School of Applied Sciences (SAS) (2006-July 2014)

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Browsing School of Applied Sciences (SAS) (2006-July 2014) by Course name "Water and Wastewater Engineering"

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    Impact of MIEX pre-treatment on floc strength and structure
    (Cranfield University, 2009-09) Burton, Vincent; Jarvis, Peter
    In drinking water treatment regulation standards for natural organic matter (NOM) are tightening owing the fact that NOM leads to formation of carcinogenic disinfection by-products which can be harmful for human health. To further remove NOM from water, advanced NOM removal processes are developed such as the magnetic ion exchange (MIEX) resin process. MIEX followed by coagulation has been shown to improve the dissolved organic carbon removal in comparison with coagulation alone. However knowledge is lacking about how the magnetic resin influences the floc structure which is of major importance for the efficiency of the downstream processes. The present thesis aimed to evaluate and understand the impacts of MIEX pre-treatment on floc properties. To meet these aims, a literature review was carried out in order to provide sufficient background on floc size, floc strength, re-growth potential and fractal dimension concepts. It showed that almost all of factors influencing these properties act on the floc structural properties via a modification of either the nature of the primary particles, or the conditions of coagulation process, or the hydrodynamic conditions of the surrounding water. De facto, these pre-cited properties are all linked together. It is especially recommended to interpret with care the floc strength concept. The pre-defined objective was to compare properties of flocs generated by coagulation alone and combined treatment (MIEX pre-treatment followed by coagulation). Coagulation steps were operated with three different coagulants - FeCl3, AlCl3, PACl - at optimum pH and dose conditions on jar test bench. A light scattering instrument measured floc size and fractal dimension during growth, breakage and regrowth periods. Additional fractal dimension measurements for ferric flocs were gathered by image analysis using a microscope. MIEX pre-treated flocs were larger but withstood less well increases in shear rate than conventional flocs. This reduced robustness to increases in shear rate was thought to be due to the initial larger size of the pre-treated flocs. Light scattering and image analysis gave different fractal dimension values for the same floc system. It is probably because they are applicable for different size ranges: light scattering for the small primary particles and image analysis for the overall flocs. Of more operational interest, image analysis results showed that MIEX pre-treated flocs were more compact. Combined treatment gave an overall improvement in floc structure quality, producing flocs with better filterability and settleability.
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    Improve the energy efficiency of a full scale integated fixed film activated sludge (IFAS) process at Thames Water Oxford WWTW
    (Cranfield University, 2009-09) Buñol i Tordera, Ramon; Jefferson, Bruce; Healey, Kieran; Barugh, Angela
    The primary objective of integrated fixed film activated sludge (IFAS) is to remove dissolved pollutants from wastewater. This process allows an increased treatment capacity in comparison with conventional activated sludge. Thames Water has been operating a full scale IFAS biological process demonstration lane of 870 m 3 at the Oxford wastewater treatment works for over two years. The aim of this study was to evaluate the benefits of using an ammonia feed back control system as an alternative to a conventional set point dissolved oxygen control in order to improve energy efficiency in the aeration system. The main finding was that the adjustment of dissolved oxygen (DO) set points regulated by the feedback control system resulted in periods of excess or deficiency of DO in the wastewater with an total over aeration of 14.7 % and under aeration of 9.8 %. Investigation into the causes of the deviations found that the speed of the control equipment was unable to match the responsiveness of the control set point leading to periods where the air control valve was either too open or too closed. A linear equation was established to relate the influent wastewater flow with the time taken by the ammonia to be detected in the settled sewage stream in the IFAS process lane. Thus a feed forward control philosophy was suggested to control the aeration system, as opposed to the feed-back control system in order, to improve the target DO set point. Alongside this study, a comparison of the energy costs between IFAS with the ammonia feed-back control philosophy and a conventional activated sludge set point DO control system was completed. The analysis found negligible difference between the two processes. In addition when compared to the calculated design aeration requirements for set point DO control for the observed actual oxygen required (AOR) loadings, it was found that operating the IFAS system in NH4-N feed back control requires significantly less air. It can therefore be concluded that, with further research, the IFAS process can still be optimised to reduce aeration energy costs.
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    The relationship between UVT and turbidity, with respect to the disinfection of surface waters
    (Cranfield University, 2009-09) Roussillon, Guillaume; Parsons, Simon
    UV transmittance (UVT) is a parameter very closely related to the efficiency of any UV processes. Firstly a theoretical model of UVT proposed by Gregory (2006) was assessed and secondly an experimental approach was undertaken to find out any relationship between turbidity and UVT. UVT calculations were made for a range of particle diameter from 100nm to 5µm and to assess the impact of particle characteristics (particle size, particle number concentration and refractive index) on UVT. The theoretical model showed that the most important parameter affecting UVT was particle size and this was attenuated when the refractive index of the particle was small. The sensitivity of turbidity measurement required to show a 1% variation in UVT for UVT (for UVT above 90%) was calculated to be 0.01NTU which is below the level of sensitivity of most commercial system. Experiments with made solution containing one pollutant showed that turbidity is linearly related to the UVT (turbidity range between 0.14-5 NTU), but for real and non commercial water compositions UVT cannot be estimated using turbidity measurement. The nature of turbidity cannot be directly related to the particle characteristics which are particle size, chemical composition and hence absorbance and scattering coefficient. Finally from comparison between experimental results and theoretical calculation of UVT it appears that difference in UVT prediction (given values from 0% to 100% of UVT whereas measurement are between 90-99%) was mainly due to an error in theoretical determination of particle number concentration measurement. As an improvement of the theoretical model it was proposed to calculate the particle number concentration from 4 size particle fractionated solutions and then these solutions were diluted and UVT calculations and measurement were performed. Difference between predicted and measured UVT was still too important (but lower than 4%) to get a good correlation and use the theoretical model as a tool to predict UVT.