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.

Thesis (Ph. D. - Soil, Water and Environmental Science)--University of Arizona, 1999. / Includes bibliographical references (leaves 123-128).

Thesis (Ph.D.)--Murdoch University, 2003. / Thesis submitted to the Division of Science and Engineering. "Project supported by Environmental Solutions International (ESI) ... ". Bibliography: leaves 193-208.

Thesis (MScAgric)--University of Stellenbosch, 2007. / Bibliography. Also available via the Internet.

Thesis (M.S.)--Oregon State University, 2000. / Typescript (photocopy). Includes bibliographical references (leaves 58-64). Also available on the World Wide Web.

Thesis (M.S.)--Baylor University, 2006. / Includes bibliographical references (p. 134-141).

Denitrification was investigated during 1985 and 1986 in a stagnogleyic brown earth (Macmerry) and a stagnogley (Winton), both soil types widely used for cereal production in south-east Scotland. The crop was winter barley. Much of the work was devoted to the evaluation and attempted field application of the acetylene-inhibition technique. This involves blocking the terminal reduction of nitrous oxide to elemental nitrogen by acetylene concentrations in excess of 0.1% by volume; total denitrification loss is then estimated by measuring the resultant enhanced nitrous oxide flux at the soil surface. For prolonged periods during the growing season relatively high concentrations of nitrous oxide in the soil atmosphere indicated significant rates of denitrification. It proved impossible, however, to obtain an adequate distribution of acetylene throughout the profile, resulting in improbably low denitrification estimates. This finding casts doubt on any reported measurements using the acetylene- inhibition technique in wet soils with poor structure. The method was applied with more success on a freely-drained Darvel soil. An alternative technique was investigated for the gleyed soils: laboratory incubation of intact cores in an atmosphere containing acetylene. This produced increased estimates of denitrification but was found to introduce other uncertainties. It was concluded that further development was required before the technique could be used to give reliable quantitative results. Measured gaseous diffusion constants and nitrous oxide concentrations at depth were employed in the Fick?s Law calculation of nitrous oxide flux; calculated losses during 1986 ranged from 0.2-0.5 kg N /ha for direct-drilled Winton plots to 3.6-7.4 kg N /ha for normally-ploughed M acmerry soil. These were minimum values for total gaseous loss, since losses as elemental nitrogen were not recorded. A mathematical model of denitrification was developed. It depends on numerical solution of the differential equations governing the simultaneous steady-state diffusion and reduction of oxygen, nitrate, and nitrous oxide in a spherical m icro-environm ent (an aggregate where aggregates are present - an "effective aggregate" in a structureless soil). A model aggregated soil is pictured as an assembly of spherical aggregates with log-normally distributed aggregated radius and reductive potential; the radius of the effective aggregate in a structureless soil is determined by the density of air-filled pores. Intra?aggregate diffusion constants are calculated by a method which amounts to an assumption of parallel diffusion through all possible serial combinations of intra-aggregate pores. Reductive potentials for the various reactions considered in the model are assumed to be proportional to one another. The model predicts an approximately linear relationship between the denitrification rate of an assembly of soil aggregates and its anaerobic fraction calculated according to the method of Smith (1980); such a relationship has in fact been observed (Parkin and Tiedje, 1984). Model results illustrate the importance of soil structure: a model clay soil continues to denitrify at moisture potentials much lower (more negative) than a model sand. Calculated whole-soil denitrification rates range from 0 g N/ha/d to 5.2 kg N/ha/d. Reported field measurements range from 0 g N/ha/d to 4.5 kg N/ha/d.

The wastewater treated at the Hopewell Regional Wastewater Treatment Facility (HRWTF) is very unique both because it is comprised of effluents of seven different industries in the area in addition to the domestic wastewater in the area, and because it reaches high temperatures in the basins, often above 45oC during the summer. Four different bench scale systems consisting of continuously stirred tank reactors (CSTRs) in series were operated during the summer of 1997 to quickly assess the feasibility of nitrifying and denitrifying the total flow at HRWTF down to a final effluent total nitrogen concentration of 10 mg-N/L or less. The four main treatment strategies tested were: aerobic/anoxic treatment of the final effluent of HRWTF at moderate temperatures (approximately 30oC); anaerobic/anoxic/aerobic (A2/O) treatment of the primary effluent of HRWTF at moderate temperatures; treatment of the effluent of one of the industries which had a high ammonia wastewater and which was originally believed to contain nitrification inhibitors; and fully aerobic treatment of the primary effluent of HRWTF at high temperatures (of approximately 40 to 45oC) with an activated sludge gradually acclimated to such temperatures over the course of two months. At the end of the study, a two-week high temperature study was conducted on the system which had been treating the secondary effluent all summer with the same activated sludge which was acclimated only to temperatures around 30oC. The fully aerobic high temperature system which had been nitrifying the primary effluent all summer was converted to a modified Lutzack-Ettinger (MLE) process at the end of the study to test whether the primary effluent could be denitrified as well as nitrified at high temperatures with the sludge acclimated to high temperatures. All four of the main treatment strategies demonstrated that nitrification and denitrification of either the total flow or the high ammonia side stream could be achieved down to the desired total nitrogen concentrations. The high temperature study conducted on the system which had been treating the secondary effluent all summer indicated that the sudden increase from approximately 30oC to approximately 40oC over a twenty-four hour period, similar to the sudden temperature increase which occurs every spring at HRWTF, quickly ends nitrification in a system not acclimated to high temperatures, while denitrification and COD removal is hardly affected by such a temperature change. While the nitrification performance of the gradually acclimated system treating the primary effluent at high temperatures was adequate, problems maintaining a consistent MLVSS or ETSS concentration suggested that the high temperatures seen in the basins at HRWTF are likely to make consistent treatment difficult. As a result of considering both capital cost requirements and quality of treatment, the bench scale testing suggested that the most likely candidates for successful treatment of the total flow down to desired total nitrogen concentrations would involve either the A2/O treatment of the primary effluent of HRWTF, possibly with the addition of a cooling tower, or A2/O treatment of the high ammonia side stream, possibly involving the dilution of the wastewater with one of the other flows sent to HRWTF. It was concluded that pilot scale evaluation of the two options was required for a final design decision, and pilot scale evaluation was being performed when this thesis was completed. / Master of Science

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