In order to ensure ease of operation and
maintenance, extended aeration systems should be
installed so that the tank walls extend nearly 0.15
meters (6 inches) above ground. This will supply
insulation in the winter, prevent surface runoff from
infiltrating the system, and allow the system to be
serviced readily. If a plant is installed below
ground, it must have distinct diversion ditching or
extension walls in order to prevent surface water
infiltration into the plant. When the plant is
installed completely above ground, it may be
necessary to provide insulation for cold weather and
walkways for easy maintenance (WEF, 1985).
Sequencing batch reactors
Important internal components include an aeration
system, which typically consists of diffusers and a
blower; a floating mixer; an effluent decanter; a
pump for withdrawing sludge; and a sequence of
liquid level floats. The PLC and the control panel
are usually positioned within a nearby control
building (Sloan, 1999).
When the wastewater flow rate at the site is less
than 0.05 MGD, a single, prefabricated steel tank
can be used. This tank is divided into one SBR
basin, one aerobic sludge digester, and one influent
pump well. Concrete tanks may also be used, but in
North America are not as cost effective as steel for
small systems. If the plant must be able to treat 0.1
to 1.5 MGD, multiple concrete SBR basins are
commonly used (CASS, 1999).
The design of SBR systems can be based on
carbonaceous BOD removal only or both
carbonaceous and nitrogenous BOD removal. The
system can be expanded to achieve optimum
nitrification and carbonaceous removal by operating
primarily in an oxic state with few anoxic periods
such as during settle and decant.
Denitrification and biological phosphorous removal
can be promoted by providing adequate anoxic
periods after intense aerobic cycles. This allows
DO to be dissipated and nitrate to be used by the
consuming organism and released as elemental
nitrogen. By introducing an anaerobic process after
the anoxic process, bacteria conducive to excess
phosphorous uptake will develop. Phosphorous
will be released in the anaerobic phase, but
additional phosphorous is incorporated into the cell
mass during subsequent aerobic cycles. Since the
excess phosphorous is incorporated in the cell mass,
cell wastage must be practiced to achieve a net
phosphorous removal. Anaerobic conditions should
be avoided in treating the waste sludge since they
may result in the release of the phosphorous.