HRT (HRT)

Hydraulic Retention Time (HRT) is the average number of days that liquid feedstock spends inside a biogas digester between entry and discharge, calculated by dividing the active digester volume by the daily feed flow rate. For a 1,000 m³ working-volume CSTR receiving 25 m³/day of slurry, HRT equals 40 days. HRT is one of the three primary design variables for any anaerobic digester, alongside organic loading rate (OLR) and operating temperature.

HRT directly determines the trade-off between gas yield and reactor size:

• Short HRT (10–20 days) — smaller digester, lower capex, but methanogens may be washed out faster than they can grow; suitable for high-rate granular reactors (UASB, EGSB) treating soluble industrial effluents
• Medium HRT (25–40 days) — the standard window for Indian CSTR-type CBG plants treating cattle dung, press mud, or food waste at mesophilic temperatures
• Long HRT (40–60+ days) — required for slow-degrading lignocellulosic feedstocks (paddy straw, Napier grass) and pulses biodegradation to completion; larger tank but higher methane recovery per tonne of feed

Typical Indian CBG plant design choices map feedstock to HRT:

• Cattle dung CSTR — 30 days HRT, 35–37 °C, OLR 2.5–3.5 kg VS/m³/day
• Press mud + cattle dung co-digestion — 35–40 days HRT, similar OLR
• Food waste CSTR — 25–30 days HRT due to higher biodegradability, OLR up to 4 kg VS/m³/day
• Crop residue CSTR — 40–55 days HRT, OLR 1.5–2.5 kg VS/m³/day after fibre size reduction

HRT differs from Solids Retention Time (SRT) in mixed-solids digesters where settled biomass is recirculated, allowing SRT to exceed HRT. In a true CSTR with no biomass recycle, HRT = SRT, which is why CSTRs need long HRT to retain slow-growing methanogens. The downside of long HRT is capex: a 5,000 m³/day biogas plant requires ~₹4–8 crore more in tankage and civil work at 45-day HRT than at 25-day HRT, which is why feedstock selection and pre-treatment to shorten effective HRT is a central economic lever in CBG plant design.