Page 748 - Fundamentals of Water Treatment Unit Processes : Physical, Chemical, and Biological

Biological Reactions and Kinetics 703

was not the case; the P was ‘‘stored’’ in the cells. Also, 22.6 SUMMARY
P release was observed in sludges under the anaerobic con-
Biological treatment processes, as stated, may be summarized
ditions in the final clarifier, with subsequent P increase in the
in terms of reactions and kinetics. The three kinds of reactions
reactor, i.e., higher than found in the raw wastewater. The
include (1) heterotrophic, (2) autotrophic including nitrifica-
anaerobically conditioned cells, in addition to the net release
tion and denitrification, and (3) anaerobic. A fourth reaction is
of P, were also characterized by having a higher rate of
endogenous respiration in which cell matter is oxidized. A
P uptake (as compared with cells not subject to an anaerobic
key principle is that the cells synthesized are related stoichio-
environment). The foregoing observations led to the general
metrically to the substrate degraded.
approach in P removal: (1) promote ‘‘starved’’ microorgan-
The Monod equation is the kinetic model, and has been
isms entering the reactor, and a ‘‘selection’’ of an organism
adopted, universally since the late 1960s. A limitation is that
such as Acinetobacter, that were amenable to the luxury
the constants, bm, and K s , are unique to the situation at hand.
uptake of P and at high rate; (2) get rid of the P either in
Data available are variable and depend upon the substrate and
the supernatant of sludge after being subjected to an anaer-
organisms and are not reported in consistent units.
obic environment or in the sludge. To expand further, as
Empirical parameters have been adopted for practice and
abstracted from Bowker and Stensel (EPA, 1987, p. 17),
include the substrate utilization rate, U, and the sludge age, u c .
the P removal was also observed in anoxic zones coincident
Each parameter has a rational basis.
with nitrate reduction (with N 2 as a product). As to mechan-
Nitrification, i.e., conversion of ammonia to nitrate,
ism of the luxury uptake of P, the bacteria showing
follows the same principles as for carbon-based substrate
this characteristic, stored the P as polyphosphates within
degradation. The nitrifiers have a slower growth rate
‘‘volutin’’ granules and is associated with the chemical,
than heterotrophic organisms, and do not compete well.
polyhydroxybutyrate (PHB).
Therefore, a separate reactor is recommended to follow the
22.5.9.4 Technologies carbon-substrate reactor, operated with sludge recycle.
The nitrification reactions may be carbon limited and benefit
Based on the foregoing research, several approaches
by a supplemental carbon source such as methanol. Denitrifi-
emerged to become technologies in biological treatment.
cation requires an anaerobic environment and a separate
Three of these processes, in order of development, are
reactor. Biological phosphorous removal has evolved as a
(1) the Phostrip process, (2) the modified Bardenpho process,
technology from observations in the 1950s to a rationale
and (3) the A=O process. As indicated they are proprietary.
for practice by the early 1970s, with several proprietary
In the Phostrip process, a sidestream of settled sludge from
technologies resulting.
the final clarifier is passed through an ‘‘anaerobic phosphor-
ous stripper,’’ where the supernatant is subjected to lime
precipitation and leaves the system. The sludge is returned
PROBLEMS
to the aeration basin. The other portion of the final clarifier
underflow is handled normally with a portion wasted (WAS) 22.1 Mass Balance for a Chemical Equation
and the other portion returned to the aeration basin (RAS)
Given
along with the sludge from the anaerobic P stripper. The
Equation 22.8 for oxidation of sugar and synthesis of
modified Bardenpho process (p. 21) is both a nitrogen and P
cells.
removal system. The raw-water inflow and the RAS are
contacted in an anaerobic tank to promote fermentation and
8(CH 2 ) þ 3O 2 þ NH 3 ! C 5 H 7 NO 2 þ 3CO 2 þ 6H 2 O
P release. Following this the flow passes through the four
stage Barenpho process, i.e., anoxic, aerobic, anoxic, aer-
Required

obic. In the first stage, NO 3 from internal recycle from
Calculate the mass balance for each element for each
nitrification in the second stage, is reduced to N 2 gas utiliz-
side of Equation 22.4 for both a molar basis and for a
ing the carbon in the raw wastewater (in lieu of an external
mass basis.
carbon source), which removes about 0.7 fraction of the
22.2 Calculate Cell Yield, Y for Sugar as the Substrate

NO 3 nitrogen. In the second stage, an aerobic stage,
NH 4 oxidation, BOD removal, and P uptake occur. In the Given
þ
third stage, which is anoxic, additional denitrification occurs. Equation 22.8 gives an equation for cell synthesis, i.e.,
In the fourth stage, which is aerobic, the aeration conditions
the MLSS to minimize anaerobic condition in the final
8(CH 2 ) þ 3O 2 þ NH 3 ! C 5 H 7 NO 2 þ 3CO 2 þ 6H 2 O
clarifier that may result in P release. The underflow from
(22:8)
the final clarifier is handled as with conventional activated
sludge, with a portion returned to the raw-water inflow
MW 240,113
(RAS) and a portion wasted (WAS). The A=O process has
a sequence of four anaerobic stages followed by four aerobic Required
stages, with RAS and WAS handled normally. Calculate the cell yield, Y.