**4. Biological treatment of wastewater**

## **4.1. Biological growth equation**

The biological growth can be described according to the Monod equation:

$$\mu = (\mathcal{AS}) / (K\_{\mathcal{S}} + \mathcal{S})$$

**Figure 2.** Photosynthesis and oxidation [2].

Where, *μ* is the specific growth rate coefficient; *λ* is the maximum growth rate coefficient that occurs at 0.5 *μ*max; *S* is the concentration of limiting nutrient, that is BOD and COD; and *KS* is the Monod coefficient [3].

Generally, the bacterial growth can be explained by the following simplified figure:

2 2 Organics+Bacteria+Nutrients+Oxygen ® New Bacteria+CO +H O+ResidualOrganics+Inorganics

Several bioenvironmental factors affect the activity of bacteria and the rate of biochemical reactions. The most important factors are: temperature, pH, dissolved oxygen, nutrient concentration, and toxic materials. All these factors can be controlled within a biological treatment system and/or a bioreactor in order to ensure that the microbial growth is maintained under optimum bioenvironmental conditions. The majority of biological treatment systems operate in the mesophilic temperature range, where the optimal temperature ranges from 20°C to 40°C. Aeration tanks and percolating filters operate at the temperature of the wastewater that ranges from 12°C to 25°C; although in percolating filters, the air temperature and the ventilation rate may have a significant effect on heat loss. The higher temperatures increase the biological activity and metabolism, which result in increasing the substrate removal rate. However, the increased metabolism at the higher temperatures may lead to problems of oxygen limitations.
