The organic loading rate (OLR) is the most important operating parameter of a digester: it measures how much organic matter is fed per reactor volume and day (kg VS/m³·d). It is calculated on volatile solids (VS), not on total solids (TS), because only the volatile fraction degrades and produces biogas. A mesophilic CSTR works well at 2-6 kg VS/m³·d; exceeding ~6 without acclimation is one of the most frequent causes of acidification. The golden rule is to raise it slowly (0.2-0.5 kg VS/m³·d per week). This article explains what it is, how to calculate it properly and how to use it as a sentinel variable.
The organic loading rate is, together with the retention time, the parameter that best describes how a digester is fed. And it is, by far, the variable that causes the most biological shutdowns when it is calculated wrongly.
The idea is simple: it measures how much organic matter enters the reactor per cubic metre per day. But small errors in its calculation (using the wrong solids, an outdated characterisation) have large consequences.
This is the what is of the OLR: its definition, the correct calculation, the ranges by digester type and how to use it without destabilising the process. It builds on what anaerobic digestion is.
Definition of the organic loading rate (OLR)
The organic loading rate (OLR) is the amount of organic matter introduced into the digester per unit of volume and time. It is expressed in kg VS/m³·d (kilograms of volatile solids per cubic metre of reactor per day).
In practice, it is the parameter that most conditions performance and stability: it defines how much the digester eats. Feeding it too much overloads and acidifies it; feeding it too little underuses it and wastes capacity.
Correct calculation: why it is done on VS and not on TS
The OLR is calculated with a simple formula: OLR = (Q × S) / V, where Q is the feed flow (m³/d), S the volatile solids concentration of the substrate (kg VS/m³) and V the working volume of the digester (m³).
The critical point is the S: it is calculated on volatile solids (VS), not on total solids (TS). The reason is biological: only the volatile fraction (the organic matter) degrades and produces biogas; the mineral fraction (ash, sand, salts) does not take part in the process. Within that organic fraction there can be parts that are easier to degrade and others that are harder.
Calculating the OLR on TS overestimates the genuinely biodegradable load and leads to wrong decisions, such as believing the digester is more loaded than it actually is.
Typical operating ranges by digester type and substrate
The OLR a reactor tolerates depends on its configuration. These are the usual ranges:
| Digester type | Typical OLR | Unit |
|---|---|---|
| Mesophilic CSTR | 2-6 | kg VS/m³·d |
| Plug-flow | 5-12 | kg VS/m³·d |
| UASB (wastewater) | 5-15 | kg COD/m³·d |
| Overload without acclimation | > 6 | kg VS/m³·d (risk) |
These are indicative ranges: the real value each plant can support depends on the substrate, the temperature and the degree of acclimation of the consortium. The digester type sets the starting point.
OLR and retention time: an inverse relationship
The OLR and the hydraulic retention time (HRT) are inversely related: for the same substrate, raising the load usually means lowering the time the material stays in the reactor.
That balance has limits. Too short an HRT can wash out the biomass (the methanogens, which grow slowly, leave with the effluent); too high an OLR overloads it. Designing a digester well consists, to a large extent, of adjusting both parameters at the same time.
Common errors in determining the OLR at the plant
Most problems do not come from the formula, but from the data fed into it:
- Calculating on TS instead of VS, which overestimates the real load.
- Using an outdated characterisation of the substrate: the VS concentration varies with the season and the supplier. The difference between the nominal (design) OLR and the real OLR reaches 15-25% in plants that only characterise quarterly.
- Not discounting the lost volume: crusts, sediments and foams reduce the working volume of the digester and raise the effective OLR.
- Ignoring energy co-substrates: a small amount of fat or glycerine sends the real OLR soaring even though the added volume is minimal.
How to raise the OLR without destabilising the digester
Raising the OLR is tempting (more load, more gas), but it is also the manoeuvre that destabilises the most digesters. The rule is to do it slowly: increments of 0.2-0.5 kg VS/m³·d per week, watching FOS/TAC, VFAs and methane production.
If propionic acid or FOS/TAC start to rise, the increase must be paused so the consortium can acclimate before continuing. Jumping past ~6 kg VS/m³·d without prior acclimation is one of the most frequent causes of acidification.
The OLR as a sentinel variable in diagnosis
Beyond the calculation, the OLR is a sentinel variable: tracking its evolution and comparing it with specific biogas production reveals whether the digester is well fed, underused or on the edge of overload.
It is one of the key variables of the biogas plant diagnosis and a starting point for stabilising the anaerobic digester.
Frequently asked questions about the organic loading rate
What is the OLR in a biogas plant?
The OLR (organic loading rate) is the amount of organic matter fed to the digester per unit of volume and day, expressed in kg VS/m³·d. It is the operating parameter that most conditions performance: it indicates how much the reactor eats. A well-adjusted OLR maximises biogas production without overloading the process, making the most of the biodegradable part of the substrate.
How is the organic loading rate calculated?
With the formula OLR = (Q × S) / V: the feed flow (Q, in m³/d) multiplied by the volatile solids concentration of the substrate (S, in kg VS/m³), divided by the working volume of the digester (V, in m³). The result is expressed in kg VS/m³·d. The key is to use the real, up-to-date VS concentration of the substrate, not an old design value.
What is the maximum OLR a mesophilic digester can support?
In a mesophilic CSTR digester, the usual range is 2 to 6 kg VS/m³·d. Exceeding 6 kg VS/m³·d without prior acclimation of the consortium is one of the most frequent causes of acidification. The real limit depends on the substrate, the temperature and the state of the biomass: a well-acclimated digester can go higher, but always raising the load gradually.
Why is it calculated on VS and not on TS?
Because only the volatile solids (VS), which are the organic fraction, degrade and produce biogas. Total solids (TS) also include the mineral fraction (ash, sand, salts), which does not take part in digestion. Calculating the OLR on TS overestimates the genuinely biodegradable load and can lead to dosing errors.
Smallops and organic load control
Many biological shutdowns start with an OLR that was miscalculated or raised too fast. At Smallops we verify your real OLR (on VS and working volume) and design the loading ramp with an Operational Excellence Diagnosis.
Do you know the real OLR of your digester?
Request a Smallops Operational Excellence Diagnosis: we characterise your substrate, calculate the real organic load and design the safe ramp to raise production without acidifying the reactor.
References and standards
Speece, R.E. (2008). Anaerobic Biotechnology and Odor/Corrosion Control in Municipal and Industrial Wastewaters. Archae Press.
Rincón, B. et al. (2008). Influence of organic loading rate and hydraulic retention time on the performance of anaerobic digestion. Bioresource Technology, 99 (16), 7995-8005.
Lin, L. et al. (2017). Bioenergy production and operational performance of a CSTR. Bioresource Technology, 241, 49-56.
Drosg, B. (2013). Process monitoring in biogas plants. IEA Bioenergy Task 37.