**2.3 Biological processes**

In biological processes, microorganisms such as bacteria, fungi, yeasts, even enzymes are used to remove POPs from water. In these processes, microorganisms use the contaminant as a substrate, generating the production of enzymes, transforming the contaminants into smaller molecules that are generally less toxic [39, 40].

The most widely used biological process is activated sludge, so the search for new biological treatments continues and some interesting ones have emerged, such as immobilized cells using a membrane bioreactor. Microalgae are also a developing biological technology capable of removing pesticides from water. Another interesting alternative is fungal biosorption coupled to a membrane bioreactor [39, 41].

The use of yeasts such as *Candida tropicalis* is a new treatment process and is another alternative for the elimination of persistent organic pollutants in water, since these microorganisms have a high degradation potential, a high tolerance to the toxicity of the pollutant, a nearly constant reaction rate, they degrade the contaminant in approximately 66 hours and they can serve as single-cell protein as food. In these processes, the concentration of the contaminant is an important factor, because with a large amount the yeast can die [42–44].

Enzyme treatments are another of the emerging technologies, in which a biocatalyst (an enzyme) is used to transform pollutants. This technique can be applied as a primary treatment or in combination with a biological unit. Different enzymes are used, but the most common ones to degrade POPs are laccases and peroxidases [2, 15]. Although the use of enzymes is an effective technology with proven bioremediation potential, its application is currently a long way off, as it has not yet been incorporated into large-scale water treatment systems. The main reason for the lack of applications of this type of bioremediation is the cost of obtaining pure enzymes, their stability, the number of enzymes required, as well as its lack of reuse [2].

**Table 4** shows a comparison of biological processes, where the wide diversity of actions in these technologies for the elimination of POPs can be observed since to obtain the desired results using them it is necessary to have the correct temperature, pH and ionic strength. Depending on the compound and the treatment conditions, the removal efficiency values vary in a range from 40 to 100%, using a pH of 3–7, with temperatures between 25–37°C and with a removal time of 3–200 hours.

In these technologies, the application of aerobic or anaerobic conditions is related to the acceptance of terminal electrons. Some research indicates that easily biodegradable POPs can be eliminated by this technology, while those with low biodegradability may not go away completely. Then it is recommended to couple them sequentially with other tertiary treatment processes, carry out a pretreatment in the case of some very toxic compounds or find a strain that can endure those [46]. Some proposed methodologies are bioelectrochemical systems (BES), the combination of phase change, biological and electrochemical processes, as well as the use of electrochemical membrane bioreactors (EMBR) [47].

Biological treatment processes have many advantages compared to phase change technologies and advanced oxidation processes, as they are safer, less disruptive, less expensive, require less energy use, are considered green catalysis processes, generate biomass and can be used with contaminants that have very low concentrations, which cannot be achieved by physicochemical techniques. One disadvantage of this method is the high variation in each treatment that depends on a load of organic matter, the concentration of toxic compounds, changes in pH and


**Table 4.**

*Biological treatment for POPs removal.*

temperature. However, a major disadvantage of biological treatments is the time required, as shown in **Table 4**, so it is possible that microorganisms cannot survive and grow in hard and adverse environmental conditions [48, 49].
