copies/ng DNA. The addition of Fe³⁺ significantly enhanced the dewaterability of the sludge (p < 0.05), which in turn improved its settleability, with the sludge volume index after settling for 5 min (SVI₅) decreasing from 306.83 ± 6.65 to 50.73 ± 0.82 mL/g. Applying the extended Derjaguin-Landau-Verwey-Overbeek theory, the energy barrier between microorganisms before and after the addition of Fe³⁺(at 10 mg/L) decreased from 1787.67 to 474.93 KT, facilitating easier microbial aggregation. In addition, Fe³⁺ induced bacteria such as Paracoccus, TM7x, TM7a, Hoeflea, and Lactococcus to secrete more extracellular polymeric substances, enhancing cell hydrophobicity and reducing electrostatic repulsion. This study demonstrated that the addition of Fe³⁺ is a feasible strategy to restore the disintegrated halophilic AGS, due to its low cost and wide application in the operation of wastewater treatment plants.