Control strategy for filamentous sludge bulking: Bench-scale test and full-scale application

Area of Science:

• Environmental microbiology
• Wastewater treatment engineering

Background:

• Sludge bulking, often caused by Microthrix parvicella overgrowth, is a persistent issue in wastewater treatment plants (WWTPs), particularly during cold periods.
• Filamentous bacteria proliferation negatively impacts sludge settleability, leading to operational challenges and reduced treatment efficiency.

Purpose of the Study:

• To investigate the impact of varying sludge loads on the in situ growth of Microthrix parvicella.
• To evaluate the effectiveness of sludge load manipulation as a strategy for controlling filamentous bulking at low temperatures (15°C).

Main Methods:

• A bench-scale anaerobic-anoxic-aerobic reactor was operated for 500 days, fed with raw sewage.
• Sludge load was systematically varied (0.04-0.12 kg COD·kg MLSS⁻¹·d⁻¹).
• Microthrix parvicella abundance was quantified using quantitative polymerase chain reaction (qPCR) and fluorescence in situ hybridization (FISH); sludge settleability was assessed via sludge volume index (SVI).

Main Results:

• Reduced sludge loads (0.04-0.07 kg COD·kg MLSS⁻¹·d⁻¹) correlated with increased M. parvicella abundance (up to 18.53% by FISH) and high SVI values (135-164 mL/g).
• Increasing sludge load to 0.12 kg COD·kg MLSS⁻¹·d⁻¹ significantly reduced M. parvicella abundance (1.06% by qPCR) and SVI (105 mL/g).
• A full-scale WWTP validation confirmed that maintaining sludge load above 0.1 kg COD·kg MLSS⁻¹·d⁻¹ (specifically 0.14 kg COD·kg MLSS⁻¹·d⁻¹) successfully controlled SVI (<120 mL/g) and M. parvicella abundance (<7% by FISH) over two winter seasons.

Conclusions:

• Sludge load is a critical operational parameter for controlling Microthrix parvicella growth and preventing sludge bulking.
• A sludge load-based strategy, specifically maintaining loads above 0.1 kg COD·kg MLSS⁻¹·d⁻¹, is a feasible and efficient approach for managing filamentous bulking in WWTPs, especially during low-temperature periods.