Micropollutants removal with activated carbon : Adsorption and regeneration by biodegradation
The presence of micropollutants in surface water threatens the production of high quality and safe drinking water. Micropollutants are contaminants present in the environment at trace concentrations (ng/L - µg/L). These contaminants originate from the use of organic chemicals in anthropogenic activities. Micropollutants reach the environment due to incomplete removal in wastewater treatment plants or via diffuse sources, such as agricultural and urban water runoff and sewer overflow. Despite their occurrence at trace concentrations, they can negatively impact the environment. Moreover, they can impact human health when water contaminated with micropollutants is used to produce drinking water. To minimize these risks, it is crucial to prevent micropollutants from reaching the environment and our drinking water.Micropollutants can be removed from water using adsorption to granular activated carbon (GAC) in fixed-bed filters. GAC filters are often used in the production of drinking water from surface water. GAC adsorption capacity reduces with time, hence GAC needs to be reactivated regularly. GAC reactivation is achieved by oxidizing the adsorbed contaminants at high temperatures (>700°C), which makes it an energy intensive process, resulting in large carbon footprints. Hence, efforts to increase sustainability in GAC filtration should focus on reducing the need for GAC reactivation.During the use of GAC filters, biomass inevitably grows on the GAC surface and contributes to removing the biodegradable fraction of organic contaminants, including biodegradable micropollutants. Biodegradation of micropollutants from the liquid phase reduces the amount of molecules that adsorb to the GAC. Furthermore, biodegradation of previously adsorbed micropollutants releases adsorption sites, regenerating GAC adsorption capacity and reducing the need for reactivation. This thesis addresses the potential of combining adsorption and biodegradation of micropollutants to bioregenerate GAC.In Chapter 2, the adsorption capacity and rate of adsorption of hydrophilic micropollutants to fresh GAC is assessed. Micropollutants removal in GAC filters is not only determined by equilibrium characteristics, such as affinity for the GAC and the adsorption capacity, but also by the adsorption rate. The rate-determining step in adsorption using GAC is usually the adsorbate diffusion inside the granules. The presence of mesopores can facilitate diffusion, resulting in higher adsorption rates. In this thesis, we use two different types of GAC, with and without mesopores, to study the adsorption rate of hydrophilic micropollutants. Ten micropollutants are selected as model compounds: benzotriazole, desphenyl-chloridazon, diclofenac, guanylurea, hexamethylenetetramine, iopamidol, iopromide, melamine, metformin and pyrazole. A pore diffusion model is applied to the kinetic data to obtain pore diffusion coefficients. This chapter shows that adsorption rate is influenced by the molecular size of the micropollutant as well as the GAC pore size. Pore diffusion coefficients correlate negatively with the adsorbate size for most micropollutants and this correlation is stronger for the largest adsorbates. Furthermore, diffusion of the largest micropollutants is hindered in the GAC micropores. Micropore surface area increases GAC adsorption capacity for micropollutants, although size-exclusion effects were observed for the largest molecules, i.e., iopamidol and iopromide. Finally, affinity for GAC correlates to molecular structure: micropollutants with cyclic structures, e.g., benzotriazole and melamine, adsorb to GAC to a larger extent than micropollutants with linear or globular structures, e.g. metformin and hexamethylenetetramine.Chapter 3 describes the potential of biomass obtained from full-scale GAC filters to biodegrade micropollutants. Assessing micropollutants biodegradation in the presence of GAC requires distinguishing adsorption from biodegradation. This is done by assessing micropollutants removal at 5°C and 20°C with biologically active and autoclaved GAC. At 5°C micropollutants are removed mainly through adsorption whereas at 20°C they can also be biodegraded. Autoclaved GAC is used to determine the effect of temperature on micropollutants adsorption. Here, the same micropollutants mixture as applied in Chapter 2 is used. Three of these micropollutants, iopromide, iopamidol and metformin, are biodegraded by the GAC biofilm. Additionally, temperature increases adsorption of some micropollutants, e.g. iopromide and iopamidol, whereas it decreases adsorption of others, e.g. metformin and guanylurea. Finally, the adsorption capacity of fresh and used GAC are compared with each other. GAC use in a treatment plant results in decreased adsorption capacity for most micropollutants, except for the positively charged ones (metformin and guanylurea) and hexamethylenetetramine.In Chapters 4 and 5, the potential of biodegradation to regenerate GAC adsorption capacity is assessed. This is studied in batch and in column experiments using a model micropollutant, melamine, and biomass capable of degrading it. In Chapter 4, favourable conditions for melamine biodegradation are presented. Melamine biodegradation is assessed in fully oxic and anoxic, as well as in alternating oxic and anoxic conditions. Moreover, the effect of an additional carbon source on the biodegradation is determined. The most favourable conditions for melamine biodegradation are applied to bioregenerate GAC, loaded with melamine. Melamine can be biodegraded in either oxic or anoxic conditions and melamine degrading biomass can restore at least 28% of the original GAC adsorption capacity in batch systems. Furthermore, high adsorption rates obtained for bioregenerated GAC indicate that bioregeneration occurs mainly in the largest pore fraction of GAC.The contribution of melamine biodegradation to the total removal in lab-scale GAC filters is assessed in Chapter 5. The filters are inoculated with the same melamine degrading biomass used in Chapter 4. The effect of an additional carbon source (methanol) and contact time on melamine removal efficiency using two different inoculation methods is studied. Inoculation of GAC filters with melamine degrading biomass increases melamine removal efficiency by at least 29% in the absence of an additional carbon source. When the additional carbon source methanol is supplied to inoculated filters, melamine removal is almost complete (up to 99%) and no breakthrough is observed. Furthermore, the supply of methanol to an inoculated filter that is close to saturation can stimulate bioregeneration up to 98%.The conclusions and implications related to the experimental chapters are discussed in Chapter 6, and put in perspective of the current knowledge on micropollutants removal with GAC. Finally, recommendations for future research and opportunities for using bioregeneration to improve water treatment are presented.
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| Format: | Doctoral thesis biblioteca |
| Language: | English |
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Wageningen University
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| Subjects: | Life Science, |
| Online Access: | https://research.wur.nl/en/publications/micropollutants-removal-with-activated-carbon-adsorption-and-rege |
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