Ks is the half-saturation constant in the Monod equation, which describes the growth rate of microorganisms in a substrate-limited environment, such as an activated sludge system. Ks represents the concentration of substrate (such as organic matter) at which the specific growth rate of microorganisms is half of the maximum rate. Ks is an indicator of the affinity of microorganisms for the substrate: a lower Ks means a higher affinity and a faster growth rate at low substrate concentrations.

The value of Ks depends on various factors, such as the type and composition of substrate, the characteristics and diversity of microorganisms, the temperature and pH of the system, and the presence of inhibitors or competitors. Therefore, Ks is not a fixed parameter but rather a variable that can change under different conditions. According to some sources, typical values of Ks for activated sludge systems range from 10 to 100 mg/L for soluble organic matter and from 50 to 500 mg/L for particulate organic matter. However, these values are only approximate and may not apply to your specific system.

To determine the value of Ks for your system, you may need to perform some experiments using batch reactors or continuous reactors with different substrate concentrations and measuring the growth rate of microorganisms. Alternatively, you may use some mathematical models or software tools that can estimate Ks based on your system data and parameters. However, you should be aware that these methods may have some limitations and uncertainties and may not reflect the actual behavior of your system.