# Foams in Wastewater Treatment Plants: From Causes to Control Methods
## 2 Bibliometric Research and Structure  
In order to focus the problems of foams in wastewater with the causes and circumstances associated, the literature search was performed in the Scopus and Google Scholar databases by entering \"foam wastewater\" and \"foaming wastewater\" as keywords. To get a review on the currently known methods for the evaluation of foaming and the most applied strategies for controlling foaming directly in WWTPs, \"foam test wastewater\" or \"foam control wastewater\" were used as keywords in the same databases. In Scopus database the analysis has been conducted searching the keywords on fields \"Article title, Abstract, Keywords\" to focus attention on works closely related to the management application field in WWTPs. The research focused mainly on works published since 1990. A first screening of the literature was carried out, discarding the articles and reviews that do not analyze (i) the causes and inconveniences related to the formation of foams in WWTPs, (ii) the evaluation methods or (iii) the control mechanisms that operators can apply to manage the problem in WWTPs. At the end of this screening, about 120 papers were found. Focusing only on the field of environmental science and engineering rather than on chemical engineering and chemistry, almost 90 articles were selected for the review.  
The spatial and temporal distribution of publications concerning the foam formation in the WWTPs carried out since 1990 is shown below. In Figure 1 the period from 1990 to today is divided into time intervals of 5 years and for each one, the total number of international scientific publications on foaming in WWTPs is reported. It is possible to notice how over the years there has been a growing interest in the problem, this has led the world of research to focus more on the issue. In fact, since 2015 there have been 30 publications, 20 more than the first five years considered.  
The map of Figure 2 shows the origin of the scientific publications on foams in the WWTPs. The country reported in a scientific publication indicates the nationality of the first author. The largest number of research and review papers on foams in WWTPs was written in the United States and China, respectively 21 and 20. Italy and Australia are placed in third and fourth position with 11 and 10 publications, respectively.  
Figure 1: Number of publications per year on Scopus introducing the words “foam wastewater” and “foaming wastewater”.  
Figure 2: Number of publications by country on Scopus introducing the words “foam wastewater” and “foaming wastewater”.  
In conclusion, it is possible to state that this topic is not yet very studied in the world, but over the years a greater interest is developing more and more. Publications are on the rise and on a geographical level this issue is being addressed in more and more countries. Foaming in the WWTPs is a very sensitive issue that is only dealt with at the management level, so in recent years there has been a growing perception of the need to deepen the study through a methodological research approach.  
The review consists of four parts; Section 3 reports the causes of the formation of foams, focusing mainly on the biological ones in CAS, MBR processes and ADs. Instead, in Section 4 the main methods of evaluation of the foams are explored, describing both the direct methodologies (including tests applicable directly on-site) and the indirect ones of characterization of the mixed liquor. Section 5 discusses the application of foam control methods, mainly describing the physical and chemical ones most used on the real scale. Finally, in Section 6 a brief discussion is presented on the main advantages and disadvantages of the evaluation and control methods, focusing on possible future outlooks.