Nyamutswa, Lavern Tendayi ORCID: 0000-0003-0662-1134 (2020) Light Transmitting Photocatalytic Membrane For Chemical-Free Fouling Control In Water Treatment. PhD thesis, Victoria University.

Abstract

Membrane filtration has revolutionised water treatment, enabling safer provision of drinking water due to its high efficiency to block human infectious pathogens commonly present in raw water sources. Accumulation of substances on membrane surfaces and pores during operation, referred to as fouling, is considered one of the biggest barriers to wider adoption of membrane technology in water treatment. Maintaining continuous low-pressure filtration requires significant amounts of chemicals to clean off the accumulated fouling substances. Chemical use comes with economic and environmental costs associated with acquisition, transportation, storage, usage and disposal of chemicals, especially in disadvantaged and remote communities. By conservative estimates, supply of household water to a remote community of 100 people using a membrane system would require continuous supply of at least 10 L of polyaluminium chloride coagulant and 4 L of sodium hypochlorite (in concentrated form) every month. The main aim of this thesis is to demonstrate a sustainable, innovative, low cost membrane solution harnessing conveniently available solar energy to offset these chemical demands. Coating membrane substrates with semiconductor photocatalysts such as titanium dioxide (TiO2) is an effective method for mitigating fouling in membranes through induced superhydrophilicity, enabling cleaning from the available water without chemicals. TiO2 also enables water contaminant degradation and pathogen inactivation through reactive oxygen species (ROS) facilitated advanced oxidation. Despite these well- known effects, a major challenge limiting practical adoption comes from light absorption and scattering by the turbid contaminants in the feed stream before reaching the TiO2. This thesis proposed a novel solution to this challenge by transmitting light to the TiO2 through cheap porous borosilicate glass substrates with between 10% and 80 % transmission in the 340-400 nm wavelength range relevant to activating commercial Degussa P25 TiO2 photocatalyst. The concept novel membrane was produced using commercial glass substrates modified by simply dip- coating and heat sintering Degussa P25. The formed asymmetric membrane’s mean pore size was measured at 0.5 μm, which classifies the membrane as a microfiltration (MF) membrane, which are utilised in the industry as a barrier to water-borne pathogens such as protozoa and bacteria, and partially to viruses. To demonstrate the membrane’s photocatalytic ability, photocatalytic reactions stimulated by a UV lamp (365 nm peak) facing the glass substrate side in an ex-situ setup led to a 52% degradation of methyl orange in aqueous solution, being only slightly lower than the 58% degradation when the TiO2 active layer faced the UV light source. The membrane was then operated in-situ using a custom module with a quartz window and UV LED installed on the permeate side, enabling simultaneous microfiltration of model fouling solutions. Results showed significant reductions in trans-membrane pressure (TMP) rise rates directly linked to UV light application. Specifically, UV light was responsible for up to 3.0-fold reduction in total filtration resistance and up to 4.2-fold reduction in irreversible fouling indices. Testing continued on simulated indirect solar light with a real non-potable water. The membrane itself showed up to 94% turbidity removal and up to 80% total organic carbon (TOC) rejection. The sunlight was directly responsible for an 8-fold reduction in the irreversible fouling index. The significant practical findings were followed by an investigation to confirm the fundamental basis for improvement. Analysis by scanning electron microscopy (SEM) coupled with fouling modelling showed the beneficial photocatalytic fouling reduction effects during microfiltration stemmed from reduced intrusion of organic fouling material inside the TiO2 membrane pores, as well as reduced cake layer resistance. Analysis of results and photocatalysis mechanisms from literature led to the conclusion this was due to both superhydrophilicity minimising organic attractions to the surface and photocatalytic oxidation of organics approaching the surface. The potential for advanced oxidation to participate in reacting with organic matter surfaces attracted to the membrane was confirmed from a measurable increase in the presence of hydroxyl radicals using para-chlorobenzoic acid (pCBA) probe experiments. The practical benefits for industry towards chemical consumption and energy reduction were also measured. For example, a 4.5-fold extension to the time needed for a clean-in-place (CIP) was realised when the membrane was operated in photocatalytic mode. A 50% reduction in filtration pump electricity demand was also calculated, which translates to a reduction in height of the feed water for a flux of 300 L/m2/h from 8.6 m to 3.7 m over a 5 hour run. Future work suggested includes using recycled glass to improve affordability and minimise glass manufacture environmental impact, as well as experimentally establishing the relationship hydroxyl radical concentration and TOC reduction. Optimisation of the glass material for enhancing light transmission efficiency and development of porous glass monoliths like current commercial ceramic membranes for full-scale use, as well as optimisation to increase contaminant degradation are also suggested.

Search Google Scholar

Repository staff login