HRT and SRT: retention time in the digester and how to choose it

HRT (hydraulic retention time) is the average residence time of the liquid in the digester, calculated as the working volume divided by the daily flow, and SRT (solids retention time) is the average residence time of the microbial biomass. In a typical fully mixed tank without recirculation the two are equal, so HRT must be longer than the doubling time of the slowest-growing methanogens. In agricultural biogas plants it is generally several dozen days.

Formulae and example

HRT = V / Q, where V is the working volume (m³), Q — the daily volume of substrate fed (m³/d).

Example: a digester of 3,000 m³, feeding of 65 t/d of substrate with a density of approx. 1 t/m³ → Q ≈ 65 m³/d. HRT ≈ 3,000 / 65 ≈ 46 days. With two digesters (6,000 m³ in total) and the same feed, the total HRT would be approx. 92 days. Increasing the feed by 30% would shorten HRT to 35 days in the first configuration.

In practice the flow is greater than the mass fed if digestate recirculation is used for dilution; the HRT calculated relative to the total flow then differs from that calculated relative to fresh substrate. The operator should state clearly which indicator is being reported.

Why it cannot be shortened indefinitely

Methanogens have a low growth rate. If HRT is shorter than the time needed to regenerate the biomass, the loss in the outflow exceeds the growth and the population shrinks (washout). The consequences: a rise in VFA, a fall in pH, a fall in methane production. The safe margin depends on the regime (thermophilic tolerates a shorter HRT) and on the substrates (slurry with low dry matter requires a different approach than silage).

Typical ranges

The ranges given are indicative; each project must be verified for the specific mix.

Degradation and time

The curve of methane yield over time has a shape close to an exponential function: most of the potential is released in the first weeks, and the last per cent requires much longer. That is why practice combines the main digester with a second digester (or a post-digestion tank with gas collection), which increase the total retention time without too great an increase in the volume of the main one.

Cascade arrangement

Two or more digesters in series give a residence time distribution closer to plug flow: short particle paths are less likely, so less under-degraded material ends up in the digestate. With the same total volume this usually gives a higher yield than one large digester, especially for substrates that are hard to degrade.

Practical rules

1. Know the current HRT calculated from actual feed rates, not design ones.
2. When increasing the feed rate, make sure HRT does not fall below the value at which the plant operated stably.
3. Extend the time if there is a lot of undegraded material in the digestate or a lot of gas is released from the digestate store.
4. Document HRT together with OLR; together they describe the load on the digester.
5. Carry out a residual methane potential test of the digestate (BMP or a 28–60 d test) to assess whether shortening or extending the time is justified.

A residual potential test is the best way to turn a discussion about the optimal HRT into numbers from your own plant.

Recirculation and separation as a way to get SRT > HRT

In reactors without biomass retention SRT is equal to HRT. In plants with recirculation of thickened biomass (e.g. after digestate separation) some of the microorganisms return to the digester, so SRT is longer than HRT. This gives higher stability and allows a greater load, but also increases the concentration of ammonium nitrogen and salts, which must be taken into account in inhibition control.

A practical sanity test

Compare HRT with the time at which the BMP test of the substrates reaches a significant part of the potential (e.g. 80–90%). If HRT is clearly shorter, energy remains in the digestate; if much longer — the digester may be oversized or the feed too low. The check is worth repeating after every major change of mix.

Example: effects of increasing the feed rate

A plant with a 3,000 m³ digester and an HRT of 46 days increases the feed by 30% (from 65 to approx. 85 m³/d). HRT falls to approx. 35 days, which means a shortening of the reaction time by almost a quarter. If previously a considerable undegraded fibrous fraction was present in the digestate, after shortening HRT its share will rise and the yield per unit of VS will fall. The gain from the additional feed may therefore be smaller than a simple calculation would suggest.