One of the most fundamental control parameters for the activated sludge process is the relationship between the load (i.e. kg/day as opposed to mg/l) of BOD (or bacterial ‘food’) entering the aeration plant, and the ‘mass’ of bacteria in the aeration tank available to treat the incoming BOD. This is therefore known as the Food to Mass ratio (F:M ratio), also often referred to as the Sludge Loading Rate (SLR).

The amount of biomass within the reactor is known as the Mixed Liquor Suspended Solids (MLSS). Having established the MLSS by filtration and drying at 105 °C to constant weight, it is then possible to calculate the F:M ratio using the following simple model.
FB:M = [(Bi x Qi)/SMLx Va)] x 10-3 x 24

Influent flow = Qi (m3/hr)

Influent BOD = Bi (mg/1)

Aeration tank volume = Va (m3)

MLSS = SML(g/l)

In this instance, FB:M refers to the loading measured as BOD5as opposed to FC:M which is when COD is the measured parameter. This very important ratio is perhaps the single most important parameter in controlling the activated sludge process, and is defined as the kg of BOD5applied per kg MLSS per day.
The ABC Paper Co Ltd operates an activated sludge plant for the treatment of their tissue mill waste effluent. Discharge is to an estuary which requires high BOD removal but there is no ammonia element to the consent and hence no requirement for nitrification. The plant operates with the following parameters: Flow rate: 6000 m3/day (250 m3/hour), Influent BOD: 300 mg/l, MLSS: 3000 mg/l (3.0 g/l) and Aeration tank volume: 3000 m3.
Simon J Binyon, ARL Consulting Ltd, Tel: 01829-741524.