Design and Development of a Pilot-Scale Industrial Wastewater Treatment System with Plant Biomass and EDTA

Abstract

The impact generated by the indiscriminate disposal of heavy metals into the different bodies of water is not only environmental but also social due to the health effects it produces in several organisms, including ourselves. Therefore, treatment systems around the world are the subject of continuous research to find treatment systems that are economical, efficient, and easy to implement in the industries that generate these increasingly harmful impacts on society and the environment in general. One way to design and develop systems of water treatment is that which takes advantage of the waste generated, such as the waste from the E. crassipes plant. The conditions of this plant make it perfect due to its abundant biomass and important content of cellulose and hemicellulose. Nevertheless, in almost all the investigations that characterize the way in which the biomass of this plant adsorbs heavy metals, it does so under laboratory conditions, being very far from the reality of industrial discharges. The objective of this project is to design and develop a pilot-scale industrial wastewater treatment system with plant biomass and EDTA. Three pilot-scale systems were built with EDTA-modified biomass in different concentrations, giving the parameters of the design for the development of a system that can treat around 80 L of Chromium (VI) contaminated water. This treatment system with E. crassipes biomass and EDTA with proportions of 9:1 costs around USD 10, which is quite cheap compared to conventional ones.

1. Introduction

Global environmental policies are becoming more concise and strict in enforcing heavy metal dumping laws. The Sustainable Development Goals encompass different objectives in order to conserve water resources already deteriorated due to the lack of clear policies on the treatment of this valuable resource. Goal number six establishes “basic water and sanitation”. To enforce this goal, worldwide research has conducted experiments on cheap, easy-to-install, and effective treatment systems to remove harmful contaminants, such as different heavy metals [1,2,3,4].

Chromium is a heavy metal used in different industries due to its effectiveness in metal alloys and the preservation of leather in the tannery industry; it has led to the exaggerated use of this heavy metal, causing serious environmental, social, and health consequences. An example of this is in the south of Bogotá, Colombia, where the use of Chromium (VI) in more than 350 tannery industries has led to the discharge of Cr (VI), with concentrations of around 1000 mg/L of this heavy metal, causing the deterioration of the Tunjuelo River, leading to its decrease in its ecosystem services to the community surrounding the river [5,6,7]. Among the methods available to treat industrial effluents, adsorption is the most widely used method for wastewater treatment thanks to its low capital cost, versatility, which allows you to remove most contaminants, and ease of regeneration of the adsorbent [8,9,10,11,12,13,14]. Recently, the chemisorption of heavy metals, especially chromium, in contaminated water bodies with different types of vegetables has drawn the attention of researchers due to its high removal capacities, abundant availability in nature, and low cost, which has been demonstrated in experimental work in the field of water treatment [15,16,17,18,19]. The presence in the cellulose of hydroxyl (OH), amino (NH₂), and carboxylic (COOH) groups increases the removal of heavy metals by adsorption via cation exchange or chemistry mechanisms [20,21,22,23,24].

In experimental processes at the laboratory scale, the dry and crushed biomass of E. crassipes has been contacted with water contaminated with different heavy metals, with removals that do not exceed 75% [25,26]. Trials through pilot-scale processes provide design support for the construction of treatment systems adapted to discharge regulations. The development of treatment systems for adsorption columns (continuous system), with a fixed bed made up of biomass that adsorbs heavy metals, is a way to have disruptive projects because this type of treatment system provides a practical application in wastewater treatment [27]. The Fick diffusion model has been used successfully to predict rupture curves, calibrate the inlet load, and comply with the discharge standards, being the main parameter to model and define the designs of treatment systems, allowing to establish the operation time in which the column is saturated under certain operating conditions such as feed concentration, feed flow, and adsorbent mass [28,29,30,31,32]. One way to improve the removal of heavy metals on the biomass of E. crassipes is through their chemical or physical modification [28,29,30] to increase their affinity with the polluting agent molecules. For example, EDTA, this agent chemistry that enhances the pollutant adsorption process, is used as a chelating agent in industrial processes due to its great capacity to attract heavy metals through cation exchange [33,34,35].

