Page 747 - Fundamentals of Water Treatment Unit Processes : Physical, Chemical, and Biological

702 Fundamentals of Water Treatment Unit Processes: Physical, Chemical, and Biological

temperature; dissolved-oxygen concentration; and pH (Parker where
et al.=EPA 1975, pp. 3-6–3-29). The kinetic rate constant, m, M is the methanol concentration (mg methanol=L)
for ammonia follows the Monod equation, i.e., with K M is the half-saturation constant for methanol (mg
C(NH 4 ) as the S. The cell concentration, X(nitrifiers), methanol=L)
þ
depends on the fraction, f(nitrifiers), of X (the MLSS in the
reactor), which depends on the BOD=TKN ratio (Parker They give K M ¼ 0.1 mg methanol=L; in other words an
et al.=EPA 1975, pp. 3–22). Oxygen is a reactant, per the excess of ethanol above the stoichiometric requirement is not
stoichiometric equation, Equation 22.9, and affects the spe- necessary as about 1 mg methanol=L in the reactor effluent is
cific growth rate as defined by a ‘‘secondary’’ Monod equa- sufficient for the kinetic coefficient to approach the maximum.
tion, i.e., by the factor [C(O 2 )=(K s (O 2 )þC(O 2 )], or Concerning pH, the highest rate is in the range 7.0 pH .5.
1
m 0.19 [C(O 2 )=(2.0þC(O 2 )] day ; the curve starts to As evident from Equation 22.10 the reaction is anoxic, which
level at C(O 2 ) > 4mg O 2 =L, with about 0.8 fraction of bm means that oxygen has a negative effect since it has a stronger
at 4 mg=L. The effect of pH is due to the alkalinity require- affinity for electrons than does NO 3 .

is available at Since the reaction must occur without oxygen, an anoxic
ment, per Equation 22.9, in which HCO 3
7.0 < pH < 8.4. reactor must be added to follow the nitrification reactor. A
Nitrification is done most effectively in a separate nitrifi- carbon supplement is usually required.
cation activated-sludge reactor, then the cell-separator stage
that follows the organic carbon (BOD) reactor and cell separ-
ator. In the design of the system, from u c 1=(m b), then 22.5.9 PHOSPHOROUS UPTAKE
m
u 1=(bm b) 1=(0.32–0.05) day 1 3.7 day (note that
c
m
b 0.05 day 1 was assumed). The SRT(design) > u ;thus, 22.5.9.1 Occurrence in Wastewaters
c
SRT(design)> 3.7 day. Parker et al.=EPA (1975, pp.3-19–3-20) The three forms of phosphorous in wastewater are orthopho-
recommended SRT(design) 5 day, with an associated reduc- sphate, polyphosphate, and organically bound phosphate
tion in ammonia concentration >0.95 fraction. (Orhan and Artan, 1994, p. 471). The orthophosphates
A major factor in successful nitrification is that nitrifiers are easily assimilated by microorganisms. McCarty (1970,
must comprise a significant fraction of the biomass, i.e., X,in 227) gives the formulae of condensed phosphates as
4
5
the reactor. This requires a low BOD=TKN ratio in the tripolyphosphate, P 3 O 10 ; and pyrophosphate as P 2 O 7 ,
reactor, calculated as BOD removed in mg=L and TKN which must be hydrolyzed to the PO 4 3 form for assimilation;
oxidized in mg=L; to illustrate, if the numbers are 20 and 25, about 0.5–0.8 fraction are hydrolyzed during biological treat-
ment (p. 228). The total P in wastewater ranges 6–20 mg=L
respectively, BOD=TKN 0.8. Further, if Y(net nitrifiers)
3
0.15 kg nitrifiers synthesized=kg NH 4 as N oxidized and if and the orthophosphate, PO 4 -as-P ranges 4–15 mg=L
þ
Y(net-cell-mass) 0.55 kg cells synthesized=kg BOD oxidized, (Orhan and Artan, 1994, p. 472). Table 22.2 provides add-
then (f nitrifiers) (0.15 25)=[(0.15 25)þ(0.55 20)] ¼ 0.25. itional resolution on concentrations and forms. Black and
The low BOD=TKN ratio is achieved by adding a nitrifying Veatch=EPA (1971, p. 2-1) gave a yearly mass P output per
reactor with sludge recycle, after the carbon reactor. In other capita in municipal wastewater as 1.6 kg (3.5 lb) with about
words, to emphasize, the treatment train adds a nitrification 0.5 kg (1.2 lb) from human excretions and about 1.1 kg
reactor with a cell separator and recycle. For most systems, (2.3 lb) from detergents with phosphates and gave an average
1 BOD=TKN 3, for which 0.21 > (f-nitrifiers) > 0.08. total P in domestic raw wastewater as 10 mg P=L.
Parker et al.=EPA (1975, pp. 3–25) showthatat208C, the

nitrification rate, U(N)¼ f(nitrifiers) [(m(nitrifiers)=Y(nitrifiers)] 22.5.9.2 Uptake to Cells
1
(0.25kgnitrifiers=kg all cells) [0.32 day =0.15 kg cells synthe-
Phosphorous is an integral part of cell matter such as DNA,
size=kg NH 4 -N oxidized] 0.53 kg NH 4 -N oxidized=kg all
þ
þ
RNA, ADP=ATP, etc., and therefore is an essential nutrient.
cells=day. The ‘‘all cells’’ is measured as MLVSS. Thus, the rate
From the P-based cell formula by Stumm, C 106 H 180 O 45 N 16 P,
of substrate utilization rate, U, depends to a large extent on the
Equation 22.5, MW ¼ 2429 g=mol and MW(P) 31, giving
fraction of nitrifiers.
fraction(P) 0.013. Bowker and Stensel (EPA, 1987, p. 15)
gave 0.015 < fraction(P) < 0.02. They gave an estimated
22.5.8.2 Denitrification: NO 3 to N 2 Gas 0.10 < fraction=removal(P) < 0.30. Their historical review

Concerning denitrification, i.e., conversion of NO 3 to N 2 referred to a 1955 finding that P uptake could exceed the
gas, kinetic constants are given in Table 22.9. Parker et al.= cell fractions normally expected, and 1965 observations at a
EPA (1975, pp. 3–40) use a secondary Monod relationship to full-scale plant of P 1mg=L with adequate aeration, which
calculate the effect of other variables on the maximum spe- was referred to as ‘‘luxury uptake.’’ For such sludges, 0.02 <
cific growth rate. For the effect of methanol as carbon source fraction(P) < 0.07.
the relationship given was
22.5.9.3 Theory
M
_
m(nitrifiers) ¼ m(nitrifiers) (22:59) In the early 1970s the additional uptake was thought by
K M þ M some to have been due to chemical precipitation, which