Digester Stability Phase Portrait: VFA vs Alkalinity

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 to read the phase portrait

This is the same digester stability model as the 3D tank view, drawn as a 2D state-space diagram instead of a rendered tank. The top panel plots the reactor's live state as a point at (VFA, Alkalinity); a fading trail shows where it has been. The background is shaded by the operational stability ratio:

VFA/TA ratio  =  VFA / Total Alkalinity
  < 0.3   healthy, well-buffered   (green)
  0.3–0.8  caution, buffer eroding  (amber)
  > 0.8   souring — pH crash       (red)

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

Because the ratio is a straight line through the origin (VFA = k·Alkalinity), the stability bands are two rays fanning out from (0,0) — so a trajectory that swings from the green wedge into the red wedge is visually the exact same "souring" event as the 3D tank's pH crash, just seen as geometry in the reactor's own two state variables instead of a camera view of the vessel. The bottom strip chart scrolls pH, the VFA/TA ratio, methanogen population and CH₄ rate over the last ~60 simulated days, so you can see the causal order: ratio rises first, pH follows, then methanogens die back and methane collapses.

Push organic loading rate past what methanogens can consume and VFA outpaces alkalinity; raise alkalinity dosing to buy back buffer capacity; temperature sets how fast the mesophilic consortium works (optimum ≈ 37 °C); shock-load dumps a sudden VFA slug to test recovery. Simplified logic in the spirit of the IWA Anaerobic Digestion Model No. 1 (ADM1).

Stable region Caution region Souring region