**5. Peat as a packing material**

Biofiltration using peat as the filter medium is widely used for wastewater treatment processes in small communities and has been used to remove various pollutants and lately also used to remove emerging pollutants, due to its adsorption properties, ability to retain moisture, buffering capacity, and abundance in nature. Peat is an organic material, dark brown in color, and rich in carbon. It is formed as a result of the rotting and partial carbonization of vegetation in the acidic water of swamps, marshes, and wetlands [27]. It is formed in poorly oxygenated wetlands, where the rate of accumulation of plant matter is greater than that of decomposition. It is a very complex material, with lignin and cellulose as major constituents. The polar functional groups of lignin, which include alcohols, aldehydes, ketones, acids, phenolic hydroxides, and ethers, are involved in the formation of chemical bonds during the adsorption processes. As it has a high adsorption capacity for polar organic molecules and is a highly porous material (approx. 95% and a specific area of 200 m2/g), it is usually washed and sieved before being used in wastewater treatment [36].

Four stages in the adsorption process using porous peat are identified: (i) transport of impurities from the bulk of solution to the exterior surface of the peat; (ii) movement of pollutant across the interface and adsorption onto external surface sites; (iii) migration of pollutant molecules within the pores of the peat; and (iv) interaction of pollutant molecules with the available sites on the interior surfaces, bounding the pore and capillary spaces of the peat [36].

### **6. Design criteria for biofilter scale-up**

The textile industry requires a large amount of water, between 100 and 200 L per kg of textile products. The wastewater obtained from the various processes is highly polluted because it contains dyes, surfactants, inorganic salts, and chemical compounds used in the production process [37]. To scale up the processes implemented in the laboratory, the hydraulic retention time (HRT) and the flow rate generated in the production systems must be considered. The following Eq. (1) establishes the volume required for the biofiltration system.

$$\mathbf{V} = \mathbf{Q} \, \*\mathbf{t} \tag{1}$$

Where: V: Usable volume of support medium (m3 ), Q: Flow rate (m3 /s), and t: HRT (s).

The total effective volume of the biofilter will be affected by the porosity of the specific packing medium selected; with this information, the flow rate of the wastewater generated, the hydraulic conductivity of the packing medium, the hydraulic gradient have to be determined and applying Darcy's Law, the surface area of the treatment system calculated. This information is necessary before the scale up of these laboratory systems to a full scale.

### **7. Experimental procedure**

#### **7.1 Materials and methods**

#### *7.1.1 Packaging materials, inoculum, and biofilter*

The reactor was built of acrylic, with the dimensions shown in **Figure 1**. These proportions between the biofilter measurements will need to be considered when scaling is required (geometric similarity). The packaging materials were selected: peat and perlite (**Table 1**). These materials are characterized by having high porosity, adsorption capacity, and availability, which implies that they are low-cost, have ideal characteristics for suitable packaging material. Peat is an organic material, dark brown in color, and rich in carbon. It is formed as a result of the rotting and partial carbonization of vegetation in the acidic water of swamps, marshes, and wetlands [9]. Perlite is a mineral of volcanic origin, whose chemical components are silica and oxides of aluminum, iron, calcium, magnesium, and sodium [11].

The packing materials were washed with plenty of water, to eliminate the color in the case of peat or powders in the case of the rest of the materials. Subsequently, they were dried by exposure to the sun. For the case of all inorganic packing materials, an


#### **Table 1.** *Physical characteristics of packing materials.*


#### **Table 2.**

*Characterization of the inoculum.*


#### **Table 3.**

*Synthetic wastewater composition.*

average particle size of 8 mm in diameter was selected. As a filter medium, peat and perlite were used alone and, a combination of both, in a 50:50 (v/v) ratio.

The biofilters were inoculated with activated sludge (**Table 2**) from the ECCACIV Wastewater Treatment Plant, located in Jiutepec, Morelos. In total, 20% of the volume of sludge and 80% of the volume of synthetic municipal wastewater were used (**Table 3**), added with a solution containing the azo dye Direct Blue 2. The biofilters were left for up to 7 days, in order for the biofilm formation to take place (**Figure 2**).

#### **7.2 Characterization of packaging materials**

#### *7.2.1 Adsorption and desorption isotherms*

For each filter medium, adsorption kinetics was performed, based on the methodology proposed by OECD [41]. Known volumes of the test solution are added to the packing material (previously equilibrated with CaCl2 0.01 M). The mixture is stirred for an appropriate time. Subsequently, the packing material is separated by centrifugation, and the aqueous phase is analyzed by spectrophotometry. The amount of substance adsorbed on the packaging material is calculated as the difference between the amount of test substance initially present in the solution and the amount remaining at the end of the experiment.

In order to investigate whether the adsorption of the dye to the packaging material is reversible or irreversible, a desorption kinetics was carried out. From the adsorption test, once the aqueous phase is separated by centrifugation, the volume of solution removed is replaced by an equal volume of CaCl2 0.01 M (without containing dye) and stirred again, for an appropriate time. The aqueous phase is recovered (as much as possible), and it is analyzed spectrophotometrically.

**Figure 2.** *Design of the biofilter used.*

### **7.3 Determination of the porosity of the filter medium**

To determine the hydraulic retention time (HRT) of the biofilters, the methodology described by Garzón-Zúñiga et al. [42] is used, which generally consists of the following steps: 1) determination of the volume of voids in the filter bed layer; 2) determination of the porosity of the filter medium and; 3) determination of HRT, based on the following Eq. (2):

$$\mathbf{HRT} = \mathbf{V}\_{\mathbf{t}} / \mathbf{Q} \tag{2}$$

Where, Q = flow rate (L/d). Vt = Porosity Volume of voids in L. Y = Volume of empty spaces (L).

The flow rate was obtained by doing emptying tests, for which it was previously necessary to fill the biofilters with water (until full coverage of the filter medium). Then, the biofilter was drained, and the volume obtained was measured at different time intervals.

#### **7.4 Evaluation of color removal and degradation of organic matter**

The removal of color was followed spectrophotometrically, in the case of the Direct Blue 2 dye, the absorbance in the effluent at 576 nm was measured. On the other hand, the removal of organic matter was determined considering the removal of COD [43].
