Biological organisms within the system need
sufficient contact time with the organic material in
order to produce effluent of an acceptable quality.
Typical contact time for extended aeration package
plants is approximately 18-24 hours. The contact
time, daily flow rate, influent parameters, and
effluent parameters determine the size of the
aeration tank where air is used to mix wastewater
and to supply oxygen to promote biological growth.
A package extended aeration system is sized based
on the average volume of wastewater produced
within a twenty-four hour period. Although
provisions are made for some peaking factor, a flow
equalization system may be necessary to prevent
overloading of the system from inconsistent flow
rates in the morning and evening. Equalization
allows the wastewater to be delivered to the
treatment plant at more manageable flow rates
(WEF, 1985).
Systems should be installed at sites where
wastewater collection is possible by gravity flow.
In addition, the site should be stable, well drained,
and not prone to flooding. The facility should be
installed at least 30 meters (100 feet) from all
residential areas and be in accordance with all
health department regulations or zoning restrictions
(WEF, 1985).