Question

Define (i.e. physical meaning) SRT, HRT, F/M ratio, SVI and indicate their importance during the operation of an activated sludge process in wastewater treatment plants.

Answer 1

The flow diagram of a typical activated sludge system is shown below:

SRT, HRT, F/M ratio and SVI are important process parameters which needs to be optimized for an efficient wastewater treatment system.

[1] Sludge Retention Time [SRT]: Sludge or solid retention time is the average time spent by the activated sludge solids in the system. For a retention time interval t for solids, the mass balance will result:

(t . Q_w . X_r) + t.(Q-Q_w ). X_e = V X

If SRT is too low, the process may not remove sufficient organic matter from the wastewater.

If SRT is to high, the process will be prone to bulking of solids and the treatment system can become overloaded.This can cause the dissolved oxygen content to drop and an excessive aeration will be required which wastes electricity.

[2] Hydraulic Retention Time [HRT]: HRT is a loading parameter, which indicate the amount of time spent by sewage or wastewater in aeration tank. It is defined (in days), as

where V= volume of aeration tank (in m³), Q = Volumetric flow rate of sewage or wastewater (m³/day).

With an increase in HRT, the amount of sludge that is converted to suspended organic matter will increase but the operation time will also increase. Therefore an optimum HRT is desired.

[3] Food to microorganism (F/M) ratio: The F/M ratio is a process control number that helps you to determine the proper number of microorganisms for system. It is the ratio of the amount of organic matter to the microorganism in the system.

High value of HRT and SRT may result in small F/M ratio and vice versa.

[4] Sludge Volume Index (SVI): It is a process control parameter to determine the recycle rate of sludge. It is a standard measure of the physical characteristics of activated sludge processes. It is defined as 'the volume (in ml) occupied by 1 gram of activated sludge after settling the aerated liquid for 30 minutes'.