Strategies to re-establish stable granulation after filamentous outgrowth: Insights from lab-scale experiments

• Published in: Process safety and environmental protection 2018-07, Vol.117, p.606-615
• Main Authors: Moura, L.L., Duarte, K.L.S., Santiago, E.P., Mahler, C.F., Bassin, J.P.
• Format: Article
• Language: English
• Subjects: Aeration; Anaerobic conditions; Bacteria; Biomass; Degranulation; Disintegration; Filamentous outgrowth; Granular materials; Granular sludge stability; Granulation; Iron; Iron addition; Nitrogen removal; Physical properties; Shear stress; Sludge; Stability; Strategy; Wastewater treatment; Water treatment
• ISSN: 0957-5820; 1744-3598
• DOI: 10.1016/j.psep.2018.06.005

•Uncontrolled sludge retention time (SRT) led to large and filamentous granules.•Excess of filaments led to deterioration of granules properties and disintegration.•Strategies were adopted to suppress filamentous overgrowth and recover granules.•Shear stress by enhanced aeration intensity could not reduce filamentous outgrowth.•Prolonged shear stress combined with iron addition led to stable granulation. Aerobic granular sludge (AGS) is a promising technology for wastewater treatment. However, maintaining the granular biomass stability is not trivial. During the long-term operation of AGS systems, we observed that uncontrolled sludge retention time led to the appearance of substantially large and fluffy granules characterized by a dark anaerobic inner core. Such conditions favoured the proliferation of filamentous bacteria and deterioration of AGS properties. Consequently, granules disintegration became inevitable. We decided to assess two operational strategies to supress filamentous overgrowth and recover the granular biomass stability. Strategy 1 involved the enhancement of shear stress by increasing the aeration intensity (from 1.34 to 1.87cms−1), while strategy 2 relied on the combination of enhanced aeration and iron (10mgL−1) addition. The implementation of strategy 1 contributed to decrease the average diameter of AGS (4.7–4.1mm) and sludge volume index (SVI) (300–110mLg−1). Nevertheless, filamentous bacteria remained intact. Conversely, prolonged shear stress combined with iron supplementation (strategy 2) led to significant improvements in the physical characteristics of the granules. Very small particles acting as precursors of mature AGS were observed while big-size granules (>4mm) were no longer detected. Moreover, filamentous overgrowth was controlled and the growth of new granules was stimulated. No adverse effect on the conversion processes (COD and nitrogen removal) was observed in the AGS system and stable granulation was achieved from this period onwards. To better explain the role of the implemented strategies, some hypothesis are presented and discussed.