NITROGEN REMOVAL ACTIVATED SLUDGE SYSTEMS
Nitrogen is one of the principal nutrients of concern in wastewaters. Organic matter
removal is not enough to solve all the problems. Most of these problems, such as
eutrophication, excess depletion of oxygen, fish toxicity, etc. are associated with
PARTICULAR FORMS OF NITROGEN. The activated sludge process is a
biochemical tool to secure the necessary conversion and removal of nitrogen forms.
The relevant transformations that are likely to occur in the activated sludge process:
1) The microbial degradation of nitrogen- containing compounds and release of
ammonia
2) The incorporation of nitrogen into cells by synthesis and its removal by excess
sludge
3) The microbially mediated redox transformations including NITRIFICATION and
DENITRIFICATION
(As mentioned in the previous lectures, significant nitrogen removal can not be
achieved by biological incorporation into activated sludge; 20-30 % only)
Nitrogen Content of Sewage and Its Removal by Conventional Treatment
Concentration Raw waste Effluent Primary Effluent from Secondary
(mg/l) Settling Biological Treatment
mg/l % removal mg/l % removal
Organic nitrogen 10-25 7-20 10-40 3-6 50-80
Dissolved 4-15 4-15 0 1-3 50-80
Suspended 4-15 2-9 40-70 1-5 50-80
Ammonia nitrogen 10-30 10-30 0 10-30 0
Nitrite nitrogen 0-0.1 0-0.1 0 0-0.1 0
Nitrate nitrogen 0-0.5 0-0.5 0 0-0.5 0
Total nitrogen 20-50 20-40 5-25 15-40 25-55
FORMS OF NITROGEN
4 major forms are defined to depict nitrogen in wastewater discharges and receiving
waters:
Organic nitrogen
Unoxidized TKN
+
Ammonia nitrogen (NH3-N or NH4 -N)
Total
Nitrogen
Nitrite nitrogen (NO2- -N)
Oxidized
Nitrate nitrogen (NO3- -N)
Only the first two are significant for wastewaters with the exception of a few specific
industrial discharges.(See the above Table)
Unoxidized nitrogenous compounds are experimentally determined by Total Kjeldahl
Nitrogen (TKN) and ammonia nitrogen (NH3-N) tests.
The oxidized nitrogen compounds (NO2- -N, NO3- -N) are not accounted for by the
TKN test. There are different analysis methods for these. Wastewaters do not contain
appreciable amounts of oxidized nitrogen.
NITRIFICATION (First step in biological nitrogen removal)
Nitrification, as a controlled ammonia oxidation process, oxidizes ammonia first to
nitrite (NO2- -N) and then to nitrate (NO3- -N) .
DENITRIFICATION (Second step for overall nitrogen removal systems)
Denitrification reduces nitrate, NO3- -N, to nitrogen gas and returns it to the
atmosphere. So nitrogen removal is completed.
Denitrification is only applicable when the nitrogen content of the wastewater is
predominantly in the nitrate form. Domestic sewage, however does not generally
contain oxidized nitrogen. Therefore, in municipal applications the organic and
ammonia nitrogen in sewage must first be oxidized to nitrate before the nitrogen can
be removed. This necessitates coupling biological denitrification with nitrification.
Since the effluent limitations for nitrogen are given in terms of total nitrogen (organic
nitrogen + ammonia + nitrite + nitrate), nitrification itself is not enough for complete
nitrogen removal unless it is combined with denitrification.
Optimization of the activated sludge process with regard to a rational control of
nitrification and denitrification mechanisms, depends upon a better understanding of
the stoichiometry and the kinetics of these two processes. We will only see the
stoichiometry and the kinetics of NITRIFICATION as well as the design of
nitrification systems combined with carbon removal activated sludge systems.
NITRIFICATION
Microbial Description:
Nitrification is associated with the metabolism of a group of
CHEMOAUTOTROPHIC bacteria.
Principle C source : Inorganic carbon (CO2)
Energy source : Chemical energy (inorganic nitrogen oxidation)
Electron donor : NH4+
Electron acceptor : O2
NH4++3/2 O2 NO2-+2H++H2O Nitrosomonas
NO2-+1/2 O2 NO3- Nitrobacter
Stoichiometry of Nitrification
The oxidation of ammonia (NH4+) is involved both in the energy and biosynthesis
reactions. A portion of the electrons released by the oxidation of ammonia is
transferred to O2 in the energy reaction, while the remaining part is used as the
reducing power to convert CO2 to the oxidation level of cellular constituents
(C5H7NO2).
Energy Reaction:
+
NH 4 + 2 O 2 NO 3 + 2 H + + H 2 O
123 {
14 64
64/14=4.57 g O2/g NH4+-N
It should be noted that nitrification generates protons (H+) as indicated in energy
reaction. Consequently the process will consume alkalinity equivalent to the amount
of protons released. Two moles of HCO3- are consumed per mole of NH4+-N
oxidized. This is equivalent to an alkalinity of 7.14 g Ca CO3/g NH4+-N.
Biosynthesis reaction:
+
2.5 NH 4 + 5CO2 + NH 3 C 5 H 7 NO 2 + 2.5 NO3 + 5 H + + 1 / 2 H 2 O
1424 3 123 {
electron donor C source N as nutrient
Reported values for YA range from 0.07 to 0.28 gr cell COD/g N. The autotrophic
yield, YA, is very low comparing with that of heterotrophs.
