Water purification using photocatalysis, ozonation and ultraviolet disinfection

Abstract

The rapid urbanization is creating a rising demand of clean water while producing a huge amount of wastewater. A special issue in Hong Kong is that sea water is extensively used for toilet flushing and street cleaning, which generate contaminated sea water. Wastewater treatment plants often rely on physical, chemical and biological methods to remove the contaminants, but the state of the art is still limited by several technical challenges. For instance, (1) many dissolved toxic chemicals (e.g., detergents) cannot be treated efficiently by the prevailing treatment methods; (2) the treated wastewater still contains significant amounts of residual chemicals and bacteria, which will affect water activities; (3) the contaminated sea water cannot be cleaned effectively using those methods developed for the contaminated freshwater. The high concentration of salt ions affects the microorganism growth and the flocculation in the treatment pools. This study aims to overcome the existing challenges using solar photocatalysis, ultraviolet (UV) irradiation, ozone and their combined effects. More specially, this research consists of three parts: (1) solar photocatalytic reactors to degrade organic pollutants in freshwater, (2) photocatalytic ozonation of sea water and (3) UV ozonation of residual bacteria in wastewater effluent. The logic behind is that sunlight is abundant while photocatalysis can decompose a wide range of chemicals, and thus the solar photocatalysis is energy-saving and environmental friendly. In sea water, the existence of Na⁺, Clˉ and other ions invalidates many biochemical methods and reduces the efficiency of photocatalysis; nevertheless, the addition of ozone produces OH* radicals, enhances the capture of photoexcited electrons, and at the same time, provides dissolved O₂ to improves the photodegradation. To kill bacteria, UV irradiation and ozonation each has its own weaknesses, but the successive use of both can well complement each other to kill the bacteria more effectively. The first part of this research starts with a chip-size solar reactor (footprint of reaction chamber 1 cm × 1 cm) for the laboratory tests and mechanism studies. Then, it is scaled up to the meter size (footprint of reaction chamber 60 cm × 40 cm). It is made of PMMA plates. A simple soaking and spraying method has been developed to form the TiO₂ film on the PMMA, which is the first method that can make large-area, uniform photocatalytic film on cheap, durable, lightweight and non-brittle substrates. Field tests show that the large-size reactor can degrade MB by 30% in sunlight for two hours, which is 23.6 times faster than the photolysis. Although the performance has much room to improve, this large solar reactor is the first demonstration of the planar reactor directly scaled up from the microfluidic reactor; and it also moves one step forward to process large amount of water sample that is a typical requirement of practical applications. The second part of this research synergizes photocatalysis and ozonation for the decontamination of sea water. Mechanism studies and experiments using a home-made setup show that the maximum synergistic effect is achieved when the ozone concentration is about 50 ppm. Under this condition, the reaction rate is improved by more than 2 times as compared to that of only photocatalysis. This is the first study of sea water decontamination using the photocatalytic ozonation and is worth further development to tackle the existing waste sea water problem in Hong Kong. The third part aims to solve another problem of the treated waste water, the residual bacteria in the effluent, which result from the extensive uses of various types of bacteria in the biological treatment method. The effluent often has E. coli > 100 cfu/mL. To reduce the count to < 10 cfu/mL with minimal cost, the UV irradiation is first applied to damage the nucleic acids inside the bacteria, followed by the ozonation to destroy the external membrane of bacteria. The combined effect enables to completely kill the bacteria and prevents the resurrection of bacteria under visible irradiation, which is a common problem of UV-only treatment. This work has developed a new gas-liquid exchange plant for fast dissolution of ozone in the water sample. Experiments start with the sample of 3 × 10⁶ cfu/mL and obtain 46 cfu/mL right after the UV irradiation for 0.6 s, 270 cfu/mL if the UV-irradiated sample is then irradiated by visible light for 24 hours, and < 5 cfu/mL if the UV-irradiation is followed by the 5-ppm ozonation and the 24-h visible irradiation. The large difference between 270 cfu/mL and < 5 cfu/mL shows that a low concentration of ozone is already able to reduce the bacteria count by a factor of more than 50. As the UV irradiation time is short and the ozone concentration is low, the electricity cost is estimated to be HK$0.15 for the treatment of 1 ton of water. In summary, this M. Phil. study has three major contributions to science and applications: the new solar reactors for the photocatalytic degradation of residual chemicals in waste fresh water, the photocatalytic ozonation method for the decontamination of waste sea water and the UV ozonation for killing the residual bacteria in the wastewater effluent. The novelty and originality of this work lie most in the new design of solar reactors, the first study of photocatalytic ozonation of sea water and the new design of gap-liquid exchange plant for the disinfection of residual bacteria. The designs and techniques developed in this work are of highly application values and may be incorporated into the current waste water treatment works.