Anaerobic Digester Souring: VFA/Alkalinity Balance

Digester Controls

Organic loading rate 4.0 kg VS/m³·day

Alkalinity dosing 40%

Reactor temperature 37 °C

Reactor State

Stable

7.10

VFA : Alkalinity

0.20

CH₄ rate

0.00 m³/m³·d

Methanogens

60%

VFA

800 mg/L

Alkalinity

3000 mg/L

How it works

Anaerobic digestion runs in stages: hydrolysis and acidogenesis break organic matter into volatile fatty acids (VFA), then methanogens slowly consume those VFAs to produce CH₄ + CO₂. Methanogens grow far slower than the acid-formers that feed them, so overloading the reactor creates a classic engineering failure mode: digester souring.

VFA/TA ratio  =  VFA / Total Alkalinity
  < 0.3   healthy, well-buffered
  0.3–0.8  caution, buffer capacity eroding
  > 0.8   souring — pH crash imminent

Monod uptake:  rate = Vmax · [VFA] / (Ks + [VFA])
Methanogen growth is inhibited as pH drops below ~6.8
and stops almost entirely below pH 6.0.

Bicarbonate alkalinity is the buffer that neutralizes VFA and keeps pH near neutral. Push the organic loading rate faster than methanogens can consume the acids they're fed, and VFA accumulates while alkalinity is consumed — the VFA/TA ratio climbs, pH falls, methanogen growth is inhibited (further slowing VFA uptake), and the loop reinforces itself. Alkalinity dosing (e.g. sodium bicarbonate) buys buffer capacity back; temperature sets how fast the mesophilic consortium can work (optimum ≈ 37 °C, falling off sharply outside ~30–42 °C); the shock-load button dumps a sudden slug of VFA to test recovery. This mirrors the stability logic used in real digester operations and simplified versions of the IWA Anaerobic Digestion Model No. 1 (ADM1).

Acid-forming bacteria Methanogens Biogas (CH₄)

⚠ Couldn't load the 3D engineThree.js failed to load from the CDN. Check your connection and reload.