Decreasing activated sludge thermal hydrolysis temperature reduces product colour, without decreasing degradability

Area of Science:

• Environmental Science
• Chemical Engineering
• Wastewater Treatment

Background:

• Activated sludge is increasingly recalcitrant to anaerobic digestion due to operational changes.
• Thermal hydrolysis is used to improve sludge degradability but can create problematic colored compounds.
• These colored compounds, identified as melanoidins, negatively impact downstream processes.

Purpose of the Study:

• To investigate the impact of thermal hydrolysis operating temperature on melanoidin formation.
• To assess the effect of reduced temperature on sludge anaerobic biodegradability.
• To optimize thermal hydrolysis for reduced downstream impacts.

Main Methods:

• A full-scale thermal hydrolysis system was operated at two different temperatures (165°C and 140°C).
• Sludge effluent color was measured using mg-PtCo L⁻¹.
• Anaerobic biodegradability was assessed by measuring chemical oxygen demand (COD) biodegradation and hydrolysis rates.

Main Results:

• Decreasing thermal hydrolysis temperature from 165°C to 140°C significantly reduced effluent color from 12,677 to 3837 mg-PtCo L⁻¹.
• The temperature reduction did not significantly affect the rate or extent of anaerobic COD biodegradation.
• Average first-order hydrolysis rate remained 0.19 d⁻¹ and conversion extent was 0.43 g-COD(CH4)g-COD⁻¹.

Conclusions:

• Reducing thermal hydrolysis operating temperature is an effective strategy to minimize melanoidin production.
• This operational adjustment mitigates downstream issues associated with colored compounds.
• Optimized thermal hydrolysis preserves sludge biodegradability while improving effluent quality.