Calculating the organic loadings for a given activated sludge system is an important aspect of process control. Measuring the BOD of the incoming wastewater gives a value for the amount of biodegradable matter available for microbial use, which can be used together with an estimate of the resident biomass to derive a relationship termed the food to micro-organism (F/M) ratio. This, which is also sometimes known as the organic loading rate, is given as follows:
The F/M ratio is a useful indication of anticipated micro-organism growth and condition, a high F/M value yielding rapid biomass increase, while a low one suggests little available nutrients and consequently slow growth results. Clearly, the total active biomass content in an activated sludge system, which is termed the mixed liquor suspended solids (MLSS), is an important factor in process efficacy. Accordingly, it is routinely measured at sewage works being important in the calculation of the F/M ratio, which can be more properly defined as:
Although the preceding systems are the most common forms likely to be seen in use, a number of other systems exist which may sometimes be encountered, some of which will be briefly outlined for the sake of completeness.