OLR: loading the digester with organic load — how to calculate and choose

OLR (organic loading rate) is the daily mass of volatile solids (VS) introduced into the digester relative to its working volume: OLR = ΣmVS / V, expressed in kg VS/(m³·d). In agricultural biogas plants it usually lies in the range of about 2–4 kg VS/(m³·d), and the limits depend on the composition of the mix, temperature, quality of mixing and buffering capacity; values above approx. 4–5 usually require special control.

Formula and input data

OLR = (Σ mᵢ · TSᵢ · VSᵢ) / V, where mᵢ is the fresh mass of substrate i per day (kg), TSᵢ — the share of dry matter (as a fraction), VSᵢ — the share of volatile solids in the dry matter, V — the working volume of the digester (m³). The TS and VS data come from laboratory analyses of each batch (drying at approx. 105 °C and ashing at approx. 550 °C); catalogue values are only a preliminary approximation.

Worked example

Assumptions (simplified, for illustration): a digester with a working volume of 3,000 m³, daily feed:

OLR = 10,398 / 3,000 ≈ 3.5 kg VS/(m³·d). With two digesters of 3,000 m³ each and the same feed, the load would be approx. 1.7. Note that slurry adds a lot of volume but little VS: this is why a high share of slurry lowers OLR and shortens HRT (at a constant digester volume a greater flow means a shorter retention time).

Relationship with yield

Methane production rises with OLR up to a point, but the yield per unit of VS falls when the reaction time is too short: part of the material leaves undegraded. Conversely, too low a load means that the digester is oversized relative to the feed.

Effects of overloading

• accumulation of VFA, a rise in FOS/TAC, a fall in pH,
• foaming,
• a fall in CH₄ content, a rise in CO₂,
• insufficient mixing (increased viscosity at high TS),
• a higher residual potential in the digestate and greater methane emissions from storage.

Practical rules

1. Increase the feed rate gradually (e.g. by 0.2–0.5 kg VS/(m³·d) per week depending on the plant), watching pH, FOS/TAC and gas.
2. Calculate OLR for the whole plant and for individual digesters, if there are several, especially in a series arrangement.
3. Take the working volume into account, not the geometric one — foam, sediments and the gas zone reduce the usable volume.
4. Spread the feed over the day to avoid peaks (time-controlled feeders, not one large dose).
5. Use TS/VS analyses: silage with variable dry matter can change OLR by more than ten per cent without any change in fresh mass.

OLR and other parameters

When assessing a biogas plant it is worth looking at all four numbers together; high volumetric productivity achieved at the cost of a low yield from VS can be unprofitable, because it consumes more substrate.

Yield and load — completing the example

In the digester from the example (3,000 m³, feed of 10,398 kg VS/d) let us assume a yield of 0.30 m³ CH₄ per kg of VS fed. Production is then approx. 3,120 m³ CH₄/d, that is a volumetric productivity of approx. 1.04 m³ CH₄/(m³·d). Such a calculation is also a data check: if the result comes out above the theoretical potential of the substrates, there is an error in the data (e.g. an overstated gas measurement or understated VS).

When OLR is misleading

The same OLR calculated from silage with a high dry matter content and from kitchen waste does not mean the same thing: waste degrades much faster and is quicker to threaten acidification. It is therefore worth supplementing OLR with the share of easily degradable fractions (fats, sugars) in the feed.

Changing OLR with the season

In spring and summer ambient temperature rises, and in winter the plant uses more heat. The same OLR can therefore have different stability depending on the time of year. Good practice is to record the limit OLR values at which the plant operated stably in winter and summer, and to use a lower, conservative level during mix changes or mixer failures.