3.4.5 Hydraulic and solid retention time

Video: https://youtu.be/qJKoW8d8frc

Hydraulic retention time (HRT) refers to the duration that raw material remains within a digester in anaerobic digestion systems. It is a crucial factor in the design of AD systems. On the other hand, solids retention time (SRT) indicates the amount of time the solid particles or sludge remain in the digester. While HRT primarily focuses on the overall material flow, SRT specifically targets the retention of solids.

When a reactor is not specifically designed to retain solids, it is generally assumed that the HRT and SRT are equal. In other words, the retention time for both the raw material and the solids is the same.

To ensure the optimal growth of microorganisms and prevent their loss from the system, it is crucial to maintain a suitable solids retention time (SRT) in anaerobic digestion processes. The SRT can vary depending on the temperature and will differ between seasons for digesters without heating systems. Therefore, it is essential to consider the average temperature during the coldest month when designing such systems, as it is shown in the following example from Vietnam.

Mean temperature
in winter (°C)
Retention time (day)
10 – 1555
15 – 2040
Retention Time According to Sectoral Standards in Vietnam (Biogas Project Division/ Department of Livestock Production, 2011)

However, it is important to note that under psychrophilic and mesophilic conditions (moderate temperatures), pathogens are unlikely to be completely inactivated regardless of the retention time. Therefore, complete pathogen elimination cannot be guaranteed even with extended retention times.

Pathogens53 – 55 °C35 – 37°C8 – 25°C
Salmonella1 – 2100%7100%44100%
E.coli210-1 – 10-22110-440 – 6010-4 – 10-5
Hookworm ova1100%10100%3090%
Ascaris ova2100%3698.8%10053%
Effect of Retention Time and Temperature on Pathogen Reduction (Yongfu, 1992)

Considering this, additional treatment of the effluent and sludge may be required depending on the specific application or disposal method to ensure complete pathogen removal. Further treatment steps can address any remaining pathogens and minimise potential health risks.

It is important to prioritise public health by considering these factors and taking appropriate measures to safeguard against pathogen transmission in anaerobic digestion systems. By implementing necessary treatment procedures, we can ensure organic waste’s safe and effective management while minimising potential health hazards.

HRT (Hydraulic Retention Time) and SRT (Solids Retention Time) are typically expressed in hours (h) or days.

Hydraulic Retention Time (HRT) is calculated by dividing the volume of the anaerobic digester by the influent flow rate. The formula to calculate HRT is as follows:


HRTHydraulic Retention Timedays
VDVolume of Digester
INInfluent Flow Rate m³/day