Environmental Engineering Reference
In-Depth Information
and disposal. The volume of sludge returned to the aeration basins is
normally 40 to 60% of the wastewater flow. The rest is wasted.
8.4 faCTors affeCTing oPeraTion
of The aCTivaTed sludge ProCess
A number of factors affect the performance of an activated sludge
system. These include the following:
• Temperature
Return rates
•
•
Amount of
oxygen available
•
Amount of
organic matter available
• pH
•
Waste rates
•
Aeration time
•
Wastewater toxicity
To obtain the desired level of performance in an activated sludge
system, a proper balance must be maintained among the amount of food
(organic matter), organisms (activated sludge), and oxygen (dissolved
oxygen). The majority of problems with the activated sludge process
result from an imbalance among these three items.
To fully appreciate and understand the biological process taking
place in a normally functioning activated sludge process, the operator
must have knowledge of the key players in the process: the organisms.
This makes a certain amount of sense when you consider that the heart
of the activated sludge process is the mass of settleable solids formed by
aerating wastewater containing biological degradable compounds in the
presence of microorganisms. Activated sludge consists of organic solids
plus bacteria, fungi, protozoa, rotifers, and nematodes.
8.5 groWTh Curve
To understand the microbiological population and its function
in an activated sludge process, the operator must be familiar with the
microorganism
growth curve
(see Figure 8.2). In the presence of excess
organic matter, the microorganisms multiply at a fast rate. The demand
for food and oxygen is at its peak. Most of this is used for the production
new cells. This condition is known as the
log growth phase
. As time
continues, the amount of food available for the organisms declines. Floc
begins to form while the growth rate of bacteria and protozoa begins to
decline. This is referred to as the
declining growth phase
. The
endoge-
nous respiration
phase occurs as the food available becomes extremely
limited and the organism mass begins to decline. Some of the micro-
organisms may die and break apart, thus releasing organic matter that
can be consumed by the remaining population.
