3.8.3 High-Rate Aeration
Similar to the modified aeration process, a high-rate aeration process uses short
aeration times, 0.5–2 h, to take advantage of the fast BOD removal at the exponential
growth phase of biological sludge. A high MLSS concentration (4000–8000 mg/L)
is maintained to increase the sludge retention time in the system. Most often the
system is operated with a very high volumetric loading (up to 500 lb/1000 ft
3 ) and a
very high substrate utilization rate that poor settling of sludge in the final clarifier
occurs. Even though a BOD removal of 75–85% can be obtained, the effluent still
contains a significant amount of soluble and solid BOD that the effluent quality is no
better or inferior to that of the modified aeration.
3.8.4 Step Feed
In all standard process modifications that use plug flow mode operation, the return
sludge enters the head end of the aeration tank. Balmér et al. [37] developed a
process in which the pre-aerated return sludge is returned to the aeration tank in
equal portions at various locations. In the standard step aeration process, part of the
wastewater entering the outlet end of the aeration tank will have a short retention
time compared to the part entering the outlet end of the aeration tank. The BOD
removal will not be uniform, and a lower efficiency should be expected at the outlet
end. With step feeding of sludge, more biomass exists at the outlet end to ensure
efficient BOD removal. Compared with conventional and step aeration processes,
the step feed process maintains a higher MLSS concentration with a better adsorption property and thus can receive a higher BOD loading.
Balmér’s experience showed good-quality effluent at high F/M or volumetric
loading. An 85% BOD removal was obtained at 4 kg BOD/m
3 /d (250 lb
BOD/1000 ft
3 /d) loading and 66–79% removal at 9.6 kg BOD/m
3 /d or 600 lb
BOD/1000) ft
3
À/d loading. The process is able receive ten times as much volumetric
loading as an extended aeration process with equivalent treatment performance.
3.8.5 Oxidation Ditch
Using a shallow channel with a circular path, the wastewater is introduced into the
ditch continuously or intermittently for a long hydraulic retention time. The screened
wastewater is aerated by an aeration rotor and circulates at about 30–60 cm/s (1–2 ft/
s). The process is operated in a similar fashion to an extended aeration process or an
aerated lagoon with equivalent performance [38].
108
L. K. Wang et al.
Similar to the modified aeration process, a high-rate aeration process uses short
aeration times, 0.5–2 h, to take advantage of the fast BOD removal at the exponential
growth phase of biological sludge. A high MLSS concentration (4000–8000 mg/L)
is maintained to increase the sludge retention time in the system. Most often the
system is operated with a very high volumetric loading (up to 500 lb/1000 ft
3 ) and a
very high substrate utilization rate that poor settling of sludge in the final clarifier
occurs. Even though a BOD removal of 75–85% can be obtained, the effluent still
contains a significant amount of soluble and solid BOD that the effluent quality is no
better or inferior to that of the modified aeration.
3.8.4 Step Feed
In all standard process modifications that use plug flow mode operation, the return
sludge enters the head end of the aeration tank. Balmér et al. [37] developed a
process in which the pre-aerated return sludge is returned to the aeration tank in
equal portions at various locations. In the standard step aeration process, part of the
wastewater entering the outlet end of the aeration tank will have a short retention
time compared to the part entering the outlet end of the aeration tank. The BOD
removal will not be uniform, and a lower efficiency should be expected at the outlet
end. With step feeding of sludge, more biomass exists at the outlet end to ensure
efficient BOD removal. Compared with conventional and step aeration processes,
the step feed process maintains a higher MLSS concentration with a better adsorption property and thus can receive a higher BOD loading.
Balmér’s experience showed good-quality effluent at high F/M or volumetric
loading. An 85% BOD removal was obtained at 4 kg BOD/m
3 /d (250 lb
BOD/1000 ft
3 /d) loading and 66–79% removal at 9.6 kg BOD/m
3 /d or 600 lb
BOD/1000) ft
3
À/d loading. The process is able receive ten times as much volumetric
loading as an extended aeration process with equivalent treatment performance.
3.8.5 Oxidation Ditch
Using a shallow channel with a circular path, the wastewater is introduced into the
ditch continuously or intermittently for a long hydraulic retention time. The screened
wastewater is aerated by an aeration rotor and circulates at about 30–60 cm/s (1–2 ft/
s). The process is operated in a similar fashion to an extended aeration process or an
aerated lagoon with equivalent performance [38].
108
L. K. Wang et al.