In order to meet the requirement of a low effluent substrate concentration,
therefore, a lower specific substrate utilization rate (or a longer sludge retention
time) should be used. The terms food-to-biomass ratio (F/M) and specific substrate
utilization rate have been used interchangeably by engineers. The two have slightly
different meaning in definition in that F/M is the mass of substrate applied for unit
biomass per day, whereas U is the mass of substrate actually utilized by unit biomass
per day. The value of U approximates F/M only when nearly all of the substrate
applied to the system is utilized, a situation found in the secondary treatment of
organic waste in which a very high treatment efficiency is to be expected. However,
it is not correct to assume such relationship when the system is not achieving high
treatment efficiency.
At this point, it is necessary to reiterate the fact that all equations presented in the
previous section and relationships between process variables aforementioned apply
mainly to soluble substrates. With a significant portion of the substrate in the solid
form of organics, the system may behave differently in its treatment performance.
This fact is manifested in the contact stabilization operation in which the kinetics of
growth and substrate removal are completely different from that of other modifications of activated sludge processes.
The effect of sludge retention time θ c on effluent substrate concentration can be
found in Eq. (3.9), as is illustrated in Fig. 3.8.
The aforementioned kinetics also assumes a near-complete removal of biomass
from the final clarifier. In other words, the settling characteristics of the activated
sludge are completely overlooked. Sludge retention time has significant effects on
the sludge settling characteristics. A short sludge retention time and the resulting
Fig. 3.6 Relationship
between sludge retention,
specific growth rate, and
substrate utilization rate.
(Source: US EPA)
3 Biological Processes
93
therefore, a lower specific substrate utilization rate (or a longer sludge retention
time) should be used. The terms food-to-biomass ratio (F/M) and specific substrate
utilization rate have been used interchangeably by engineers. The two have slightly
different meaning in definition in that F/M is the mass of substrate applied for unit
biomass per day, whereas U is the mass of substrate actually utilized by unit biomass
per day. The value of U approximates F/M only when nearly all of the substrate
applied to the system is utilized, a situation found in the secondary treatment of
organic waste in which a very high treatment efficiency is to be expected. However,
it is not correct to assume such relationship when the system is not achieving high
treatment efficiency.
At this point, it is necessary to reiterate the fact that all equations presented in the
previous section and relationships between process variables aforementioned apply
mainly to soluble substrates. With a significant portion of the substrate in the solid
form of organics, the system may behave differently in its treatment performance.
This fact is manifested in the contact stabilization operation in which the kinetics of
growth and substrate removal are completely different from that of other modifications of activated sludge processes.
The effect of sludge retention time θ c on effluent substrate concentration can be
found in Eq. (3.9), as is illustrated in Fig. 3.8.
The aforementioned kinetics also assumes a near-complete removal of biomass
from the final clarifier. In other words, the settling characteristics of the activated
sludge are completely overlooked. Sludge retention time has significant effects on
the sludge settling characteristics. A short sludge retention time and the resulting
Fig. 3.6 Relationship
between sludge retention,
specific growth rate, and
substrate utilization rate.
(Source: US EPA)
3 Biological Processes
93