The principle aim of this study was to gain an understanding of the conditions and processes governing the occurrence of simultaneous nitrification and denitrification (SND). SND is the process that combines nitrification and denitrification in the same reactor (at the same time) under fully aerobic conditions. From various studies, two main hypotheses, one physical and one biological, have been proposed to explain SND (Simultaneous Nitrification and Denitrification). Significant research has been performed on the biological aspects, whereas relatively little is known about the physical explanation. Therefore, further investigations of physical explanation on SND (Simultaneous Nitrification and Denitrification) are carried out in this thesis. To fulfill this principal objective, two major tasks were preformed: experimental studies and model development. The experimental investigation was conducted using lab scale sequencing batch reactors (SBR). The operating conditions of the reactors were varied corresponding to the aim of each experiment. The influent wastewater was collected from the effluent of an anaerobic pond at an abattoir wastewater treatment plant. The main experimental studies focused on three factors, the effect of soluble organic carbon, floc size and dissolved oxygen (DO) concentrations, on the SND activity. The results revealed that all these factors had a significant influence on the degree of SND achieved. Almost 50% of inorganic nitrogen lost by SND (Simultaneous Nitrification and Denitrification) could be achieved when operating at a soluble COD:TKN ratio of 6. A dramatic increase in SND activity to 85% was found when this ratio reached 10. With a soluble COD:TKN ratio of 15, complete nitrogen removal by SND could be achieved. The effect of dissolved oxygen (DO) was equally strong. SND could completely occur at very low DO concentrations (0.2 mg/L). However, the nitrogen removal in this range was substantially limited by the low nitrification rate. To improve the nitrification rate but still achieve effective denitrification, a DO concentration of around 0.4-0.5 mg/L seems to be an optimal value to maintain a significant degree of SND. In this range, the nitrification rate reached 50% of the rate found at DO of 1.1 mg/L and 60% SND activity was achieved. The effect of bacterial floc size on SND was also quite remarkable. It was found that an SBR operating with a median floc size of 80 mm could achieve 80% SND, whereas the SND activity decreased to only 50% after the median floc size was reduced to 40 mm in the following treatment cycle. A complete nitrogen balance over the whole process was performed to confirm the occurrence of SND in such systems. Under typical operating conditions, it was found that the nitrogen gas was the major nitrogen product of the treatment process (approximately 58% of the total output). 14% of nitrogen was assimilated to biomass whereas 23 % of nitrogen at the end of the process was in the soluble form (organic nitrogen, nitrite, nitrate and ammonium). The mathematical dynamic model was developed to gain a better understanding of SND in the situation that is difficult to investigate experimentally. The overall model structure can be divided into 4 main areas : 1. a micro level model for a single floc 2. the reaction rates for a single floc size 3. the reaction rates for the entire reactor considering the floc size distribution 4. a macro model for the whole reactor including the operational changes throughout the cycle. It was found that the model can predict the SND behavior well for the system operating under typical influent characteristics (SCOD:TKN of 10). However, poor predictions were found at different levels of SCOD:TKN. Two crucial reasons can be given. Firstly, this model did not include intracellular carbon storage by bacteria. Secondly, many parameters, especially floc and microorganism characteristics (i.e. intra floc biomass distribution, growth and decay of the microorganism, etc.) could not be determined or estimated accurately. However, under normal operating conditions of this study, the model advances the fundamental understanding of SND process in activated sludge system. The simulation results showed that both floc diameter and liquid phase concentration are important factors influencing the internal floc concentrations. It was also predicted that an anoxic microzone, caused by oxygen diffusion limitation, potentially occurs in the floc center. This microzone therefore enhances denitrification activity inside the floc. A number of major conclusions can be drawn from this thesis: 1. SND potentially occurs as a result of physical phenomenon 2. high soluble COD is beneficial to SND activity 3. suitable floc size distribution (with more large flocs) can enhance SND 4. major nitrogenous product of the treatment process is nitrogen gas 5. dissolved oxygen optimization is critical to get good nitrification rate and SND.
| Identifer | oai:union.ndltd.org:ADTP/253740 |
| Creators | Pochana, Klangduen |
| Source Sets | Australiasian Digital Theses Program |
| Detected Language | English |
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