What is the FM ratio
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| 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)/SML
x Va)] x 10-3 x 24
| Where |
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 BOD5
as 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 BOD5
applied 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
butthere 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. |
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|
Simon J Binyon, ARL Consulting Ltd, Tel: 01829-741524. |
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