Synthesis and characterisation of Pd-modified N-doped TiO2 for photocatalytic degradation of natural organic matter (NOM) fractions.

INTRODUCTION: The removal of natural organic matter (NOM) from water is becoming increasingly important in order to prevent the formation of carcinogenic disinfection by-products. The inadequate removal of NOM has a bearing on the capacity of the other treatment processes to remove organic micro-pollutants or inorganic species that may be present in the water. New methods are therefore currently being sought to effectively characterise NOM and also to ensure that it is sufficiently removed from drinking water sources. METHODOLOGY: Nitrogen- and palladium-co-doped TiO(2) was synthesised by a modified sol-gel method and evaluated for its photocatalytic degradation activity on NOM fractions under simulated solar radiation. The photocatalyst was characterised by FT-IR, Raman, XRD, DRUV-vis, SEM, TEM, EDS, XPS and TGA. FT-IR confirmed the presence of OH groups on thermally stable, nearly spherical anatase nanoparticles with an average diameter of 20 nm. PdO species appeared on the surface of the TiO(2) as small uniformly dispersed particles (2 to 3 nm). A red shift in the absorption edge compared to commercial anatase TiO(2) was confirmed by DRUV-vis. In order to gain a better insight into the response of NOM to photodegradation, the NOM was divided into three different fractions based on its chemical nature. RESULTS AND DISCUSSION: Photodegradation efficiencies of 96, 38 and 15 % were realised for the hydrophobic, hydrophilic and transphilic NOM fractions, respectively. A reasonable mechanism was proposed to explain the photocatalytic degradation of the NOM fractions. The high photocatalytic activity could be attributed to the larger surface area, smaller crystalline size and synergistic effects of the co-dopants N and Pd in the TiO(2) crystal.

An improvement on the power law for the description of particle size distributions in potable water treatment.

In potable water treatment, the use of the power law to describe particle size distributions (PSDs) in particle counting practice is common. The power law is popular because it allows the reduction of numerous data bits to two meaningful parameters that completely describe the size distribution characteristics of a particle suspension. The model is however flawed. This paper presents the further development of an improved model (the variable-beta model) first proposed by Lawler (1997). Both the power law model and variable-beta model are used to describe the PSDs of a large number of potable water treatment samples taken from full-scale plants and the resulting correlations are compared. The findings from the comparison of data reduction methodologies support the argument that the variable-beta model is fundamentally more correct than the power law model and consequently describes the PSDs better.

Sludge and washwater management strategies for the Vaalkop water treatment plant.

The Vaalkop plant, owned and operated by Magalies Water, provides a valuable South African case study of sludge and washwater management at a large water treatment plant. Starting out as a small plant of 18 Ml/day about thirty years ago, it has steadily grown to a plant with treatment capacity of 210 Ml/day; fairly large by South African standards. During the preceding years, it has not only been subject to a vastly larger scale of operation, but it also had to adapt to a tremendous increase in the cost of raw water, an increased environmental awareness amongst water treatment professionals and general public alike, and a much more sophisticated and complicated legislative framework. It is the objective of this paper to track the sludge and washwater management practices adopted over the years at Vaalkop, and to present the current strategies adopted for the medium to long term. The paper will summarize the previous methods of sludge and washwater disposal, with reasons why they were adopted. The multitude of technical analyses and alternatives that were performed over the years will be summarized, and may provide valuable pointers for other applications in South Africa. The current system, which has just been commissioned, will be presented; its technical design parameters, the anticipated mode of operation, its costs and how the current environmental and legislative requirements are being met.

Modelling of floc-bubble aggregate rise rates in dissolved air flotation.

The use of relatively simple, but conceptually sound mathematical models, is a powerful tool to identify and understand parameters that are critical to a process. In this paper, a model is presented which addresses the rise rate of floc-bubble aggregates in the DAF separation zone. The model uses Stoke's Law as a point of departure, which is then progressively extended to incorporate the non-sphericity of the aggregates, the non-laminar nature of their movement, the fractal nature of the flocs, the physical constraints of attaching bubbles to a floc, and the limit on bubble numbers imposed by a typical air dosing system. The main findings are: There are two distinctly different DAF domains, namely a small floc domain and a large floc domain. In the small floc domain, the bubble size, the air volume, chemical dosing and the degree of flocculation have to be optimized and accurately controlled, while the large floc domain is less sensitive to these parameters. The calculated rise rates are significantly lower than some of the latest pilot testing results reported from the USA; a surprising finding at first. There are, however, numerous modelling simplifications that may explain this discrepancy. The most probable reasons are the further agglomeration of the aggregates in the separation zone due to differential rise rates or the recirculating flow within the white-water blanket, and the complex and poorly understood flow patterns within the separation zone.

Evaluation of the performance of full-scale packed saturators.

An evaluation of the performance of full-scale packed saturators was made. Measurements of saturator effluent air concentrations, saturator efficiency and precipitated are reported for both cold water and warm water operating conditions. Packed saturators were analyzed over a full range of operating pressures and loading rates. Measured results compared to predicted results based on a comprehensive model for the performance of packed saturators prove the model to be a useful tool in designing efficient and cost effective saturation systems.

Relatedness of Thermomyces lanuginosus strains producing a thermostable xylanase.

Properties of an endo-beta-xylanase produced by a locally isolated Thermomyces lanuginosus strain SSBP was compared to seven other T. lanuginosus strains isolated from different geographical regions. Strain SSBP produced the highest xylanase activity of 59600 nkat ml(-1) when cultivated on corn cobs (maize) medium, whereas the seven other strains produced xylanase activities ranging from 6000 to 32000 nkat ml(-1). No cellulase activity was produced by the strains. Despite the variability in the production of xylanase, little difference in the other characteristics of the strains could be found. The optimal temperature and pH for xylanase production by the strains was either 40 or 50 degrees C and between pH 6 and 7, respectively. Optimal xylanase activity of the strains was observed at 70 degrees C and at pH 6 or 6.5. Culture supernatant analysis by SDS-PAGE and isoelectric focusing PAGE of all strains revealed the presence of a single 24.7 kDa and pI 3.9 xylanase. Phylogenetic analysis by PCR amplification and sequencing of the internal transcribed spacer of nuclear rRNA repeat units and 5.8S rDNA revealed no strain diversity. However, random amplified polymorphic DNA analysis pointed to greater diversity and with one primer (5'-GCCCGACGCG-3'), a relationship was established between xylanase levels and the RAPD pattern.

Diseño de floculadores hidráulicos abiertos de flujo horizontal

El procedimiento de diseño desarrollado en este artículo demuestra el hecho que estos floculadores se pueden diseñar para trabajar satisfactoriamente en condiciones variables de gradiente de velocidad, caudal y calidad de agua. El procedimiento se base en dos pasos: se determina primero la disposición horizontal (en planta) con los valores promedio del gradiente de velocidad y de la profundidad del agua y luego se definen la pendiente del fondo y del pelo de agua, para asegurar una floculación satisfactoria para todos los escenarios diseñados

Diseño de floculadores hidráulicos abiertos de flujo horizontal

El procedimiento de diseño desarrollado en este artículo demuestra el hecho que estos floculadores se pueden diseñar para trabajar satisfactoriamente en condiciones variables de gradiente de velocidad, caudal y calidad de agua. El procedimiento se base en dos pasos: se determina primero la disposición horizontal (en planta) con los valores promedio del gradiente de velocidad y de la profundidad del agua y luego se definen la pendiente del fondo y del pelo de agua, para asegurar una floculación satisfactoria para todos los escenarios diseñados