The objective of this investigation was to design and develop a pilot-scale industrial wastewater treatment system with plant biomass and EDTA, where a mathematical equation was developed based on Fick’s laws to establish the design criteria of this system; this was supported with three different experimental systems at a pilot scale, where it developed a system of treatment to scale industry for treating around 80 L of water contaminated with Cr (VI).

2. Materials and Methods

This investigation was developed in two experimental phases. In the first phase, three pilot-scale treatment systems were used to treat one liter of water contaminated with Cr (VI) and, based on the data obtained using Fick’s laws, the design and assembly of the second phase, which consisted of an industrial-scale treatment system for the treatment of 80 L of water, was carried out.

Chromium measurement. Chromium (VI) laboratory measurement: 200 and 400 mg/L of Cr (VI). In sampling, aliquots of the reaction mixture were analyzed for residual chromium concentration using a UV84. Samples were taken in the flask at each interval, analyzing the residual chromium concentration. Adsorption experiments were carried out in a 100 mL glass vessel with constant stirring (IKA Ks 4000 shaker) at 20 °C, 250 rpm. All experiments were carried out in triplicate, with an average of the final values. Twenty µm samples were taken and then placed in the centrifuge (KASAI MIKRO 200, Hettich, Föhrenstr, Germany) [27].

Chromium determination. Using the diphenylcarbazide method, the amount of chro-mium (VI) residue is estimated. For this purpose, the phosphate buffer solution was prepared by adjusting it to a pH equal to 2 with 90% of the grade of purity (H₃PO₄). In an eppendorf tube, 200 µL of 0.5% diphenylcarbazide (with 97% grade purity) and W/V acetone (with 97% grade of purity), 900 µL of phosphate buffer and 100 µL of the residual sample were added. A suitable portion is transferred to an absorption cell and the absorbance is measured at 540 nm.

Spectrophotometer. The Evolution 300 spectrophotometer was used to monitor changes in absorbance. All procedures for the determination of chromium, for water and substrates, were carried out using the implementation of APHA (Procedure of the American Public Health Association) for standard tests (standard methods for the examination of water and wastewater). Method 3500-Cr: The uncertainty of the measurements estimated in this study showed that the measurements of the heavy elements, specifically Cr(VI), can be made with a level of uncertainty around 3.95%.

The detection limits of the method obtained for a primary wastewater effluent water of 0.3 µg/L: Initial Performance Demonstration: Prior to sample analysis, the instrument was set up and analyzed enough known samples to determine the method detection limit and calibration range estimates, after every 10 Cr(VI) samples, and then the final sample, an independent control sample, and a calibration blank were analyzed.

Phase 1. Pilot scale experimentation: Use of E. crassipes in the wetland located in the municipality of Mosquera, in the outskirts of Bogotá: E. crassipes (EC) plants discarded by the authorities that clean this type of wetlands were collected.

Each plant generates about 50 g of biomass. Subsequently, EC was washed with water, not distinguishing the stem, leaves, and roots. It was then dried at 70 °C for 48 h to remove moisture and ground to a 0.212 mm diameter. The pulverized biomass was sieved through meshes to obtain different particle sizes.

Creation of pilot-scale treatment systems using PET plastic containers. The treatment systems were built using recycled PET containers (400 mL). Two compartments were built to hold the dry and crushed biomass of E. crassipes, each containing 25 g. Each compartment was connected to the next through a serial, with openings in the lids at the bottom of each capsule to allow the flowy of treated water to the next capsule. Figure 1 shows the system of treatment.

The flow was guaranteed by dripping over the upper capsule, preserving the system flow rate of 15 mL/min; also, the pH was kept constant in the feed flow rate. The bed density was constant, and, at temperature and pressure conditions of 20 °C and 1 bar, respectively, the system had manual flow control.

