**1. Introduction**

The textile industry is one of the most important worldwide; however, the large number of chemical compounds used in the dyeing and washing process cause its wastewater discharges to have a high content of organic and inorganic compounds that are toxic to the environment [1].

The dyes used by the textile industries contain different structures, which are in greater abundance: the acidic, basic, disperse dyes, azo, basic, anthraquinones, and metal-complex dyes [1]. Currently, the exact number of colorants produced

worldwide is not known. Still, there are an estimated 10,000 colorants, with production greater than 7X105 tons, and an approximate 5–10% of the colorant remains in the effluents [2].

The main problem derived from the contribution of color to the waters of rivers and lakes is due to the reduction in transparency and the decrease in dissolved oxygen, due to the fact that high color loads hinder the photosynthetic function of plants [1]. Additionally, some problems associated with textile effluents are due to the presence of heavy metals or sulfur, which cause environmental problems due to their toxic nature. Some dyes of azoic nature have been found to have potential carcinogenicity, and at least 3000 commercial azo dyes have been classified as carcinogenic [3].

The production and manufacture of denim are important activities within the textile industry. However, the rise of the blue jeans maquiladora has dramatically deteriorated the environment that surrounds them. Its wastewater discharges are characterized mainly by presenting a blue color, as well as high organic loads [4].

Direct Blue 2, used mainly in denim dyeing, is an azo dye and chromophore since it involves two nitrogen molecules linked by a double bond and contains two aromatic rings in its structure (**Figure 1**). Due to its properties, it is difficult to degrade, and its discharge into the water can interfere with various biological processes that take place in bodies of water [5].

There are different physical, chemical, and biological processes that can be applied to remove colorants from wastewater; however, each process presents technical and economic limitations. Biological treatments are recognized as effective methods for the discoloration and degradation of colorants in highly polluted industrial wastewater [1, 4].

Biofiltration is a technology of easy operation, low investment, and maintenance; the influent is fed in the upper part of the biofilter and infiltrates through the filter medium; the processes that are achieved during the infiltration of the influents are slow filtration and passive, adsorption, absorption, ion exchange, and biodegradation, the latter being a destructive process through the use of microorganisms, predominantly heterotrophic bacteria, which degrade the pollutants present in industrial wastewater [6, 7]. Microorganisms are immobilized by adhering to the surface of a support medium through the formation of a film, which is in contact with wastewater continuously and intermittently [6].

There are various support materials that can be used, among the substrates that have been used for this type of technology is peat, which is partially fossilized plant material, generally dark brown, which is formed with little oxygenation and plenty of water, in places where the rate of accumulation of plant matter is greater than that of decomposition. Being a complex material, whose major constituents are lignin and cellulose, it has a surface area > 200 m<sup>2</sup> /g and a porosity of 95% [8, 9]. These properties, together with their ability to adsorb the different compounds, make peat a material that can be used as a support for the formation of biofilms. With respect

**Figure 1.** *Chemical structure of direct blue 2 dye.*

to expanded perlite, it is a hydrated amorphous volcanic glass material with a water retention capacity of 2–5%, maintaining its original structure, it has a density of 30–150 kg/m3 , it is used to modify soils reducing its firmness and facilitating water drainage and moisture retention [10]. The composition of perlite is 70–75% silicon dioxide: SiO2 12–15% aluminum oxide, Al2O3 3–4% sodium oxide, Na2O 3–5% potassium oxide, K2O 0.5–2% oxide iron, Fe2O3 0.2–0.7% magnesium oxide, MgO 0.5–1.5% calcium oxide, and CaO 3–5% [11].

The main objective of this work was to design, build, and operate a prototype of a biofiltration system to remove direct blue dye 2 present in wastewater using peat, perlite. and a mixture of peat. Perlite as packing materials.

### **2. Characteristics of textile wastewater**

The textile industry is one of the main sources of pollutants for water worldwide due to the volume and composition of its effluents, which are characterized by being typically alkaline, hot, and colored. These effluents represent a danger to living organisms, as well as to the environment since they carry various types of toxic pollutants [1].

Textile effluents are characterized by a high level of dissimilarities in many parameters such as chemical oxygen demand (COD), pH, total solids (TS), biological oxygen demand (BOD), water use, and color [4]. The industrial manufacturing process rules out unsafe and colored dyes, mostly azo dyes. These colorants cause a great environmental problem, especially to aquatic life, due to their low biodegradability, strong color, high COD, and low BOD/COD ratio [12].

Dyes are classified into synthetic and natural. Synthetic dyes are easy to produce in a wide variety of colors and are very stable molecules; that is why they are widely used compared with natural colorants [1]. Synthetic colorants can be classified according to their mode of application and chemical structure. Based on the mode of application, they can be reactive, acidic, direct, dispersed, etc. While considering their chemical structure, they are categorized as azo, anthraquinone, triaryl methanes, among others [12].

Azo dyes are the most important family among industrial dyes, due to their ease to synthesize and their structural versatility. They are characterized by having an azo functional group (-N = N-) attached to aromatic rings. These colorants provide a practically complete range of shades and high color intensity. In addition, they are very stable to light, heat, water, and other solvents [13]. Azo dyes can be classified by the number of azo bonds they contain (monoazo, diazo, triazo, etc.) or based on the form of application in the fibers (acid, basic, direct, dispersed, mordant, reactive, and sulfurized) [14].

The typical characteristics of textile wastewater are difficult to define, because textile application methods, even the same process, are different for each industry. The concentration of colorants in textile wastewater varies in a wide range from 10 to 250 mg/L [12, 15–17] and in some cases, up to 800 mg/L [18].

Textile industries consume more than 100,000 tons/year dyes, and about 100 tons/year of dye enters the effluent water [19]. There is no exact information on the amount of dye released from various processes to the environment, but the release of the actual amounts of artificial colors into the environment has been identified as an environmental challenge.
