Research activity in this area covers a great many topics and the aim of this review is to quantify the level interest in each of these areas. This quantification of research has shown that interest in nanofiltration technology has grown over the past decade, particularly over the past five years. The primary journals reporting articles on nanofiltration are the Journal of Membrane Science, Desalination and Separation and Purification Technology, although articles have been spread across a further 139 journals. There has been clear growth in the areas of organic solvent nanofiltration, pharmaceutical and biological applications, design and economics of nanofiltration processes and review articles. The aim of this review is to collate and highlight the trends in research progress related to nanofiltration membranes and processes over the past decade, from 2007 onwards. To this end, the ScienceDirect platform has been used to collate academic papers across the spectrum of journals available. The subsequent processing of articles has removed any papers that claim keywords or text such as ‘ nanofiltration ’ but does not actually include work in the respective field. For example, the term nanofiltration may appear quite legitimately in the text of a paper, but on closer inspection the paper actually deals specifically with ultrafiltration. Where this is the case, these papers have not been included in this review with the goal to only include relevant research specific to nanofiltration. At the time of writing, there have been 1902 journal articles published on Science Direct related to the topic of nanofiltration. Due to the versatility of nanofiltration membranes, these articles cover a range of topics, for example membrane fabrication, membrane modification, desalination, and Organic Solvent Nanofiltration ( OSN – formerly known as Solvent Resistant Nanofiltration [ SRNF ] ) all showing that nanofiltration is an innovative technology that has a wide range of applications. This number, barring small spikes in 2009 and 2012, was almost constant until 2013 ; beyond which there has been a significant year on year increase to 266 papers in 2016. Figure 2 indicates the major journals that have published articles related to nanofiltration. Collectively these three journals account for more than 60 % of all nanofiltration publications with the remainder of articles spread across a further 139 different Journals. A breakdown of the publication topics from the total amount of publications from 2007 to present is provided in Figure 3. Waste water applications total some 18. 30 % of the papers reviewed, the trend is then: Pharmaceutical and biotechnology ( 14. 04 % ), economics and design ( 13. 72 % ), membrane modification ( 12. 83 % ), solvent nanofiltration ( 11. 25 % ), membrane fabrication ( 10. 52 % ), desalination ( 8. 94 % ), fouling studies ( 7. 83 % ), modelling ( 6. 78 % ), reviews ( 5. 47 % ) and food ( 2. 52 % ). Given that waste water applications and desalination could be grouped together as ‘ water processing ’ then this combined subject area is the predominant area of research interest and represents some 27. For example, the largest single industrial application for nanofiltration seen in this review is that of water applications, both waste treatment and desalination. By contrast, the food industry is the lowest observed area of research found in this review. Retrofitting to include NF processes for this industry requires substantial capital investment to upgrade the pumps and pipework to take the increased operational pressure. Waste water processing is the single largest area of research interest found in this study. The directive constitutes the culmination of water legislation within the European Union and is aimed at establishing a framework for action in the field of water policy, i. e. improving and maintaining water standards across the region. The level of research interest in this field has remained almost constant across the period studied, as illustrated in figure 4, and has only seen an increase in activity since 2013. Within this sub-topic studies include removal of dyes from actual textile industry waste waters [ 8 ] and the study of synthetic systems representative of the industrial effluent such as the removal of Congo Red from water samples [ 9 ]. There has been a steady supply of papers in this area at the level of 10-12 papers per year from 2007 to 2012 and then a significant increase year on year to 32 papers in 2016. Out of the total publications in this area, seawater desalination is the most published topic ( 40. 43 % ) followed by desalting waste water ( 30. 5 % ), then brackish water ( 18. 44 % ), pure water ( 9. 22 % ) and finally river water ( 1. 42 % ). The most common support layer found in this review was polyether sulfone ( 7. 17 % ) that may be used in pure form or combined with other materials [ 20 ]. A list of the top 25 materials for fabrication and modification of nanofiltration membranes, ranked by number of papers, is provided in table 2. Similarly, the vast majority of new or modified membranes identified in this research have only ever been fabricated at bench and pilot scale. This is an obvious issue and for the impact of this colossal amount of research to be fully realised, then a new generation of commercially available NF membranes translated from this early research should be expected soon. Marchetti et al. [ 26 ] have reviewed the field of OSN, and while at 72 pages their review is not particularly ‘ critical ’ as the title would suggest, the review is certainly comprehensive and provides a full overview of this area. Pharmaceutical and biological research has also shown a steady level of increase over the period studied. As nanofiltration technology and applications grow, naturally there will be attempts made in order to design more effective and efficient separations with the ultimate aim of reducing costs. Since then the number of papers has risen steadily to 42 papers in 2016, while the total numbers are modest this change represents a 300 % increase in research activity ! Nanofiltration is no exception and figure 11 shows that the number of reviews for nanofiltration topics has been steady with between 5 and 10 papers published each year for the period 2007 to 2013, with a year on year increase following this date to 28 papers in 2016. In this review, the only area of nanofiltration that has experienced stagnation or decline is the topic of modelling nanofiltration processes. Clearly the overall growing trend in nanofiltration research indicates that the technology remains popular and this interest should materialise into further applications for a robust and sustainable future. One would suspect that as existing plants reach their end of life, then the opportunity to select NF processes will become more widespread and further research to compliment the technology uptake should be expected. While some of these latter applications have only been proven at bench scale, further development into large scale processes would be expected. By the same token, 101 different chemicals is only the very tip of the iceberg in terms of available materials for fabrication and modification and demonstrates that there is potentially a huge amount of research still to be completed in this area. The key question dictating how much of this future potential will be realised is related to the effort and cost expired in relation to the improvements in membrane performance gained and the supply of new applications for nanofiltration technology to justify the experimental cost. Thus, nanofiltration modelling is inherently coupled to measurement technology and until there is a step change in measurement at the nanoscale, interest in modelling nanofiltration membranes is likely to amble along or decline yet further.
