What Is Hydraulic Retention Time (HRT)?
HRT is the average time liquid stays inside a reactor. It is calculated as volume divided by flow rate: HRT = V / Q.
Hydraulic retention time (HRT), TRH in Spanish, is the average time liquid stays inside a treatment reactor or tank. It is calculated by dividing the reactor's working volume by the incoming flow rate: HRT = V / Q. If a biodigester has 500 m³ of working volume and receives 25 m³/day of substrate, its HRT is 20 days. It is one of the most important design parameters in wastewater treatment and anaerobic digestion, because it determines how much time microorganisms have to break down the organic matter.
Too short an HRT gives the biology no time to act and the system fails; an excessive HRT makes the plant more expensive by requiring larger volumes than needed. Design seeks the balance.
How to calculate it: the formula
The formula is direct:
- HRT = V / Q
- V = working volume of the reactor (m³)
- Q = feed flow rate (m³/day)
- HRT = time, usually in days (or hours for fast reactors)
The same formula gives the volume when flow and target HRT are known: V = Q × HRT. That is why HRT is the gateway to sizing: once the flow and the HRT required by the chosen technology are defined, the reactor volume is set.
Worked example
A dairy plant generates 40 m³/day of effluent and a reactor with a design HRT of 6 hours (0.25 days) is chosen:
- V = Q × HRT = 40 m³/day × 0.25 day = 10 m³ of working volume.
If instead it were treated in an anaerobic lagoon with an HRT of 20 days, the volume would be 40 × 20 = 800 m³. The same water, different technologies, very different volumes.
Typical HRT ranges by technology
Each technology works within a characteristic HRT range. As a rough guide, common engineering values:
| Technology | Typical HRT | Application |
|---|---|---|
| UASB reactor | 4 to 12 hours | High-strength effluents |
| CSTR reactor (mixed digester) | 15 to 30 days | Manure, sludge, co-digestion |
| Covered anaerobic lagoon | 20 to 50 days | Agro-industrial effluents |
| Activated sludge (aerobic) | 4 to 24 hours | Polishing, domestic effluent |
A key point: in retained-biomass reactors such as the UASB, HRT can be a matter of hours because the bacteria are not washed out with the liquid; their residence time (solids retention time) is much longer than the HRT. In a CSTR, by contrast, HRT and solids retention coincide, which is why it needs days.
Where it applies in Tech Tank projects
HRT is the first number that defines the size of a reactor or biodigester. At Tech Tank we use it to:
- Size biodigesters and biogas plants according to substrate and daily flow.
- Define reactor volumes in effluent plants.
- Optimize CAPEX: choosing the technology with the right HRT avoids oversizing tanks.
To see how HRT combines with organic load and gas production, read our practical guide on how to size a farm biodigester and the pillar on what is an industrial biodigester. If you want to size your system with real flow and substrate data, you can request the technical analysis from Tech Tank.
Frequently Asked Questions
What is the HRT formula?
HRT = V / Q, where V is the reactor's working volume and Q the incoming flow. The result is expressed in days or hours. Rearranged, the same formula gives the volume: V = Q × HRT.
How does HRT differ from solids retention time?
HRT measures how long the liquid stays; solids retention time (SRT) measures how long the biomass stays. In reactors like the UASB, SRT is much longer than HRT, which allows treating high flows in a small volume.
What happens if the HRT is too short?
Microorganisms cannot break down the organic matter and the effluent leaves untreated. Insufficient HRT is a common cause of failing the discharge limit or of low biogas production.
Is a longer HRT always better?
No. A longer HRT improves removal up to a point, but requires larger, costlier tanks. Design seeks the minimum HRT that guarantees the required performance so the plant is not oversized.
How does temperature affect the required HRT?
In anaerobic digestion, higher temperature lets bacteria work faster and lowers the required HRT. That is why mesophilic digesters (around 35 °C) need less HRT than an ambient-temperature process.