Microbial Community Shifts during Biogas Production from Biowaste and/or Propionate

Abstract

Propionate is the most delicate intermediate during anaerobic digestion as its degradation is thermodynamically unfavorable. To determine its maximum possible degradation rates during anaerobic digestion, a reactor was fed Monday to Friday with an organic loading rate (OLR) of 12/14 kg COD_biowaste·m^-3·d^-1 plus propionate up to a final OLR of 18 kg COD·m^-3·d^-1. No feed was supplied on weekends as it was the case in full-scale. To maintain permanently high propionate oxidizing activity (POA), a basic OLR of 3 kg COD_propionate·m^-3·d^-1 all week + 11 kg COD_biowaste·m^-3·d^-1 from Monday to Friday was supplied. Finally a reactor was operated with an OLR of 12 kg COD_biowaste·m^-3·d^-1 from Monday to Friday and 5 kg COD_propionate·m^-3·d^-1 from Friday night to Monday morning to maintain a constant gas production for permanent operation of a gas engine. The propionate degradation rates (PDRs) were determined for biowaste + propionate feeding. Decreasing PDRs during starvation were analyzed. The POA was higher after propionate supply than after biowaste feeding and decreased faster during starvation of a propionate-fed rather than a biowaste-fed inoculum. Shifts of the propionate-oxidizing and methanogenic community were determined.

Keywords: anaerobic co-digestion; biowaste; community shifts; methanogenic community; propionate; propionate-oxidizing bacteria (POB).

Figure 1

Scheme of reactors.

Figure 2

Biogas production and fatty acid levels in a 10 L biowaste reactor 1 after start at an OLR of 12 kg COD_biowaste·m⁻³·d⁻¹ (a) and in reactor 2 for increasing organic loading rates up to 18 kg COD·m⁻³·d⁻¹, maintained by 12 kg (day 1–55) or 14 kg (new batch biowaste from day 55 onwards) COD_biowaste·m⁻³·d⁻¹ plus respective amounts of propionate (b). No feeding between days 75–85 due to maintenance work.

Figure 3

Biogas production (a) and fatty acid levels (b) in a 10 L biowaste digester (reactor 3) fed constantly with propionate (2.68 mM, 80 mL·d⁻¹) at an OLR of 3 kg COD·m⁻³·d⁻¹ and additionally with biowaste from Monday to Friday (1 L·d⁻¹) to reach an OLR of 14 kg COD·m⁻³·d⁻¹.

Figure 4

Periodic feeding of biowaste (BW, 1 L·d⁻¹) or propionate (Prop, 2.68 M, 120 mL·d⁻¹) to maintain an almost constant gas production in the 10 L biowaste reactor 4 over weekends, when no biowaste was available. No fatty acids were detected at any time.

Figure 5

Propionate degradation rates (a) and degradation activity immediately following starvation in effluent of reactor 4 or after 1–5 days, after biowaste (b) or propionate feeding (c).

Figure 6

Total Bacteria and Archaea (a), propionate-oxidizing bacteria (POB; b, c), and methanogenic archaea (d) in reactor 2, during feeding of biowaste at an OLR of 12 (day 1–55) or 14 (day 55 onwards) kg COD_biowaste·m⁻³·d⁻¹ and propionate up to 4 kg COD_propionate·m⁻³·d⁻¹. For sample preparation see Section 2.4.