# Application of Machine Learning Models in Optimizing Wastewater Treatment Processes: A Review
## 2 Wastewater Treatment Processes in an Industrial Wastewater Treatment Plant  
### Stages of the Wastewater Treatment Process  
The treatment processes from a WWTP are vital in obtaining a quality plant effluent that does not negatively affect the plant, as well as in the efficient removal of inorganic or organic pollutants. These are complex and, in the majority of cases, highly nonlinear dynamic processes (as in wastewater pH neutralization, activated sludge, etc. ), which consist of three treatment stages: the mechanical (physical) stage, the chemical stage, and the biological stage. In the mechanical stage, the removal of solids (total suspended solids--TSS) takes place through gravitational separation, sedimentation, and pre-aeration [11]. Additionally, fats and oils are separated using fat separators and decanters. In the plant chemical stage, the wastewater pH neutralization process (the removal of excess acidity or alkalinity from the treated wastewater through the use of a chemical reagent of opposite composition, such as hydrated lime Ca(OH)2 and sulfuric acid H2 SO4) occurs. Also, in this stage wastewater chemical precipitation takes place (the removal of heavy metals, phosphates, sulfides, fluorides, nitrates, etc.) through the use of a set of reagents (such as aluminum sulfate--Al2(SO4)3, aluminum chloride--AlCl3, ferric chloride--FeCl3, ferric sulfate--Fe2(SO4)3, hydroxide (OH), and sulfides (S2-)), and also fine particle coagulation and flocculation. Chemical oxidation and reduction are other essential processes that occur during the chemical stage, as toxic organic pollutants (such as chromium, chlorine, hypochlorite, hydrogen peroxide, and nitrite) are transformed into less harmful compounds using different oxidants (like chlorine and hydrogen peroxide) that are more easily removed. In addition, through chemical oxidation, BOD (Biochemical Oxygen Demand) and COD (Chemical Oxygen Demand) reduction, ammonia removal, chemical oxidation of non-biodegradable organic compounds, reduction of residual organic matter concentration, oxidation of iron sulfate, etc., take place [12; 13]. In the third and final stage (before the plant effluent is discharged into the emissary), the biological one, which includes organic matter degradation and nutrients removal, the metabolic activity of microorganisms (such as bacteria, fungi, and protozoa) is considered the most cost-effective biological treatment procedure [11; 14]. Biological treatment is made through both aerobic and anaerobic processes. In industrial applications, anaerobic biological treatment is frequently used for highly organically loaded wastewater, which is treated using Upflow Anaerobic Sludge Blanket (UASB) reactors, anaerobic membrane bioreactors, and anaerobic fluidized bed reactors. This approach offers several advantages, including reduced plant operational costs, energy recovery, and reduced sludge production. The use of aerobic or anaerobic biological treatment depends on the characteristics of the treated wastewater (BOD or COD concentration, etc. ), space constraints, and energy requirements [15; 16; 17]. In addition to conventional biological treatment methods, advanced oxidation processes (AOPs) are sometimes employed, which produce reactive oxygen species for the degradation of pollutants [18]. For treating industrial wastewater that contains non-biodegradable or toxic organic compounds, AOP technology (utilizing highly reactive hydroxyl radicals, referred to as \(\bullet\)OH) is employed to achieve the complete mineralization of these difficult-to-treat organic pollutants through various mechanisms, including ozonation, Fenton oxidation, UV-based photolysis, and electrochemical methods [19]. In Figure 1, a block diagram of a WWTP from the author's point of view is presented. As can be observed in Figure 1, the input is represented by the plant influent, which is wastewater with specific characteristics (such as pH, TSS, COD, BOD, extractables, chlorides, phenols, etc.).