aerobic lagoons are not well suited where
land costs are extremely high or for
extremely large waste loads.
The treatment principle underlying
lagoons is biological oxidation and solids
sedimentation. Dissolved, suspended, and
settled solids are converted to volatile gases,
such as oxygen, carbon dioxide, and nitro-
gen; water; and biomass, such as microflora,
macroflora, and fauna. Anaerobic and other
lagoons equalize the discharge flow to fur-
ther treatment facilities or receiving waters.
The depth of anaerobic lagoons varies
from 2.5 m to 3.0 m. Surface area-volume
ratios should be minimal. Anaerobic condi-
tions are created throughout the entire
lagoon, through heavy organic loads. Under
anaerobic conditions, anaerobes digest the
organic matter. Loading rates are expressed
as BOD 5 , COD, SS, and other measurements
per unit volume of the lagoon. BOD 5 load-
ings range from 225 to 1,120 kg/ha/day.
Operating temperatures of 22ºC or higher are
needed, with 4 to 20 days of detention. BOD
reduction efficiency is typically 60 to 80% but
is a fraction of the influent BOD and the
determination time. Anaerobic lagoons are
used as primary or secondary treatment of
primary effluents containing high organic
loads or as sludge treatment systems. Anaer-
obic lagoons are normally followed by aero-
bic lagoons or by trickling filters because
their effluents remain high in organic matter
(i.e., more than 100 mg of BOD 5 ).
Some treatment processes incorporate a
combination of anaerobic and aerobic treat-
ment. A completely mixed anaerobic tank
reactor provides an environment for break-
ing down complex organic compounds into
CO 2 ,CH 4 , and simple organic compounds.
The anaerobic tank reduces BOD 5 by 85% to
95%. The gases separate from the water and
contain approximately 65 to 70% CH 4. The
effluent flows on to an aerobic reactor for
further treatment.
The previously described process involves
the flow of anaerobically treated water to a
degasification and flocculation tank, fol-
lowed by a lamella clarifier, where the anaer-
obic microorganisms are separated and
returned to the anaerobic tank. The super-
natant flows by gravity to an aeration basin,
where oxygen is supplied through mechani-
cal aerators. Because the aeration step of
the process has to remove only 5 to 15% of
the original BOD 5 , aerobic energy require-
ments are reduced. This process further
involves settling out of aerobic sludge in the
final clarifier, with a return to the aeration
basin. Surplus sludge is recirculated into the
anaerobic tank, where it enhances the bacte-
rial activity and undergoes decomposition.
A combination of anaerobic and aerobic
treatment can handle wide effluent varia-
tions. Anaerobic treatment responds slowly
to flow variation because of the slow growth
rate of the anaerobic microorganisms, but
the faster growing aerobic microorganisms
can generally treat the higher loads in the
anaerobic effluent. (Note: It is then no
longer anaerobic.)Aerobic Lagoons
Aerobic lagoons use mechanical aerators
to supply atmospheric oxygen for aiding bio-
logical oxidation. Mechanical agitators,
designed to pull air under water and circulate
it horizontally, can maintain a dissolved oxy-
gen concentration of 1 to 3 mg/L at a BOD
loading rate of up to 450 kg/ha/day. Because
oxygen transfer occurs under water, neither
freezing nor clogging occur. Aerated lagoons
are classified as either aerated facultative
lagoons (which have enough mixing to dis-
pense dissolved oxygen but not enough to
keep all the solids suspended) or as completely
mixed aerated lagoons, which are mixed
enough to keep all solids suspended. Approx-
imately 20% of the BOD sent to an aerobic
lagoon is converted to sludge solids, and the