**3.3. Biological treatment processes**

The secondary treatment can be defined as "treatment of wastewater by a process involving biological treatment with a secondary sedimentation". In other words, the secondary treatment is a biological process. The settled wastewater is introduced into a specially designed bioreac‐ tor where under aerobic or anaerobic conditions the organic matter is utilized by microorgan‐ isms such as bacteria (aerobically or anaerobically), algae, and fungi (aerobically). The bioreactor affords appropriate bioenvironmental conditions for the microorganisms to reproduce and use the dissolved organic matter as energy for themselves. Provided that oxygen and food, in the form of settled wastewater, are supplied to the microorganisms, the biological oxidation process of dissolved organic matter will be maintained. The biological process is mostly carried out bacteria that form the basic trophic level (the level of an organism is the position it occupies in a food chain) of the food chain inside the bioreactor. The biocon‐ version of dissolved organic matter into thick bacterial biomass can fundamentally purify the wastewater. Subsequently, it is crucial to separate the microbial biomass from the treated wastewater though sedimentation. This secondary sedimentation is basically similar to primary sedimentation except that the sludge contains bacterial cells rather than fecal solids. The biological removal of organic matter from settled wastewater is conducted by microor‐ ganisms, mainly heterotrophic bacteria but also occasionally fungi. The microorganisms are able to decompose the organic matter through two different biological processes: biological oxidation and biosynthesis [1]. The biological oxidation forms some end-products, such as minerals, that remain in the solution and are discharged with the effluent (Eq. 1). The biosyn‐ thesis transforms the colloidal and dissolved organic matter into new cells that form in turn the dense biomass that can be then removed by sedimentation (Eq. 2). Figure 1 summarizes these processes. On the other hand, algal photosynthesis plays an important role in some cases (Figure 2).

$$\begin{aligned} \text{Oxidation:}\\ \begin{array}{l} \text{COHNS+O}\_{2} + \text{Batteria} \rightarrow \text{CO}\_{2} + \underset{\text{+ Energy}}{\text{NH}} \\ \text{+ Energy} \end{array} \end{aligned} \quad \begin{aligned} \text{NH}\_{3} &\quad \begin{aligned} \text{NH}\_{3} &\quad \begin{aligned} \text{(OH)} &\quad \begin{aligned} \text{(OH)} &\quad \begin{aligned} \text{(OH)} &\quad \begin{aligned} \text{(OH)} &\quad \begin{aligned} \text{(OH)} &\quad \begin{aligned} \text{(OH)} &\quad \begin{aligned} \text{(OH)} &\quad \end{aligned} \end{aligned} \end{aligned} \end{aligned} \end{aligned} \end{aligned} \end{aligned} \end{cases}$$

Biosynthesis:

$$\begin{array}{c} \text{COOHNS+O}\_{2} + \text{Batteria} \rightarrow \text{C}\_{5}\text{H}\_{7}\text{NO}\_{2} \\ \text{(Organic matter)} \end{array} \tag{2}$$

#### *3.3.1. Useful terms*

The following terms are the most used in biological treatment processes [2]:


**Figure 1.** Biological synthesis and oxidation [3].
