Activated Sludge Process - Wastewater Operators - page 300

Note: The activity level of bacteria within the activated sludge process
increases with rise in temperature.
8.7.6 Toxicity
Sufficient concentrations of elements or compounds that enter a
treatment plant that have the ability to kill the microorganisms (the
activated sludge) are known as toxic waste (shock level). Common to
this group are cyanides and heavy metals.
Note: A typical example of a toxic substance being added by operators is
the uninhibited use of chlorine for odor control or control of filamentous
organisms (prechlorination). Chlorination is for disinfection. Chlorine is
a toxicant that should not be allowed to enter the activated sludge pro-
cess; it is not selective with respect to the type of organisms damaged or
killed. It may kill the organisms that should be retained in the process
as workers; however, chlorine is very effective in disinfecting the plant
effluent after treatment by the activated sludge process.
8.7.7 hydraulic loading
Hydraulic loading is the amount of flow entering the treatment pro-
cess. When compared with the design capacity of the system, it can be
used to determine if the process is hydraulically overloaded or under-
loaded. If more flow is entering the system than it was designed to
handle, the system is hydraulically overloaded. If less flow is entering
the system than it was designed for, the system is hydraulically under-
loaded. Generally, the system is more affected by overloading than by
underloading. Overloading can be caused by stormwater, infiltration of
groundwater, excessive return rates, or many other factors. Underloading
normally occurs during periods of drought or in the period following ini-
tial startup when the plant has not reached its design capacity. Excess
hydraulic flow rates through the treatment plant will reduce the effi-
ciency of the clarifier by allowing activated sludge solids to rise in the
clarifier and pass over the effluent weir. This loss of solids in the effluent
degrades effluent quality and reduces the amount of activated sludge in
the system, in turn reducing process performance.
8.7.8 organic loading
Organic loading is the amount of organic matter entering the treat-
ment plant. It is usually measured as biochemical oxygen demand (BOD).
An organic overload occurs when the amount of BOD entering the sys-
tem exceeds the design capacity of the system. An organic underload
occurs when the amount of BOD entering the system is significantly
less than the design capacity of the plant.
Organic overloading can occur when the system receives more waste
than it was designed to handle. It can also occur when an industry or
other contributor discharges more wastes to the system than originally