## 1 Introduction  
Tertiary wastewater treatment involves the removal of organics, inorganics and pathogens [1]. It produces improved quality effluent that can be used for irrigation and that is safe to discharge into water bodies. This process is therefore crucial to the advancement of society, as it provides us with the water we require to live and to thrive.  
Some of the mature technologies used on purification plants include chlorination, ultraviolet radiation and ozonation [2]. These processes offer low treatment times and highly effective purification, and they may be used individually or together.  
Despite being well established, these technologies have drawbacks. These include, but are not limited to: high energy consumption, high operating and capital costs and significant chemical demands [2]. Furthermore, these technologies were introduced at a time when little was known about the behaviour and impact of endocrine-disrupting chemicals and persistent organic pollutants [3, 4].  
Since composition analysers have become more sophisticated, more data have been collected on the concentration and ubiquity of these endocrine-disrupting and persistent organic compounds in water. While their concentrations may be too low to affect us at present [5, 6], our currently ineffective treatment methods can lead to their accumulation in our environment. This could have a major impact on the health and safety of future generations.  
This dilemma places considerable demand on advanced oxidation processes [7]. Non-thermal plasma water purifiers exhibit the potential to remove these persistent organic pollutants. This nascent technology eliminates the aforementioned drawbacks with its low energy demands (depending on the plasma source), low chemical demands, and, by application, lower cost. It is also able to compete with the strengths of mature technologiesin terms of treatment times and the effectiveness of purification [8]. However, it has yet to reach acceptable maturity to suit large industrial applications.  
This review was compiled to determine whether it would be feasible to retrofit existing wastewater treatment plants with non-thermal plasma technology, to effectively remove persistent pollutants. It was also compiled to determine which facets of plasma purification could be improved upon, such that this technology becomes more feasible and thus more readily implemented on a large scale.  
To achieve these aims, an overview of each technology type is presented. Thereafter, the strengths and weaknesses of each technology are evaluated in terms of effectiveness, capital cost, operating cost, energy efficiency and chemical demand. The scope of the review was limited to only include plasma reactors that operate non-thermally.  
At present, there are no comparison studies between plasma and mature technologies that simultaneously consider a broad range of pollutants and criteria (such as those mentioned above). However, the data do exist in isolation, and ongoing attempts are being made by researchers to compile this information. The lack of published comparison studies is likely due to the variability within the plasma field. There are dozens of different plasma reactor types, with different designs and power requirements. This makes it difficult for scientists to build on the work done by previous researchers. Furthermore, plasma is being applied to water, food and textiles, which introduces more variability into the field. Finally, there are hundreds of wastewater contaminants that can be passed through a plasma reactor, which implies that the experimental results are often not corroborated by others.