Nitrogen
Nitrogen problems in water quality
Cause of problems –
nitrification: oxygen depletion
denitrification: loss of nitrogen
eutrophication: algae or plant growth, oxygen depletion, scums, clogging of
waterways, etc…
Problem itself (toxic effects) –
nitrate: blue baby symptom (methemoglobinemia, ~10mg/L of nitrate)
ammonia (NH3): at high pH (>9) and moderate temp (~ 20oC), toxic to fish
Nitrogen
Nitrogen processes
Nitrogen fixation –elemental nitrogen → org. N, blue-green algae
Ammonification – org. N → ammonia, bacterial decomposition, zooplankton excretion, cell death
Nitrification – oxidation of ammonia to nitrate via nitrite (1st order reaction)
Ammonia and nitrate assimilation – uptake of ammonia and nitrate by phytoplankton
Denitrification – dissimilative reduction of nitrate to free nitrogen under anaerobic conditions
Nitrification
Oxidation of ammonia to nitrite, to nitrate
Nitrogen in sewage = organic N (proteins, urea, etc.) + ammonia N
Sewage N → org. N → ammonia N → nitrite → nitrate
NH4+ + 1.5O2 → 2H+ + H2O + NO2NO2- + 0.5O2 → NO3NH4+ + 2O2 → NO3- + H2O + 2H+
1.5  32
 3.43 gO / gN
1 14
0.5  32
roi 
 1.14 gO / gN
1 14
2  32
ron 
 4.57 gO / gN
1 14
roa 
It occurs when 1) 암모니아 존재, 2) nitrifying bacteria, 3) alkaline pH 4) oxygen (>1~2mg O2/L)
Slow growth of nitrifying bacteria, less competitive than OC oxidizing microbes for the substrate
and DO utilization
→ Takes place usually farther downstream than the discharge point where OC is decomposed
→ tc moves to further downstream (between tc for OC oxidation and tc for nitrification)
→ Dc increases
Nitrogen modeling (NBOD approach)
NBOD approach – simplify all nitrogen oxidation processes as a single reaction
NBOD (LN) – oxygen demand for the oxidation of nitrogen compounds
LN = 4.57 TKN
TKN (Total Kjeldahl nitrogen) = oxidizable N = org. N + ammonia N
Mass balances for NBOD and deficit in a stream at steady state
dLN
 k N LN
dx
dL
0  U N  k a D  k N L
dx
0  U
LN  L N 0 e
D  D0 e


kN
x
U
ka
x
U
k
k
 Nx
 ax
k L
 N No (e U  e U )
ka  k N
Shortcomings
No considerations of org. N → ammonia N
No considerations of the sequential reaction from ammonia N to nitrate
No considerations of inhibitory cofactors (the number of nitrifying bacteria, pH, level of
oxygen)
 result in unrealistic simulation (tc is too close to the effluent point and Dc is exacerbated)
Nitrogen modeling (alternative approach)
Each of nitrogen oxidation processes handled as a single component comprising the sequential
reactions
dN o
 koa N o (org. N  ammonia N),
dt
dN i
 k ai N a  kin N i
(nitrite  nitrate),
dt
dN a
 koa N o  k ai N a
(ammonia N  nitrite)
dt
dN n
 kin N i (nitrate accumulati on)
dt
Mass balances for the deficit in a stream at steady state
dD
r oa k ai N a  roi kin N i  k a D
dt
Nitrification inhibition
Inhibitory cofactor (oxygen) – correction for the nitrification rate constants
(k ai or kin )  f nitr
where f nitr  1  e  ( knitr DO)
knitri = 1st order nitrification inhibition coeff. ( 0.6 L/mg)
fnitr ~ 1 at DO > 3 mg/L
 result in more realistic simulation (sag curve becomes more spread and the DO recovery
delays)
Phosphorus
Critical role in genetic systems and in the storage and transfer of cell energy
Naturally scarce – present in insoluble form, easily settles
Human activities stimulates a large quantity of P discharges – wastewaters, agricultural land use
(fertilizer), urban runoffs, soil erosion, etc.
Usually, serves as a growth limiting factor for algal growth – critical factor for eutrophication
Types of phosphorus in modeling
1. Soluble reactive phosphorus (SRP) – inorganic orthophosphate (H2PO4-, HPO42-, PO43-)
2. Particulate organic P (POP) – living plants, animals, bacteria, organic detritus
3. Dissolved organic P (DOP) – dissolved compounds containing P, decomposed from particulate org. P
4. Particulate inorganic P – phosphate minerals, sorbed orthophosphate, phosphate complexed
compounds
5. Nonparticulate inorganic P – condensed phosphate such as in detergent, available for plant growth
The level of total phosphorus (TP) has been used for the indication of eutrophication.
Water quality constituents
The items simulated by QUAL 2K
Simulation processes for the model constituents
JPO43-