**3.4. Chemical treatment processes**

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

2 2 3

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

COHNS+O +Bacteria CO + NH +Other end products ® (1)

2 57 2

COHNS+O +Bacteria C H NO ® (2)

(Organic matter) (New cells)

(Organic matter) +Energy

Biosynthesis:

(Figure 2).

*3.3.1. Useful terms*

Oxidation:

4 Wastewater Treatment Engineering

**a.** DO: Dissolved Oxygen (mg L-1)

**b.** BOD: Biochemical Oxygen Demand (mg L-1)

**d.** COD: Chemical Oxygen Demand (mg L-1)

**g.** SOD: Sediment Oxygen Demand (mg L-1)

**e.** CBOD: Carbonaceous BOD (mg L-1)

**f.** NBOD: Nitrogenous (mg L-1)

**h.** TBOD: Total BOD (mg L-1)

**c.** BOD5: BOD (mg L-1), incubation at 15°C for 5 days

In early wastewater treatment technologies, chemical treatment has preceded biological treatment. Recently, the biological treatment precedes chemical treatment in the treatment process. Chemical treatment is now considered as a tertiary treatment that can be more broadly defined as "treatment of wastewater by a process involving chemical treatment". The mostly implemented chemical treatment processes are: chemical precipitation, neutralization, adsorption, disinfection (chlorine, ozone, ultraviolet light), and ion exchange.
