| Summary: | 碩士 === 國立成功大學 === 環境工程學系 === 87 === Kinetics of Nitrification/Denitrification in the Single-Sludge System
ABSTRACT
In a single-sludge system (anaerobic/anoxic/aerobic), many factors affect nitrification/denitrification and various types of microorganisms are present because single sludge flows through anaerobic-, anoxic-, and aerobic-compartments of the system. Accordingly, the development of a nitrification-denitrification model for a single-sludge system is rather complicated, compared to the development of a model for a separate-sludge system. In the last decade, numerous mathematical models of nitrification-denitrification have been developed for single-sludge systems. It turns out that these models have complex structure and a large number of unknown chemical and biological parameters. Thus, these models may be restricted to a limited use in biological wastewater treatment plant design. For benefiting design and operation of single-sludge systems, the development of a simplified, practical nitrification-denitrification model (followed by experimental verification) is therefore necessary. It is noted that, in our study, synthetic wastewater which simulates piggery wastewater pretreated by a solid-liquid separator and a three-day period of anaerobic degradation was used.
When synthetic wastewater containing high strength nitrogen (COD = 2550 - 3160 mg/L, TKN (NH4+-N) = 900 mg/L, TP = 101 - 103 mg/L) was treated by a laboratory-scale single-sludge system (anaerobic/anoxic/aerobic) under TKN (NH4+-N) loadings of 0.101 - 0.178 kg N/kg MLVSS-d, COD loadings of 0.32 - 0.50 kg COD/kg MLVSS-d, TP loadings of 0.012 - 0.020 kg P/kg MLVSS-d, sludge recycle ratios (Rs) of 1 - 2, mixed liquor recycle ratios (Rm) of 1 - 2, and observed SRTs of 10.1 - 19.4 d, the removal efficiencies of COD, TKN (NH4+-N), and TP were 99.3% - 99.7%, 93.2% - 100.0%, 66.7% - 77.6%, and 8.7% - 24.8%, respectively.
According to mass balance of TKN (NH4+-N) and NOx--N in the single-sludge system with influent COD/TKN ratios of 2.8 - 3.5, nitrification occurs mostly in aerobic compartment whereas denitrification occurs mostly in anaerobic compartment (belongs to denitrification frequently found; specific denitrification rates range from 0.142 to 0.320 mg NOx--N/mg VSS-d). Only a small part of denitrification occurs in anoxic compartment (belongs to endogenous denitrification; specific denitrification rates range from 0.003 to 0.088 mg NOx--N/mg VSS-d). With an increase in Rs, the removal efficiency of TN in the single-sludge system increases but the removal efficiency of TP decreases. The change of Rm does not affect the removal efficiencies of TKN (NH4+-N) and TN in the single-sludge system when the concentration of NOx--N is left in anoxic compartment. A higher COD/NOx--N ratio flowing into anaerobic compartment gives a higher removal efficiency of TP in the single-sludge system. In the single-sludge system, the ratios of the actual consumption of alkalinity to the theoretical range from 0.91 to 1.03.
From the independent batch experiments, nitrification of enriched culture follows the Monod-type kinetics with rate constants (kn and Ksn) of 1.15 d-1 and 17.3 mg N/L. Denitrification of enriched culture follows the zero-order kinetics with a rate constant (kd) of 1.03 d-1. When denitrifiers in anaerobic compartment of the single-sludge system are not inhibited or inhibited by NO2-, the activity coefficients of denitrifiers (determined by independent batch experiments) are 0.51 - 0.58 (average value of 0.54) or 0.18 - 0.40 (average value of 0.29). The activity coefficients of denitrifiers taken from anoxic compartment are 0.022 - 0.058 (average value of 0.043). The activity coefficients of nitrifiers taken from aerobic compartment are 0.13 - 0.33 (average value of 0.2).
In a biological phosphorus release/uptake experiment using batch reactors (aerobic/anoxic/anaerobic/aerobic in sequence), if the initial COD/NOx--N ratio in the anoxic stage is controlled at 8.13, denitrification and remarkable phosphorus release occur simultaneously. If the initial COD/NOx--N ratio is too low (2.71 - 4.07), denitrification occurs only.
Finally, by placing biological parameter values (obtained by independent experiments) and operational conditions of the single-sludge system into the nitrification-denitrification model, the calculated TKN (NH4+-N), TN and TKN (NH4+-N), NOx--N removal efficiencies of the system and its various compartments are in good agreement with the experimental data. Therefore, the proposed model can be used to predict treatment performance of the single-sludge system.
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