1. 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 (NH 3 ): at high pH (>9) and moderate temp (~ 20 o C), toxic to fish

2. 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 (1 st order reaction) Ammonia and nitrate assimilation – uptake of ammonia and nitrate by phytoplankton Denitrification – dissimilative reduction of nitrate to free nitrogen under anaerobic conditions Nitrogen

3. 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 NH 4 + + 1.5O 2 → 2H + + H 2 O + NO 2 - NO 2 - + 0.5O 2 → NO 3 - NH 4 + + 2O 2 → NO 3 - + H 2 O + 2H + It occurs when 1) 암모니아 존재, 2) nitrifying bacteria, 3) alkaline pH 4) oxygen (>1~2mg O 2 /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 → t c moves to further downstream (between t c for OC oxidation and t c for nitrification) → D c increases

4. Nitrogen modeling (NBOD approach) NBOD approach – simplify all nitrogen oxidation processes as a single reaction NBOD (L N ) – oxygen demand for the oxidation of nitrogen compounds L N = 4.57 TKN TKN (Total Kjeldahl nitrogen) = oxidizable N = org. N + ammonia 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 (t c is too close to the effluent point and D c is exacerbated) Mass balances for NBOD and deficit in a stream at steady state

5. Nitrogen modeling (alternative approach) Each of nitrogen oxidation processes handled as a single component comprising the sequential reactions Mass balances for the deficit in a stream at steady state

6. Nitrification inhibition f nitr ~ 1 at DO > 3 mg/L  result in more realistic simulation (sag curve becomes more spread and the DO recovery delays) Inhibitory cofactor (oxygen) – correction for the nitrification rate constants k nitri = 1 st order nitrification inhibition coeff. (  0.6 L/mg)

7. 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 (H 2 PO 4 -, HPO 4 2-, PO 4 3- ) 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.

8. Water quality constituents The items simulated by QUAL 2K

9. Simulation processes for the model constituents J PO43-