Potential causes of partial-denitrification (PD) granular sludge breakdown under high nitrate loading rates

Area of Science:

• Environmental Science
• Biotechnology
• Wastewater Treatment Engineering

Background:

• Granule instability is a known issue in wastewater treatment under high loading rates.
• Anoxic partial-denitrification (PD) granules offer a novel pathway for nitrite production for anammox processes.
• Limited research exists on the stability of these novel PD granules.

Purpose of the Study:

• To investigate the impact of increasing nitrate loading rates (NLRs) on the stability of anoxic partial-denitrification (PD) granules.
• To identify the key factors contributing to PD granule instability under high NLRs.
• To understand the effect of operational parameters like cycle frequency on granule stability.

Main Methods:

• Two lab-scale sequencing batch reactors (SBRs) were operated with gradually increasing NLRs.
• Influent nitrate concentration was elevated to achieve NLRs up to 7.68 kg N/m³d.
• Extracellular polymeric substances (EPS) composition and structure were analyzed to assess changes.

Main Results:

• Nitrite production rates increased with NLRs, reaching a maximum of 5.26 kg N/m³d.
• PD granules broke down above an NLR of 3.84 kg N/m³d, leading to sludge washout.
• High NLRs altered EPS composition, decreasing protein content and tightly bound EPS (T-EPS), identified as a primary cause of instability.
• Shorter idle times in the SBR cycle (higher frequency) exacerbated granule deterioration.

Conclusions:

• Nitrate loading rates significantly influence the stability of anoxic PD granules.
• Changes in EPS, particularly reduced protein and T-EPS, are critical factors in granule breakdown.
• Operational parameters like SBR cycle frequency also play a role in maintaining granule stability.
• Findings provide insights for developing stable, high-rate PD systems for efficient nitrite production.