**8.1 Biofilter flow rates and hydraulic retention times**

**Table 4** shows the flow rates and HRT of each biofilter, calculated using the methodology described by Garzón-Zuñiga et al., [42]. It can be seen that the biofilter with peat shows a higher HRT (1.51 d); this is because the peat can expand and has low porosity, which reduces the empty spaces. In the biofilter with the peat-perlite mixture, the HRT (1.18 d) is lower than that of the peat (HRT = 1.51 d) because the perlite increases the number of empty spaces. For the perlite biofilter, the HRT (1.0 d) is lower than the peat biofilter and the peat/perlite biofilter, because perlite is an inorganic mineral material, and it does not absorb water since it retains it on its surface.


**Table 4.**

*Hydraulic retention time (HRT) calculated from biofilters.*

### **8.2 Organic matter removal assessment**

For 110 days, the biofilters were fed with a synthetic effluent with a color concentration of 50 mg / L and consequently a constant COD. For the biofilter packed with perlite, in **Figure 3**, we observe that, at 15 days, a COD removal of 14% was achieved, and at 30 days decreased to 13%. At 50 days, the removal rate was 61%, and at 110 days, 71% removal was achieved. No studies were found in which perlite is used as packaging for biofiltration of wastewater; however, there are works carried out with inorganic packaging, Villanueva et al., [44] carried out a study with a biofilter packed with gravel, obtaining removals of 27% of the COD at 21 days.

#### **Figure 3.**

*Organic matter removal was measured as COD in the biofilter with the peat, perlite, and peat-perlite mixture.*

For the peat-packed biofilter, **Figure 3** shows that, from day 15, COD removals of 27% were obtained, reaching at 30 days, removals of 33%. Obtaining removals up to 78% after 110 days, compared with the performance of the perlite-packed biofilter (71% COD removal), higher COD removal was achieved with the peat biofilter. In 2011, Velasco [45] reported a study of biofiltration with peat, obtaining an average organic matter removal efficiency of 75.5%, and another study by Mejia [45] reports organic matter removal efficiencies of 53.4%.

For the biofilter packed with peat-perlite, **Figure 3** shows that on day 15, organic matter removal was less than 30%. From this day on, the percentage of removal increased until reaching 70% removal on day 80, achieving a removal rate of 91% at 110 days. Comparing the results with the perlite (71%) and peat (78%) biofilters, the highest removals (86%) were obtained with this biofilter, which may be due to the use of two materials with a very different composition (organic and inorganic); however, the use of perlite helped the biofilter perform better in terms of removal. No works reported in the literature were found, with biofilters packed with peatperlite for wastewater. In 2011, Velasco [45] carried out a study using nanoparticles of TiO2 and MgO, in a biofilter packed with peat, reaching 97% in removal of organic matter.

#### **8.3 Color removal assessment**

For assessment of the dye concentration (mg/L), the UV–vis spectrophotometry method was used. To evaluate the dye concentration (mg/L), A UV–vis spectrophotometric method was used. First, the calibration curve for direct Blue 2 was performed with solutions of the dye from 10 to 100 mg/L concentration at a wavelength of 576 nm.

For the perlite-packed biofilter, the dye removal efficiencies are shown in **Figure 4**. On day 15, a 46% color removal was achieved, gradually increasing the removal rate. On day 25, the dye removal was 50%, reaching an 82% removal rate at 110 days. In comparison to that reported by Melgoza and De la Cruz [46], inorganic filter media such as tezontle are highly efficient (≈93%) for color removal in real textile effluent with azo dyes.

In the case of the peat-packed biofilter, **Figure 4** shows that on day 15, the removal of the dye achieved 51%, gradually increasing the removal rate. On day 25, the color eliminated was 67%, reaching an 88% removal rate at 110 days. In general, the performance of the color removal results shows a wide variation, which is probably due to color interferences from the peat or the biofilm formed on the peat. Mejía [47] reports a 50% removal of Terasil SRL black color in biofilters packed with peat and inoculated with Pleurotus ostreatus.

