What Is Organic Loading Rate (OLR)?
Organic loading rate (OLR) is how much organic matter enters a reactor per unit of volume per day. It is the key parameter for sizing a digester.
The organic loading rate (OLR, in Spanish carga orgánica volumétrica or COV) is the amount of organic matter fed into an anaerobic reactor per unit of working volume per day. It is usually expressed in kg COD/m³·day (kilograms of Chemical Oxygen Demand per cubic meter per day) or in kg VS/m³·day (kilograms of volatile solids). It is the central parameter for sizing a biodigester or a wastewater treatment reactor.
How it is calculated
The formula is straightforward:
OLR = (Q × S) / V
where Q is the inlet flow (m³/day), S is the organic concentration of the influent (kg COD/m³ or kg VS/m³) and V is the working volume of the reactor (m³). If you know the flow, the pollutant load and the volume, you get the OLR; conversely, by fixing a design OLR you can solve for the required reactor volume.
Quick example: an effluent of 100 m³/day at 4 kg COD/m³ carries 400 kg COD/day. If you design a UASB reactor for an OLR of 8 kg COD/m³·day, you need a working volume of 400 / 8 = 50 m³.
Typical ranges by reactor type
| Reactor type | Typical OLR (kg COD/m³·day) |
|---|---|
| Covered anaerobic lagoon | 0.5–2 |
| CSTR (complete mix) | 2–5 |
| UASB (sludge blanket) | 5–15 |
| High-rate reactors (EGSB, IC) | 15–30 |
Why it matters
OLR defines a delicate balance. If it is too low, the reactor is oversized: you overspend on volume and civil works. If it is too high, the methanogenic bacteria cannot consume the acids produced by the acidogenic bacteria; the pH drops, the reactor acidifies and biogas production collapses. That is why high-rate reactors such as the UASB, which retain biomass inside the system, tolerate much higher loads than a lagoon.
OLR is always analyzed together with the hydraulic retention time (HRT): the first measures how much organic matter enters, the second how long the liquid stays in the reactor. A reactor is started up by raising the OLR gradually so the bacterial population can adapt.
Where it fits in Tech Tank projects
Every biogas or wastewater treatment plant Tech Tank designs starts from a load balance: we characterize the effluent (flow and COD/VS), set the design OLR according to the technology, and from there the reactor volume follows. To dig into high-rate technology, see what a UASB reactor is and how it works; and to apply these numbers to the real sizing of a farm digester, see how to size a farm biodigester. Both topics feed into our biogas and biodigester solutions.
If you have your effluent characterization and want to estimate the reactor, contact us: with your flow and COD we calculate the design OLR and the right volume.
Frequently Asked Questions
Are OLR and COV the same thing?
Yes. OLR (organic loading rate) is the English term and COV (carga orgánica volumétrica) is the Spanish equivalent. Both measure kg of organic matter per m³ of reactor per day.
Is it measured in COD or volatile solids?
It depends on the substrate. For liquid effluents kg COD/m³·day is common; for solid waste or manure digestion kg VS/m³·day is typical. What matters is being consistent with the unit throughout the calculation.
What happens if the load is too high?
The reactor overloads: volatile fatty acids accumulate, pH drops, methanogenic bacteria are inhibited and biogas production falls. Recovering an acidified reactor takes weeks, which is why load is raised gradually.
How does it relate to HRT?
OLR and HRT are two sides of sizing. For the same volume, more flow means lower HRT and higher OLR. The design seeks a combination that is stable for the chosen technology.