# Recent Developments and Emerging Trends in Paint Industry Wastewater Treatment Methods
## 4 Comparison of Techniques and Future Trends  
In recent years, there has been an increasing demand for high-quality wastewater treatment; this coincides with stricter regulations regarding wastewater reuse. The main characteristics of the paint industrial wastewater are color, pH, high chemical oxygen demand, and low biodegradability. Suspended soils are the dominant contaminants that could be decreased by the impressive implementation of conventional treatment techniques in facilities. Traces amounts of heavy metals in pigments occur in industrial wastewater. Oil and grease are the other major contaminant indicators in the paint industry [83]. Different treatment techniques have been used for the treatment of paint industry wastewater and the main advantages and disadvantages of these techniques are summarized in Figure 6.  
In the last decade intense efforts have been dedicated to developing additive chemistries that are more environmentally friendly. Therefore, the principles of green chemistry should be accepted also in paint industry effluent treatments. It should be concluded that mostly physicochemical methods are applied. In comparison with other techniques, these methods are more simple and cheaper. However, the effect of the amount and concentration of coagulant and the pH on the quality of wastewater treated is an important factor and must be optimized [30]. To eliminate the shortcomings of chemical coagulants, which are related to a high carbon footprint associated with their production, natural plant-based coalgulants are beginning to be used, which represents a renewable, non-hazardous, degradable, potentially carbon-neutral option, to replace conventional coagulants [40]. The biological degradation of the organic pollutants included in paint industrial wastewater is very difficult. Therefore, biological methods are rarely used. However, biofiltration has been successfully applied for the treatment of different types of wastewaters and should be a promising biological alternative for paint industrial effluent treatment thanks to the advantages of a smaller foot print, shorter retention times, lower sludge production, and effective removal performances [84]. \begin{table}
\begin{tabular}{p{113.8 pt} p{113.8 pt} p{113.8 pt} p{113.8 pt}} \hline \hline
**Treatment Method** & **Treatment Parameters** & **Results** & **Source of Wastewater** & **References** \\ \hline Combination of chemical coagulation-flocculation with aerobic biological process & FeCl\({}_{3}\) and coagulant aid centrifuged at 6000 rpm 25 min & 96\% of COD removal, 97\% of color removal and 92.5\% of BOD removal & Collected from equalization tank of a paint factory & [19] \\ \hline Multilevel contact oxidation system & 25 L active sludge, 30 L painting wastewater and 25 L clean water evenly mixed and then fed to biological tank with special biological stuffing total process time 30 days & Removal of COD 84\% SF82.5\% with hydraulic retention time of 8 h & Painting wastewater pretreated by physicochemical process & [82] \\ \hline Hybrid electro-thermochemical wastewater technology & Additives—two types: organic, inorganic—added during the treatment procedure both additives were used for 2 different potential settings resulting in 4 treated water samples & Removal efficiency factor (REF) 90\% & Water discharge of paint factory & [6] \\ \hline \hline \end{tabular}
\end{table} 
Table 5: _Cont._Under the current circumstances, advanced water treatment technologies are urgently needed to guarantee high-quality water, reduce chemical and biological contaminants, and strengthen industrial production processes. Modern wastewater treatment is based on the combination of simple physicochemical processes with electrochemical oxidation, ozonation, or advanced techniques. Nevertheless, future research should focus on optimizing the process and reducing costs. At the same time, an important issue that will concern final users is the disposal of the produced sludge, especially in light of the circular economy and minimizing the total cost.