# Foams in Wastewater Treatment Plants: From Causes to Control Methods
## 3 Causes  
Foaming formation occurs either chemically or biologically [2; 14; 23; 38] and in both cases, a necessary condition is the dispersion of a gas in a liquid [12; 13; 26]. The chemical foams are caused mainly by excess surfactants (white foams), which have many industrial applications such as washing and cleaning (detergents), pharmaceuticals, cosmetics, textiles, tanneries, agriculture, biotechnology, food, paints, microelectronic, metallurgy, oil recovery and paper, among others [35; 39; 40; 41]. The presence of surface-active compounds affects the rheology of the fluid interfaces because they can cause variable viscosity, elasticity and surface tension gradients [42]. The factor that contributes to the arising of the biological ones (brown foams) is the growth of particular filamentous bacteria, the so called \"foam-former\", and/or the presence of hydrophobic high-molecular-weight substances synthesized by bacteria present in mixed liquor, although not the foam-former [2; 4; 43; 44; 45; 4]. Biological foams are more viscous and stable than foams induced by synthetic surfactants and the most foam problems in WWTPs are associated with them: a three-phase phenomenon that involves a liquid, a solid and a gas [23; 26]. The hydrophobic products described above are extracellular polymeric substances (EPSs), which have properties of surface-active agents [22]. The EPSs are present both outside of cells (bound EPS (EPSb) and soluble EPS (EPSs)) and in the interior of microbial aggregates. Their major components are carbohydrates and proteins; but humic substances, lipids, nucleic acids, uronic acids and some inorganic components can also be found in EPSs [46]. In general, their concentration plays a key role in making the foams more stable [47; 48]. The bacteria in the mixed liquor can produce EPSs through a variety of biological processes (including active secretion, substrate degradation, biomass decay, shedding of cell surface material and bacterial self-hydrolysis of floc-associated EPS) [46; 49; 50] or under biological or mechanical stress conditions [51]. These disturbing conditions for biomass can be caused by the presence of toxic substances in wastewater, such as surfactants, that can damage the cell membrane or by a strong imbalance between organic matter and nutrients with consequent production of EPSs [2; 38; 46]. A deviation from the correct balance (BOD:C:P = 100:5:1 in CAS processes), such as an important lack of phosphor significantly influence EPSs production [2; 46]. The EPS concentration is increased by the presence of foam-forming and filamentous microorganisms in the mixed liquor of which they cause a worsening of the potential foaming phenomenon [52; 53; 54]. Among the causes of the formation of filamentous bacteria, in addition to a low ratio between organic matter and microorganisms [2; 5; 23], there may also be some particular operative conditions such as low concentrations of dissolved oxygen (indicatively \(<\) 2 mg L\({}^{-1}\)) in the biological reactor and high retention times of solids that create selective conditions for the growth of foam-former microorganisms [23; 55; 56]. In CAS processes, the most observed filamentous taxa seen are _Candidatus_ 'Microthrix parvicella', a gram-positive filamentous bacterium, and members of the _Mycolata_, bacteria containing mycolic acids in their cell walls. All are members of the order Actinomycetes in the phylum Actinobacteria [57; 58; 59; 60]. Several mycolic acids containing Actinomycetes (_Mycolata_) have been found in biological foams and within this group, _Gordonia amarae_, _Skermania pinitormis_ and _Rhodococcus rhodocrous_ are the main organisms previously identified in foaming microbial communities [61]. Although foaming originally was associated with CAS plants, recently this phenomenon has been observed also in other treatment processes such as MBRs and ADs [2; 21; 22; 23; 46].