# Foams in Wastewater Treatment Plants: From Causes to Control Methods
## 3 Causes
There is little information available concerning the microbial community of MBR processes and almost nothing is known about the potential micro-organisms that attend the foam formation [23]. The predominant filamentous bacteria in the foaming MBR process appear to be the HGC bacteria, including Actinomycetes, _Nostocoida limicola_ III, _Microthrix parvicella_, Eikelboom type 1851, Eikelboom type 0041, _Gordonia_ and _Cytophaga_[54]. All these bacterial species play a fundamental role in stabilizing the foams because they are hydrophobic as in their cell walls there is the mycolic acid [62]. _Microthrix parvicella_ is one of the most frequently observed filamentous bacteria in foaming at activated sludge and the results obtained by [PERSON] et al. [63] indicate that a low sludge load condition could induce a significant increase in these bacteria. In MBR systems, foams can also form in the absence of surfactants from the incoming wastewater and without a large concentration of hydrophobic bacterial strains (foam-forming bacteria). The amount of foams, in this case, is related to the concentration of EPSs in the mixed liquor, which influenced the formation of foams but not its stability [23]. As stated jointly by the scientific community [22; 49; 51; 52; 64; 65; 66], in MBR plants, EPSs cause membrane fouling, which can also lead to the obstruction of the membrane thus blocking the operation of the whole plant. Therefore, it is advisable to study the formation of foams in MBR processes with the aim of keeping membrane fouling under control. The EPS, therefore, in addition to causing the fouling of the membranes are also foaming agents [67]. The presence of filamentous organisms and surfactants in the incoming discharge can cause an increase in the concentration of extracellular substances, leading to worsening of both membrane fouling and foaming phenomena [53]. The mechanisms that lead to the formation of foams in MBR and CAS systems are the same, the main difference between the two systems lies in the fact that MBR process tends to retain most of the EPSs, so as to become a real and own \"foam trap\" [21]. The ADs provide a favorable environment for the formation of three-phase foams as all the components are present: liquid, surface active agents and solid particles and gases (biogas). In their review, [PERSON] and [PERSON] [26] reported that the foaming of a three-phase system is directly proportional to the concentration of particles and inversely proportional to the particle size: in their three-phase foams scheme, the particles and gas bubbles are relative of the same dimensional order. Instead, in the biological foams of the CAS process, the gas bubbles are much larger than the particles. Foam formation in anaerobic digesters is still a poorly understood mechanism and the studies in this regard are less thorough than those carried out for the CAS and MBR processes: there is, therefore, an urgent need for research in this field to clarify the conditions that trigger the generation of foams in biogas reactors and to develop new solutions for the prediction and prevention of foam formation [28].