WaterSubject

Interactions between the hydrosphere and the geosphere and biosphere are responsible for the occurrence of natural organic matter (NOM) in water throughout the world, and it is these biogeochemical cycles that are responsible for the diverse and complex nature of organic matter. NOM is a complex mixture of organic compounds including humic acids, hydrophilic acids, proteins, lipids, hydrocarbons and amino acids. The range of organic components found in NOM varies from water to water and seasonally as does its reactivity with chemical disinfectants such as chlorine. NOM’s reaction with chlorine lends to the production of disinfection by-products (DBPs), of which the trihalomethanes (THMs) are the most commonly regulated DBPs for water utilities. The current UK standard for total THMs is set at 100 ?g/L, which is particularly problematic for works treating surface waters with high dissolved organic carbon (DOC). NOM has the potential to form over 100 ?g THM/mg DOC; therefore, residual DOC levels as low as 1–2 mg/L could be sufficient to fail the standards. Conventional treatment processes such as coagulation are being significantly challenged to try and achieve the removal targets required to comply with the THM standards. In addition to tighter regulations, there has also been deterioration in raw water quality. For example, over the past 10 years the colour and DOC in raw water has risen at 34 out of 42 water treatment works in Yorkshire Water (UK) (from 20 to over 90 hazen). This has consequently contributed to a 100% increase in THMs over a 5-year period. The reservoir selected for this study (Albert Reservoir, Halifax, UK) is therefore representative of upland waters sourced from soils that are predominantly peaty in composition, and if raw water DOC levels continue to rise most conventional water treatment works (WTW) processes will not be able to achieve these regulated levels.

Advanced oxidation processes (AOPs), in particular titanium dioxide (TiO₂) photocatalysis, have several benefits over conventional treatment processes. To remove NOM more efficiently using conventional coagulation/flocculation, an increase in coagulant dose is required, leading to an increase in the volume of sludge generated. AOPs offer the dual attraction of a reduction in sludge and increased DOC removal. Photocatalysis has been defined as a change in the rate of chemical reactions or their generation under the action of light in the presence of a photocatalyst that absorbs light quanta. The photocatalyst is involved in the intermediate chemical interactions, and after each catalytic reaction cycle its chemical composition regenerates. The resulting accelerated generation of the non-specific, hydroxyl radical (•OH) enables a vast array of organic compounds to be simultaneously destroyed. TiO₂ is an excellent photocatalyst because of its purity, refractive index (2.49), particle size, surface properties and inertness with respect to chemical and photocorrosion. The accepted model for photocatalytic oxidation of organics to CO₂ and H₂O is that the oxygen and target molecules are initially adsorbed onto the catalyst surface. Once adsorbed, they react with photoelectrons to form superoxide radicals (O₂•^?). The adsorbed organics are then oxidised, and this can be a direct reaction or via adsorbed hydroxide radical intermediates.

Photocatalysis has been shown to oxidise and completely mineralise pesticides, toxic compounds and organic substances to carbon dioxide and water, although limited research has been carried out on the photocatalytic treatment of drinking water. The majority of existing literature is based on the application of TiO₂ in a slurry reactor and for treating organic chemicals and wastewater streams reported that TiO₂ photocatalysis, when treating raw water from Albert Reservoir (Yorkshire Water), can achieve an 81% reduction in DOC and a 96% reduction in ultraviolet (UV)₂₅₄ absorbance, compared with a DOC removal of 50–80% by conventional coagulation/flocculation. The use of a TiO₂ slurry makes the process expensive, as the additional separation stage required makes it unsuitable for large-scale municipal application. To make the process viable, the catalyst needs to be immobilised, preferably on a material with high surface area. This immobilisation removes the need for additional separation, and the catalyst can easily be reused. This paper looks at the bench-scale preparation of a range of immobilised TiO₂ photocatalysts, and the application for removing NOM from humic-rich waters compared with commercial catalysts.