Three similar treatment systems were built, changing only the way they were distributed in the treatment system. The mixture consisted of going through the biomass in the treatment system under the concentration of 5 g/L and 10 g/L of this reagent in 300 mL of distilled water. This procedure was used in [27] in the process of elutions.

These are the samples:

• Only E. crassipes 50 g of biomass
• E. crassipes with EDTA 5 g mixed with the 45 g biomass EC₁
• E. crassipes with EDTA 10 g mixed with the 40 g of biomass EC₂

Initial concentrations of 200 and 400 mg/L of chromium were evaluated (VI). All tests were performed in triplicate, calculating the average between the data obtained and, with this, the percentage of metal removal; also, it has the standars desviations and % of variations.

Model Evaluation. Mathematical modeling used to describe the behavior of the advance curves, which helps to understand and extend the system. In studies with the biomass of crassipes, the model of Thomas is the more representative [36], and with this model, it can validate the model of Carreño.

Thomas model. The Thomas model is used to estimate the maximum adsorption capacity and predict the breakthrough curves, assuming second-order reversible reaction kinetics and Langmuir isotherm [35]. The representative equation is Equation (1).

$$\ln \frac{\mathrm{Co}}{\mathrm{Cf}}-1=\frac{\mathrm{Kth} \mathrm{q} \mathrm{m}}{\mathrm{Q}}-\mathrm{Kth} \mathrm{Co} \mathrm{Tb}$$

(1)

where:

• Co: initial concentration of Cr (VI) (mg/L)
• Cf: final concentration (VI) (mg/L)
• V: volume (mL)
• Kth: Thomas constant (mL/mg∗min)
• q: adsorption capacity (mg/g)
• M: mass of biomass in column (g)
• Q: design flow rate (mL/min)
• Tb: breakthrough time (min)

Model Carreño. This is used to determine the capacity of adsorptions and other parameters of design of a system in continuity with these characteristics [27].

$$\mathrm{q} =\frac{\mathrm{QTbCo}}{\mathrm{M}}-\frac{\mathrm{QTbCf}}{\mathrm{M}}-\frac{\mathsf{\epsilon }\mathrm{VCo}}{\mathrm{M}}$$

(2)

where:

• Co: initial concentration of Cr (VI) (mg/L)
• Cf: final concentration (VI) (mg/L)
• V: volume (mL)
• q: adsorption capacity (mg/g)
• M: mass of biomass in column (g)
• Q: design flow rate (mL/min)
• Tb: breakthrough time (min)

Design of adsorption models. The dynamics of the chemical adsorption process by biomass could be reflected through Fick’s law.

$$J=\frac{{\mathrm{AsK}}_{\mathrm{f}}}{\mathrm{V}}\left(\mathrm{Ci}-\mathrm{Cf}\right)$$

(3)

where: J is the flow of contaminated water, given in terms of the concentration of the pollutant in the water, per unit area and time; $\left(\mathrm{Ci}-\mathrm{Cf}\right)$ is the concentration variant of Chromium (VI) in liquid. K_f is the diffusion coefficient (m²/s); (V) is the volume of water to be treated, as is the external area of the biomass volume occupied in the treatment; and L is longitude and is calculated through V/As.

$$\frac{\partial c}{\partial t}=-\frac{{\mathrm{AsK}}_{\mathrm{f}}}{\mathrm{V}}\left(\mathrm{Ci}-\mathrm{Cf}\right);$$

(4)

(K_f) is the rate coefficient of chemisorption of heavy metal particles to the biomass used in the process.

$$\frac{\partial \mathrm{c}}{\left(\mathrm{Ci}-\mathrm{Cf}\right) }=-\frac{{\mathrm{AsK}}_{\mathrm{f}}}{\mathrm{V}}\mathrm{dt}$$

(5)

Integrating Equation (5), it is:

$$\mathrm{Ln}\frac{\mathrm{C}}{\mathrm{Co}}=\frac{{\mathrm{AsK}}_{\mathrm{f}}}{\mathrm{V}}\mathrm{t}$$

(6)

Plotting this term, with the natural logarithmic of initial and final Cr (VI) concentrations, with the longitude of treated water in the experimental biotreatment process, you will find the diffusion constant K_f of Cr (VI) in the biomass [36,37].

