Comparing the inhibitory thresholds of dairy manure co-digesters after prolonged acclimation periods: Part...

Area of Science:

• Biochemical Engineering
• Renewable Energy
• Environmental Science

Background:

• Co-digestion enhances biogas yields and anaerobic digester stability compared to mono-digestion.
• Limited data exists on ultimate inhibition from co-substrates at maximum loading rates.
• Controlled experiments are needed to determine optimal co-digestion limits and benefits.

Purpose of the Study:

• To investigate the ultimate inhibition and benefits of co-digesting food waste (FW), alkaline hydrolysate (AH), and crude glycerol (GY) with dairy manure (MN).
• To determine maximum loading rates and mixing ratios for stable and efficient co-digestion.
• To provide data for implementing effective co-digestion strategies.

Main Methods:

• Long-term (900+ days) controlled experiments using continuously stirred anaerobic reactors at mesophilic temperatures.
• Individual co-digestion of FW, AH, and GY with MN at varying organic loading rates (OLR) and mixing ratios.
• Monitoring of biogas yields, specific methane yields (SMY), and digester stability indicators.

Main Results:

• Co-digesting FW with MN up to 3.9 g VS·L⁻¹·Day⁻¹ (51:49 MN:FW) showed no performance reduction, yielding 297 mL CH₄·g VS⁻¹.
• Co-digesting AH with MN up to 2.7 g VS·L⁻¹·Day⁻¹ (75:25 MN:AH) yielded 299 mL CH₄·g VS⁻¹, but higher rates showed potential inhibition due to free ammonia.
• Co-digesting GY with MN yielded an optimal SMY of 549 mL CH₄·g VS⁻¹ at 3.2 g VS·L⁻¹·Day⁻¹ (62:38 MN:GY), with instability beyond this due to fatty acids and foaming.

Conclusions:

• Co-digestion of FW, AH, and GY with dairy manure can significantly enhance methane production.
• Specific loading rates and mixing ratios are critical for maximizing benefits and preventing inhibition.
• Monitoring co-substrates with similar inhibiting characteristics is essential for stable high-rate co-digestion.