In the case of the biofilter packed with the peat-perlite mixture, **Figure 4** shows a variation in the data obtained on day 15, 61% the removal, and the day 15, the removal increases gradually (>70%), the rate removal was 79% on day 55. In 110 days, the removal reached 92%. Comparing the results with the biofilters with perlite (82%) and peat (88%), with the peat/perlite biofilter, removals of 92% were obtained. It may be due to the constituents of the two materials with different compositions (organic and inorganic) and textures. Therefore, the mixture of the two materials increases the percentage removal efficiency of the dye.

#### **8.4 Sorption process assessment**

In the adsorption kinetics, perlite has a sorption capacity of 16.2% in the first 2 hours of contact with the dye, while in the case of peat, the adsorption capacity was 87.5% during the first hour (**Figure 5**).

Adsorption and desorption were described by the linearized form of the Freundlich Eq. (3)

$$\log \mathbf{C}\_{\mathbf{s}} = \log \mathbf{K}\_{\mathbf{f}} + \mathbf{1} / \ln \log \mathbf{C}\_{\mathbf{e}} \tag{3}$$

where Kf is the adsorption coefficient characterizing the adsorption–desorption capacity, and n is the Freundlich equation exponent related to adsorption intensity that is used as an indicator of the adsorption isotherm nonlinearity.

**Figure 5.** *Kinetics of sorption of direct blue 2 in packing materials.*

Kf-ads is the adsorption coefficient, and Kf des is the desorption coefficient of the Freundlich equation.

The hysteresis coefficient, H, for the adsorption and desorption isotherms was calculated according to Eq. (4):

$$\mathbf{H} = \left(\mathbf{1} / \mathbf{n}\_{\text{des}}\right) / \left(\mathbf{1} / \mathbf{n}\_{\text{ads}}\right) \tag{4}$$

where, 1/nads and 1/ndes are the Freundlich constants obtained for the adsorption and desorption isotherms, respectively.

The organic matter (OM) normalized adsorption constant (KOM) was calculated by normalizing Kf-ads to the fraction of OM Eq. (5)

$$\mathbf{K}\_{\text{OM}} = \mathbf{K}\_{\text{f}-\text{ads}} \;/\text{OM} \ge \mathbf{100} \tag{5}$$

The sorption isotherms for the two packing materials are shown in the **Figure 6**. **Table 5** shows the parameters determined with the adsorption and desorption isotherms.

Based on the parameters determined by Freundlich, peat has a higher adsorption capacity than perlite. This is confirmed by considering the amount of organic matter contained in the materials. Since it has been shown that contaminants are adsorbed on the organic fraction of the substrates. Hysteresis in peat shows that the packing material has a dye-holding capacity since the ratio of the desorption intensity to the adsorption intensity gives a value below 1, indicating that the adsorption rate is higher than the desorption rate, which favors the retention of contaminants in the

**Figure 6.**

*Isotherms sorption for direct blue 2 dye in perlite and peat (a) Adsorption. (b) Desorption.* 


#### **Table 5.**

*Parameters of Freundlich isotherms for peat and perlite.*

#### *Peat as a Potential Biomass to Remove Azo Dyes in Packed Biofilters DOI: http://dx.doi.org/10.5772/intechopen.102691*

material. Whereas with an H value such as that of perlite close to 1, it indicates that the adsorption rate is similar to the desorption rate, so the hysteresis process does not occur [48]. Based on the characteristics of the materials used in the biofilters and adsorption data, the perlite serves as a porous and inert material, which provides the packing medium with aeration capacity and support that prevents clogging due to the peat compaction, but does not favor retention. Therefore, the pollutants present in it undergoes adsorption and desorption processes at the same rate, increasing the availability of the pollutants in the perlite-packed area. While peat provides the biofilters with the necessary nutrients for biofilm formation; moreover, adsorption support allows them to retain contaminants, favoring the contact between microorganisms and contaminants, when the pollutants present in the pore water are removed, the pollutants retained in the peat are released, favoring its availability and degradation. Therefore, the biofilter with the highest removal capacity is the peat-perlite mixture.
