Greywater Treatment and Using Vegetative Green Walls for Sustainable Urban Greening in Qatar

Global climate change and population growth are increasing the stress on water availability. To meet this rising water demand with minimal environmental burden, greywater recycling is a preferred option due to its lightly polluted nature. Greywater is wastewater excluding toilet and kitchen wastewater that is released from domestic units. Many studies have focused on high-tech treatment systems that require high operator expertise and energy/chemical inputs that are not aligned with sustainable decentralized treatment. Hence, there is a growing trend toward nature-based treatment processes. Green walls are generally used as aesthetically appealing green building technology to provide various energy, environment, and health benefits. However, there is a strong potential for using green walls to treat greywater through biofiltration, adsorption, and phytoremediation processes for low-quality reuse applications like toilet flushing or irrigation.

This dissertation investigates the interactions between various media and plant species with greywater, either for greywater treatment or plant irrigation, with a focus on applications in Qatar. Through a comprehensive series of experiments, including batch studies, leaching experiments, column experiments, and a pilot green wall study, the research explores the efficacy of eight different media (spent coffee grounds (SCG), date seeds, cattle bone, coco coir, biochar, moving bed biofilm reactor (MBBR) carrier media, perlite, and zeolite) and their interactions with greywater treatment processes. The research begins by assessing the suitability of the Ruellia plant for greywater treatment, revealing that it thrives better with greywater than with freshwater, exhibiting over 15% more growth and 31% greater biomass, due to the additional nutrients beneficial for plant growth provided to the soil and plant by greywater. This positions Ruellia as a prime candidate for green wall applications in Qatar.

Kinetic studies further elucidated the media's pollutant removal capabilities based on kinetics and the challenge of leaching in organic media, with biochar showing promise due to its minimal leaching properties. SCG showed rapid removal for organics while most of the media for pollutant removal reach equilibrium by 20 - 25 minutes. A follow-up investigation into colour leaching treatment methods for organic media highlighted the effectiveness of acid treatment and semi-continuous application of tap water, with the former offering a faster solution, though the latter presents a viable option where chemical treatments are undesirable. Column experiments provided insights into the mechanisms of media and greywater interactions over the long term. This included microbial biofilm effects and the efficiency of pollutant removal through filtration, adsorption, and biodegradation under varying loading conditions. The outcome of this work package was that a semi-continuous flow systems was preferable, increasing contact time which positively impacted the pollutant removal.

Further analysis revealed that organic media such as SCG and date seeds were less effective for greywater treatment due to their negative impact on organic and nutrient removal. The inorganic media is most preferable since there is no leaching effects. When considering organic media for greywater treatment, coco coir, and biochar were the next best, while SCG and date seeds were poor due to leaching. The pilot green wall study underscored the potential of the plant Wedelia in greywater treatment, demonstrating its ability to flourish under high loading conditions. The study revealed that rest periods and freshwater application in the intermediate stage have a positive impact on the pollutant removal. Overall, this dissertation presents a comprehensive evaluation of the potential for using green walls in greywater treatment in arid regions like Qatar. By identifying optimal plant-media combinations and treatment processes, the research contributes valuable insights towards the development of sustainable, green urban environments, highlighting the promise of greywater treatment as a viable solution for water scarcity and recovery in such contexts.