|Microplastics in sea water: Fate and removal potential throughout reverse osmosis installations|
Saldi, A. (2019). Microplastics in sea water: Fate and removal potential throughout reverse osmosis installations. MSc Thesis. Universiteit Gent. Faculteit Bio-ingenieurswetenschappen: Gent. xii, 78 + App. A pp.
Microplastic (MP) particles are reported to be found across our planet, from our land’s rivers and lakes into the seas. Even down in the remote and pristine Southern Ocean surrounding Antarctica, MP pollution has been observed. Research has already been exploring the consequences and the hazards of their presence on the health of aquatic ecosystems and individual organisms, including human health. Ingestion may lead to suffocation or starvation, while the plastic products themselves contain additives such as pigments or plasticizers which are released to the environment and lead to e.g. endocrine disrupting effects. The main challenge in this issue of MP pollution is the prevention of plastic waste entering the aquatic systems. However, at the same time, the pollution has become a global concern and its removal will have to be part of solving this widespread problem. The basic idea of this research is to look into a process which treats seawater at high flow rates on a daily basis. Given the increasing scarcity of drinking water, there is a growing demand for desalination capacity where seawater is processed to drinking water. An increasingly dominant process is seawater reverse osmosis (SWRO), a membrane-based technology to physically separate salt molecules from the water. To allow decent operation, this membrane process requires various pretreatment steps which gradually purify the incoming seawater. The goal of this thesis is to describe the fate of MP, present in the intake seawater, throughout this pretreatment in SWRO installations and to assess their potential for removal from this system. The experimental part allowed to identify the reject stream of a dual media filtration (DMF) unit as a hotspot for MP that enter the SWRO installations, while the microfiltration (MF) unit acts in the same way if it is not preceded by a DMF unit. This study demonstrates that the fate of MP in these two conventional pretreatment steps is similar: a very high removal efficiency from the incoming stream and a significant fraction is flushed out during the backwash procedure of these filtration units. As a result, the MP are concentrated in the reject stream after backwashing. A series of simulation calculations based on a broad range of parameters (e.g. ingoing MP concentration or plastics composition) indicates a removal potential from 0.3 kg up to 5 tonnes of MP on a yearly basis in a conventional large-scale SWRO installation.