## 2 Materials and Methods  
### Wastewater Treatment Plant Typical Processes  
The representative processes that usually take place inside a wastewater treatment plant (WWTP) are summarized in Figure 1, and further detailed in this section. The treatment process is divided, from a logical standpoint, into multiple stages: Pretreatment, primary treatment, secondary treatment, tertiary treatment, and sludge treatment. The wastewater enters the treatment plant from the wastewater network, where the source of the water is residential, institutional, commercial, industrial, rain, or a mix of the aforementioned. After the water enters the treatment plant, the pretreatment processes are initiated. Firstly, odor treatment can be applied so that the plant surrounding areas are protected from the foil smell that naturally accompanies wastewater. The odor treatment process may not be necessary at some plants. There are two distinct methods of odor treatment: Air treatment and liquid treatment. If the air treatment method is applied, the wastewater is contained in large tanks, which are hermetically covered with specially designed odor control covers. The air trapped under the cover, inside the tanks, is extracted by a ventilation system and undergoes treatment before it is released to the environment. Regarding the liquid treatment method, different chemicals that neutralize the foil smell-producing elements are introduced in the wastewater. The second process that takes place during pretreatment is screening,where the wastewater is passed through filters in order to remove both grit and large objects such as bottles, plastics, tree branches, sanitary items, and cotton buds. It is very important to remove such objects early in the process because they can damage the plant equipment if present in future steps of the treatment process. The removed objects are either incinerated or disposed in landfills. After the pretreatment is completed, the wastewater enters the primary treatment stage, where the remaining solid matter is separated from the wastewater. In this stage, large sedimentation tanks are used, in which the wastewater clarifies. The sludge settles at the bottom of the tank, while grease and oil rise to the surface. The sludge is removed and directed to the sludge treatment process, while the grease and oil can be used for soap making. After the primary treatment, an optional bypass exists so that the treated water can be sent directly to the natural environment without entering the secondary and tertiary treatment stages. This bypass is used during heavy rainfall by plants that receive wastewater from a combined sewer system. In this case, the secondary and tertiary treatment stages are bypassed in order to protect them from hydraulic overloading, the mixture of sewage and rainwater receiving only primary treatment before being sent back to the natural environment. In some plants, the bypass is implemented directly at the inlet, so the  
Figure 1: Typical wastewater treatment plant (WWTP) processes. wastewater is not even screened. In addition, for larger plants without a complete gravitational bypass, high-energy consumer pumps are used to transfer the untreated water directly into the emissary when the large amounts of wastewater exceed the capacity of the plant. The secondary treatment stage objective is to remove the biological matter from the wastewater, by using a bioreactor tank where both oxygen (introduced with blowers) and a biological floc (bacteria and microorganisms that consume the remaining organic matter) are inserted into the wastewater. Before exiting the secondary treatment, wastewater is sent into a clarifier tank, where large particles settle down at the bottom (sludge) and are extracted for the sludge treatment process. In order to maintain the optimal process parameters during the biological treatment, a pH adjustment must take place, involving different chemicals (Ca(OH)\({}_{2}\), CaCO\({}_{3}\), Na\({}_{2}\)CO\({}_{3}\), NaOH, etc.).