Thesis - faculty.ait.ac.th - Asian Institute of Technology

ammonia could only be achieved when the N: BOD5 ratio does not exceed 3.6:100. Further,
when ammonia concentrations exceed 200 mg/L (as N), in the mixed liquor, the sludge
settling is also adversely affected (Robinson and Maris, 1985). Hence, removal of nitrogen
and nitrogen compounds from the leachate by a pre-treatment prior to biological treatment
processes is of prime importance.
Biological nitrification-denitrification is one of the most economical processes for
nitrogen removal. The successful application of this system is dependent on the microbial
population, composition, characteristic of the leachate and a variety of physical and
chemical parameters (Table 2.9). The process essentially consists of oxidation of ammonia
to nitrates with nitrite as an intermediate compound and finally nitrates to nitrogen gas.
Biological nitrification is preferred in absence of inhibitory substances which interfere with
the microbial ammonium oxidation process (Doyle, et al., 2001).
Table 2.9 Operational and Environmental Conditions for Nitrification-Denitrification
Processes (Kylefors, 1997)
Parameter Unit Nitrification Denitrification
Substance
transformed
NH4 + NO3 -
End Product NO3 - Intermediate Product NO2
N2
- NO2 - , N2O
pH 7.5 to 8.6 6 to 8
Alkalinity mmol of HCO3 - /mg of N Consuming 0.14 Producing 0.07
Oxygen mg O2/L > 2 (aerobic) < 0.5 (anoxic)
Organic Material mg COD/mg of N - 3
Phosphorus content mg of P/g of N > 4 > 11
Production of Sludge g/g of N 0.17 0.45
Temperature A 10 °C increase gives about 2 times specific rate
Denitrification processes occur generally in anaerobic activated sludge, anaerobic
filter and anaerobic lagoon. Methanol is usually added as an organic carbon source prior to
denitrification; however, dosing should be monitored to prevent hydrogen sulphide
formation and its inhibition (Reeves, 1972). Endogenous respiration is not frequently used
as it results in weak kinetics and requires larger volumes.
In an extensive study conducted by Illies (1999) to treat high ammonia leachate with
a four stage nitrification-denitrification process which is biological nutrient removal, an
initial ammonia concentration of 200 mg/L was step-wise increased in an attempt to
improve process ability to handle high ammonia concentrations. The initial trial resulted in
severe nitrification inhibition due to insufficient acclimation after increment of 300 mg/L
ammonia at each stage upto a final ammonia concentration of 2,300 mg/L. Further,
methanol was added in the denitrification zone simultaneously with increase in ammonia
concentration. This led to excess of methanol leading to inhibition of denitrification. When
the system was operated at low HRT of 1.5-1.7 h for denitrification and 3-3.4 h for
nitrification with an SRT of 20 days, removal efficiencies were found to be greater than
90%.
Bae, et al. (1997) proposed a treatment scheme consisting of an anaerobic filter (pall
rings media) and 2-stage activated sludge process for the removal of ammonia and then
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