**2.2 Biological treatments**

Although the COD/BOD5 ratio reveals that physicochemical treatments are the most suitable for inorganic industrial sewage, the literature has shown that

**Figure 1.** *Main types of adsorption isotherms according to [32].*

*Biological versus Physicochemical Technologies for Industrial Sewage Treatment… DOI: http://dx.doi.org/10.5772/intechopen.100325*

biological treatments are also efficient and promising for the treatment of these effluents. Biological treatment involves the use of non-specific microorganisms (Activated sludge), well-selected microorganisms (Bio-augmentation), algae, or plants (Phytoremediation). These microorganisms can be bacteria, yeasts, fungi, etc.

Biological treatment consists of the use of pollutants of industrial sewage as a source of nutrition and energy by these microorganisms for their growth. This type of treatment has several advantages comparing to physicochemical treatments. Among these qualities, there are:


Furthermore, the presence or absence of oxygen divides these biological treatments into two categories, which are aerobic and anaerobic treatments.

### *2.2.1 Aerobic treatments*

### *2.2.1.1 Classical process of activated sludge*

The activated sludge process is the most classic and most famous process since the 20th century. It is based on the treatment with aerobic micro-organisms. These micro-organisms are generally autotrophic or heterotrophic, and they are composed of several groups like bacteria (Gram-positive and/or Gram-negative), fungi, yeasts, protozoa, and metazoa. Consequently, activated sludge is composed of micro-organisms plus inert, organic or mineral matters. These elements are grouped through mucilaginous substances. In fact, those micro-organisms degrade the organic matter of wastewaters into carbon dioxide, water, new cells, and other non-toxic by-products. The separation between the treated effluent and the activated sludge takes place in a clarifier or decanter.

Even this process is very effective in the degradation of organic sewage like agri-food sewage; it also has some disadvantages, particularly the bulking phenomenon, the presence of high amount of recalcitrant pollutants, as well as nitrogen and phosphorus are not reduced at the same time and in the same reactor. This requires the addition of another tank to remove them, and subsequently the increase of installation and operation costs.

### *2.2.1.2 Sequencing batch reactor*

The Sequencing Batch Reactor (SBR) is an emerging approach that has been well developed recently. This technique has the same treatment principle as the previous process, so it is based on the treatment by activated sludge under aerobic conditions. However, this process differs from the other one, because all treatment steps are performed in a single reactor, including nitrogen and phosphorus removal. In addition, the separation liquid/solid is done in the same reactor, where the good separation is linked to the presence of an adequate quantity of filamentous bacteria.

The treatment through this system is by cycle that involves four successive phases, which are:


The performance of this system depends on many parameters such as the volumetric organic load, daily cycle number of the treatment, level of dissolved oxygen, sludge index, sludge age, and time of the settling phase. The optimization of these parameters leads to very high abatement rates.

The reference [33] showed that SBR is a promising technique for the treatment of several types of sewage such as agri-food, pharmaceutical, pulp/paper, textile, tannery, chemical, and petrochemical effluents, … etc.

Another study shows the efficiency of the SBR system compared to the conventional activated sludge process (**Table 2**). So, the SBR is the most efficient due to very high removal rates, and also the system is capable to remove nitrogen and phosphorus at the same time. Thus, this system is inexpensive and useful than the classic process. Furthermore, aerobic denitrification was highlighted within this reactor [10, 35]. In recent decades, this phenomenon has been very attractive and advanced according to various studies. The literature showed more than 37 aerobic denitrifying bacteria, where *Bacillus pulminus*, *Arthrobacter sp*., and *Streptomyces lusitanus* were the latest shown [36].

Heavy metals could also be removed by this biological system. The reference [11] has shown that this system is capable to remove chromium from a tannery effluent with a removal rate of 96.1% using a low volumetric organic load. As well, [37] has indicated that several heavy metals (Nickel, chromium, cadmium, cobalt, zinc, and silver) were eliminated from brassware effluent through this system with high removal rates that reached more than 60%.

This system has many advantages such as low cost, short treatment time compared to the classic process, high removal of organic and mineral matter, simplicity of the process, the possibility of spreading excess sludge, limitation of bad odors,


### **Table 2.**

*Average abatement rates obtained by the sequential batch reactor and the conventional activated sludge process [34].*

*Biological versus Physicochemical Technologies for Industrial Sewage Treatment… DOI: http://dx.doi.org/10.5772/intechopen.100325*

etc. … Despite all these advantages, it also has some shortcomings like the bulking phenomenon.