The observed stoichiometry of nitrification:
+
NH 4 + 1.856 O 2 + 1.98 HCO 3 0.021 C 5 H 7 NO 2 + 0.98 NO 3 + 1.876 CO 2 + 2.92 H 2 O
Only 0.021 mole biomass may be generated with the removal of each mole of NH4+-
N.
(0.021x113)/14=0.17 g biomass/g ammonia removed
0.17x1.42=0.24 g COD/g N
As seen from the above stoichiometry, nitrification is usually based on observation
that 4.33 gr of O2 is consumed for each gram of nitrate nitrogen formed. On the other
hand, energy equation shows that for each gram of nitrogen used as an electron
donor, 4.57 gr of O2 is required without any consideration of biomass formation.
Consequently, the O2 equivalent of biomass generation under observed conditions
may be calculated as:
4.57-4.33=0.24 gr O2/gr N
This correspons to an observed YNA of 0.24 gr cell COD/gr N, a value which is
currently accepted as the overall autotrophic yield characterizing nitrification.
Oxygen requirement
For carbon removal ORH=(1-YNH).COD removed
For nitrification ORA=(4.57-YNA).NH4-N removed
Since the YNA is too small, biomass generation can be neglected and oxygen
requirement for autotrophic growth can be practically calculated:
ORA=4.57x(removed ammonia)
Process Kinetics for Nitrification
Growth of autotrophs
Decay of autotrophs
Growth of Autotrophs
The relationship between the growth rate of nitrifying bacteria and the concentration
of energy yielding substrate (NH4+) has been formulated by a Monod-type rate
expression, as in the case of heterotrophic growth. In contrast to the organic matter
removal, substrate is NH4+in autotrophic growth.
S NH
A = A
K NH + S NH
-1
A =specific growth rate of nitrifiers [T ]
-1
A =maximum specific growth rate of nitrifiers [T ]
SNH=ammonia nitrogen concentration in the growth medium [M(N)/L3]
KNH=ammonia half-saturation constant [M(N)/L3]
Effect of pH on the maximum growth rate ( A ),
A,pH = A [1 0.833(7.2 pH) ]
A is assumed to be constant for the pH range 7.2-8.5.
Effect of temperature on the maximum growth rate, A ,
A,T = A,20 T 20 = 1.08 1.123
Effect of Dissolved Oxygen on the maximum growth rate, A ,
It is generally accepted that S0 decreases, it becomes the growth-limiting substrate.
This is expressed by a saturation function:
SO
A = A
K OA + S O
where KOA is the half- saturation constant for SO function [M(O2/L3]
Hence,
dX A S NH
= A X A = A XA
dt K NH + S NH
Ammonia removal rate, nitrate generation rate and oxygen utilization rate can be
expressed as below based on growth process:
dS NH 1 dX A
=
dt YA dt
dS NH A S NH dS NH A dS NH
= X A or = XA or = qA XA
dt YA K NH + S NH dt YA dt
XA=autotrophic biomass concentration [M(VSS)/L3]
qA= specific ammonia oxidation rate [M(N)/M(VSS)T]
The ammonia nitrogen that is oxidized in the nitrification reactions appears as nitrate
nitrogen, SNO:
dS NO dS
= NH
dt dt
Oxygen consumption on the basis of stoichiometry
dSO dS
= (4.57 YA ) NH
dt dt
Decay of Autotrophs
It is mathematically defined by the following traditional first-order rate expression:
dX A dX A
= b A X A overall = ( A b A ) X A
dt dt
bA= decay coefficient for autotrophs [T-1]
DESIGN PROCEDURE OF ACTIVATED SLUDGE SYSTEMS FOR
NITRIFICATION AND CARBON REMOVAL
Select X
K NH (1 + b A X )
S NH =
Calculate SNH A X (1 + b A X )
(effluent ammonia concentration)
check SNHSNHe if not
K S (1 + k d X )
Calculate S S=
H X (1 + k d X )
(effluent soluble BOD5 or COD concentration)
Q(S1 S)
YH
PXH =
Calculate PXH 1+ kd X
(daily amount of heterotrophic biomass produced)
PXH
Calculate NX N X = i XB
Q
(nitrogen incorporated into biomass per unit volume)
Calculate NOX SNO = NOX = SNH1 SNH NX
(ammonia nitrogen oxidized per unit volume of waswater treated)
Calculate residual alkalinity SALK=SALK1-7.14 NOX
(a minimum residual alkalinity of 100-150mg/l CaCO3 is recommended)
Calculate ORTi hesapla ORT=ORH+ORA
(total oxygen requirement)
1 YH
OR H = ( - f X YNH ) Q (S1 S) YNH =
f 1+ kd X
YA
OR A = (4.57 f X YNA ) Q NO X YNA =
1+ bA X
Calculate PXA and PXT PXA=YNA Q NOX PXT=PXH+PXA
Select XT
PXT X V
Calculate V and h V= , h =
XT Q
1 h / X
Select XR and calculate R R=
XR
1
X
PXT
Calculate QW QW =
XR
