Year

2009

Degree Name

Doctor of Philosophy

Department

School of Civil, Mining and Environmental Engineering, Faculty of Engineering

Abstract

The need for potable water treatment has become an increasing necessity in modern society. Worldwide, the most widely used water treatment technology remains a combination of coagulation, flocculation, sedimentation and filtration. As a result, water treatment operations not only produce drinking water, they also produce wet residuals as a by-product. These residuals mainly include suspended solids, any organics found in water, and chemicals used in the treatment process such as coagulants, coagulant aids and filter aid polymers. The main objective regarding residuals management continues to be the reduction of residuals for either reuse or disposal. This reduction can be achieved by optimising the treatment processes, whilst also reusing the residuals for such diverse purposes as composting, manufacturing of bricks, coagulant for sewage treatment plants, enriching agricultural lands and construction filling material. When residuals from water treatment operations are mismanaged, the economic impact is considerable, and more importantly, these residuals present a possible threat to public health and safety. As a result, there is a great need to accurately estimate and accelerate the drying time of residuals, as larger quantities of these are generated, particularly as the demand for high quality drinking water increases. Constraints, such as wet weather and land availability have prolonged the drying time of residuals, which in turn has intensified the need for accelerated drying. Although substantial information is available on drying equipment and its performance, very little information is available relating to the actual drying of residuals. One element of considerable interest continues to be the influence of meteorological conditions on the drying time and rate of residuals. From a practical standpoint, these conditions must also be taken into account during the design and operation of drying beds. Clearly meteorological conditions have a significant effect on the residuals drying time, however extensive investigation into the available literature when researching this thesis revealed that these variables were not thoroughly represented in previous models. Therefore, there is a considerable need to develop a model that incorporates the effect of meteorological conditions on the residuals drying process. The drying of predominantly ferric chloride residuals has been investigated through a series of experiments performed in a laboratory drying tunnel, as well as field experiments in experimental sand drying beds. In the field experiments, two experimental drying beds were used in order to compare the drying of residuals between normal and passive solar beds. The meteorological parameters were measured using a weather monitoring station. Data collected from the drying tunnel were used to calibrate and validate the model. A new mathematical model was developed, in order to calculate the solids content for a control volume of a given residuals application thickness and area. The model was formulated using a heat balance approach, which incorporated the meteorological parameters and the residuals application area and thickness. The model includes the heat transfer components by radiation, convection, and evaporation. A nondimensional convective heat transfer coefficient has been formulated using dimensional analysis in order to calculate the convective heat transfer term. Variance-based sensitivity analysis has been used to determine the input variables variances and their influence on the model output. This will reveal a better understanding for future designs of water treatment residuals dryers. Finally, a solar drying bed has been designed in order to accelerate the drying process of residuals. iv The field study revealed no significant advantage with the passive solar drying bed, and was therefore concluded that drying time would not be enhanced in comparison to the conventional sand drying bed. However, when the solar drying bed was provided with a fan heater and a ventilator, the drying time was significantly reduced by up to 33%. The mathematical model predicts drying time with good accuracy (r2 > 0.93 for the drying tunnel experiments and r2 > 0.8 for the field experiments) of up to 50% solids content (wet basis) for a given application thickness and prevailing meteorological conditions. The relative importance of the model parameters using the sensitivity analysis revealed that relative humidity had the highest influence on the dependent variable and the application thickness the lowest. The model and methodology presented in this thesis will enable design engineers to predict the drying time of residuals as well as sizing of the residuals drying beds. Successful prediction of residuals drying time will also help water treatment facilities in their day-to-day operation and maintenance. The newly designed solar drying bed proposed in this thesis will provide several environmental benefits including reduction in drying time, transport cost savings that can be passed on to the end user, and perhaps more importantly in contemporary society, the reduction of greenhouse gas emissions, as a direct result of embracing this free, renewable energy source.

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