Environmental Engineering Reference
In-Depth Information
Lag
Phase
Log Growth
Phase
Stationary
Phase
Death
Phase
9
8
7
6
5
4
2
12
24
36
Time (in hours)
figure 8.2 Microbe growth curve.
The actual operation of an activated sludge system is regulated
by three factors: (1) the quantity of air being supplied to the aeration
tank, (2) the rate of activated sludge recirculation, and (3) the amount
of excess sludge being withdrawn from the system. Sludge wasting is an
important operational practice because it allows the operator to estab-
lish the desired concentration of MLSS, food-to-microorganisms ratio,
and sludge age.
Note:
Air requirements in an activated sludge basin are governed by: (1)
biological oxygen demand (BOD) loading and the desired removal efflu-
ent, (2) volatile suspended solids concentration in the aerator, and (3)
suspended solids concentration of the primary effluent.
8.6 aCTivaTed sludge forMaTion
The formation of activated sludge is achieved in three steps. The
first step is the transfer of food from the wastewater to the organisms.
The second step is the conversion of wastes to a usable form. Third is
the flocculation step:
Transfer—
Organic matter (food) is transferred from the water to the
organisms. Soluble material is absorbed directly through the cell
wall. Particulate and colloidal matter is adsorbed to the cell wall,
where it is broken down into simpler soluble forms, then absorbed
through the cell wall.
Conversion—
Food matter is converted to cell matter by synthesis and
oxidation into end products such as CO
2
, H
2
O, NH
3
, stable organic
waste, and new cells.
flocculation—
Flocculation is the gathering of fine particles into
larger particles. This process begins in the aeration tank and is
the basic mechanism for removal of suspended matter in the final
clarifier. The concentrated
biofloc
that settles and forms the sludge
blanket in the secondary clarifier is known as
activated sludge
.