# Foams in Wastewater Treatment Plants: From Causes to Control Methods
## 4 Evaluation Method
### Direct Methods
#### 4.1.7 Methods Applicable on Site  
Two methods of direct measurement of foams applied directly on-site are described below. The results of these procedures must be interpreted and evaluated with caution, attention and critical spirit since above all the apparent degree of foams produced is probably influenced by the layout of the plant, the configuration of the process equipment and the operating parameters (for example, entrapment and accumulation of foams in certain areas of the reactors) [21; 62]. Foam surface covered (FSC) is a percentage index of the surface of the aeration basin covered by foams. To achieve this, the aerobic reactor is photographed daily with an instrument orthogonal to the surface, connected to a digital image processing system (CAD Software). The areas covered by the foams are thus digitized [23]. This method was also previously applied by [PERSON] et al. [78]: it was possible to evaluate the foam-covered surface of aeration tank because they were equipped with mechanical aerators, although with an approximated method proposed by [PERSON] et al. [79]. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Direct Methods** & **Equipment** & **Operative Conditions** & **References** \\ \hline \multirow{2}{*}{Foam Rating (FR)} & Glass tube (50 cm high, 4 cm diameter), air & Sample volume: 50 mL, \\  & diffuser (porosity 40-90 \(\mu\)m) on the bottom & Airflow: 200 mL min\({}^{-1}\) & [2; 13; 23] \\ \hline \multirow{2}{*}{Shaking} & 100 mL graduated cylinder & Sample volume: 14 mL, Stirring time: 10 s & [72] \\ \hline \multirow{2}{*}{Air bubbling} & Quartz column (50 cm high, 3.5 cm & Sample volume: 50 mL, \\  & diameter), 0.25 cm capillary on the bottom & Airflow: 158 mL min\({}^{-1}\) for 2 min & [72] \\ \hline \multirow{2}{*}{Waring Blender} & High speed mixer & Sample volume: 100 mL, Mixing speed \\  & & \textgreater{}1000 \(r\) min\({}^{-1}\) for 3 min \\ \hline \multirow{2}{*}{Foam Power (FP)} & Vertical acrylic cylinder (100 cm height, & Sample volume: 100 mL, \\  & 3 cm diameter, 7 cm\({}^{2}\) of cross section) & Airflow: 5 L min\({}^{-1}\) for 20 s or 30 s \\ \hline \multirow{2}{*}{Foam Power 2 (FP2)} & Glass tube (1 L, 6 cm diameter), borosilicate & Sample volume: 150 mL, \\  & sintered disc (porosity 160-250 \(\mu\)m, \\  & 100–160 \(\mu\)m, 40–100 \(\mu\)m) on the bottom & Airflow: 0.5 L min\({}^{-1}\) for 1 min & [20; 62] \\ \hline \multirow{2}{*}{Foam Stability (FS)} & Graduated cylinder, sintered glass diffuser & Sample volume: 150 mL, \\  & on the bottom & Airflow: 0.5 L min\({}^{-1}\) for 1 min & [22; 62] \\ \hline \multirow{2}{*}{Scum Index (SI)} & Flotation cell (50 cm height, & Sample volume: 2L, \\  & 8 cm internal diameter) & Airflow: 10 L (L h)\({}^{-1}\) for 15 min & [2; 78; 21] \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Instrumentation and operative conditions of foam direct assessment methods. In some cases, there was an excellent correlation between the foam coverage and the foaming potential. However, it is preferable to use the result on the foam coverage together with other methods for assessing the extent of foaming in the aeration basins [62; 78]. Foam Volume (FV)  
The second method examined is the foam volume (FV). In order to quantify the foam volume, its thickness must be determined as the arithmetic mean of ten measurements taken at different points of the aeration basin operating on a pre-established horizontal grid [23]. Note the percentage of the covered area, it is possible to estimate the total apparent volume: in the study of [PERSON] et al. [78], the foam thickness was measured with a hydrometer and the estimation of the total apparent foam volume has been expressed as foam volume ratio parameter, defined as the per-thousand part of the total volume of the aerobic reactor. [PERSON] and [PERSON] [62] applied a new method based on the observation of the difference in conductivity to identify the interface between the foam layer (with lower conductivity) and the underlying sludge (high conductivity).