BIOLOGICAL TREATMENT OF WASTEWATER 157
A supplemental source of carbonaceous BOD must be added
in this stage to reduce the nitrates to nitrogen gas in a reason-
able period of time. This has been accomplished either by
adding a cheap organic substrate like methanol or by bypass-
ing a part of the wastewater containing carbonaceous BOD
in the first stage. In some cases, the carbonaceous and nitro-
geneous oxidation steps are combined in a one-stage aerobic
biological system. Another system uses fixed-film reactors,
such as gravel beds, separately for nitrification and denitri-
fication stages. Effluent nitrogen concentrations of 2 mg/L
have been proposed as the upper limit in a biological process.
Many full scale biological nitrogen removal facilities are now
in operation. Nitrifying bacteria are subject to inhibition by
various organic compounds, as well as by inorganic com-
pounds such as ammonia. Free ammonia concentrations of
0.1 to 1.0 mg/L and free nitrous acid concentrations of 0.22 to
2.8 mg/L, start inhibiting Nitrobacters in the process.^20
The majority of phosphorus compounds in wastewaters are
soluble and only a very small fraction is removed by plain sedi-
mentation. The conventional biological treatment methods typ-
ically remove 20 to 40 percent of phosphorus by using it during
cell synthesis. A considerably higher phosphorus removal has
been achieved by modifying the processes to create “luxury
phosphorus uptake.” Factors required for this increased pho-
shorus removal are plug-flow reactor, slightly alkaline pH,
presence of adequate dissolved oxygen, low carbon dioxide
concentration and no active nitrification.^46 However, the most
effective method of phosphate removal is the addition of alum
or ferric salts to conventional activated sludge processes.NomenclatureA v = Specific surface area of filter media, Length– 1
B v = Volumetric loading rate; mass per unit volume per
unit time
D = Longitudinal dispersion coefficient, (Length)^2 per
unit time
E = Process treatment efficiency, ratio
H = Filter depth, length
K = Half velocity coefficient = substrate concentration
at which rate of its utilization is half the maximum
rate, mass per unit volume
K i = Inhibition constant, mass per unit volume
L = Substrate concentration around microorganisms in
reactor, measured in terms of BOD, mass per unit
volume
N 0 = Number of microorganisms per unit volume at
time t = 0
N t = N = Number of microorganisms per unit volume at
time t
∆ O 2 = Amount of oxygen requirement, mass per unit time
Q = Volumetric rate of flow, volume per unit time
Q a = Volumetric rate of flow per unit area, Length per
unit timeQ r = Volumetric rate of return flow, volume per unit
time
R = Recycle ratio
S = Substrate concentration, mass per unit volume
∆ S = Substrate removed, mass per unit time
S e = Effluent BOD or final substrate concentration,
mass per unit volume
S 0 = Influent BOD or in the initial substrate concentra-
tion, mass per unit volume
T = Temperature, °C
U = Process loading factor, time– 1
V = Volume of the reactor, volume
X = Mass of active microorganisms present per unit
volume
∆ X = Cell mass synthesized, mass per unit time
X e = Effluent volatile suspended solids, mass per unit
volume
X 0 = Influent volatile suspended solids, mass per unit
volume
X r = Volatile suspended solids in return sludge, mass
per unit volume
X T = Total mass of microorganisms in the reactor, mass
Y = Growth yield coefficient, dimensionless
a = Fraction of BOD removed that is oxidized for
energy
b = Microorganisms decay coefficient, time– 1
b = Oxygen used for endogenous respiration of biologi-
cal mass, time– 1
c 1 = Constant
f = Fraction of volatile suspended solids present in
the influent which are non-degradablek f ,k f ,k (^) f = Rate coefficient in filters, time– 1
k 0 = Logarithmic growth rate constant, time– 1
k t = Growth rate factor, time– 1
k = Growth rate factor, (time)– 1 (mass per unit volume)– 1
l = Length dimension in reactor, Length
m = Constant
n = Trickling filter exponent
q = d S / X d t = Substrate utilization rate per unit biomass
q max = Maximum substrate utilization rate per unit
biomass
t = Contact time in filter or any other reactor, time
t - = V / Q = Mean retention time, time
u = Mean displacement velocity in reactor along
length, length per unit time
w = Volumetric rate of flow of waste sludge, volume
per unit time
u = Temperature coefficient for microbial activity
C002_001_r03.indd 157C002_001_r03.indd 157 11/18/2005 10:15:55 AM11/18/2005 10:15:55 AM