Equation (7) establishes the treatment area As.

$$\mathrm{As}=\frac{3\mathrm{Vb}\left(1-\mathsf{\epsilon }\right)}{\mathrm{rp}}$$

(7)

where:

• $\mathrm{As}$ = Area m²
• Vb = Volume of biomass mg/L
• $\mathsf{\epsilon }$ Relations of densities
• $\mathrm{rp}=$ Radio of process mL

3. Results

Results of process. The following three figures show the results obtained and reflect the arithmetic average between the three results; the data were very similar; therefore, the standard deviation was low along with the margins of error of the coefficients of variation. In the first mechanism, Cr (VI) is reduced to Cr (III), due to the interaction of this metal with OH groups, which act as electron donors. The biomass is oxidized because Cr (VI) reacts with its hydrogen ions, generating OH and H₂O, going from dichromate (Cr₂O₇)-² or Cr (VI) to chromium oxide Cr₂O₃ or Cr (III) [25].

When Cr (III), which at that time is chromium oxide Cr₂O₃, is reduced and begins a process of chemical diffusivity with the biomass, the electron-charged biomass reacts with chromium, adsorbing it while oxygen reacts with protons from (H⁺) in the biomass [25,27].

Figure 2 shows the different removal processes in the EC biomass. For the initial Cr (VI) concentration of 200 mg/L, there were removals of 85% of Cr (VI), the break point being around the treated volume of 900 mL.

The removal with an initial concentration of 200 mg/L was 90%, with the breakpoint being around 1400 mL treated. The concentrations in the effluent in the first six samples reached an average of 22 mg/L of final Cr (VI), this for the initial concentrations of 200 mg/L and in the initial concentrations of 400 mg/L, the final concentrations in the first four samples. They were around the final 60 mg/L Cr (VI). The removals of the E. Crassipes biomass are characteristic due to their removals, but chemical modifications must be made to improve their adsorption capacities [36,37]. Figure 3 shows the Cr (VI) removals for a biomass transformed with EC₁.

From Figure 2, it is possible to identify that there were interesting removals of Cr (VI) in the initial concentrations of 200 and 400 mg/L. For the treatment of 200 mg/L, it was above 99% of this metal removed, treating about 2 L of water. This biomass collapsed at this point of the treatment volume. The initial concentrations of 400 obtained similar behaviors, but the difference was that it had around 96% of Cr (VI) removed and had its breaking point when treating 1800 mL of water. The concentrations in the effluent in the first nine samples reached an average of 0.1 mg/L of final chromium, this for the initial concentrations of 200 mg/L and in the initial concentrations of 400 gm/L, the final concentrations in the first eight samples were around the final 6 mg/L Cr (VI). Figure 4 shows the Cr (VI) removals for a biomass transformed with EC₂:

Like the previous biomass, there were interesting removals of Cr (VI), but the biomass loaded with double EDTA removed 200 mL more Cr (VI) in the two types of concentrations due to the high loading of this reagent, though it was not significant. The concentrations in the effluent in the first ten samples reached an average of 0.1 mg/L of final chromium, this for the initial concentrations of 200 mg/L and in the initial concentrations of 400 mg/L, the final concentrations in the first nine samples were around the final 1 mg/L of Cr(VI).

EDTA improves adsorption capacities due to its high chelating capacity, having twice the concentration of this reagent, EC₂ compared to EC₁ has more active sites where Cr (VI) managed to adhere chemically to this biomass [33,34,35]; also, EDTA, being doubled in the EC₂ sample, only slightly increased its adsorption capacity due to limitations in the formation of active sites in the E. crassipes biomass, affecting the adsorption capacity [27].

Mathematical model. Through Equation (6), the fundamental parameters such as K_f and volume are obtained, and the area is obtained through Equation (7).

Where: the occupied volume is 80 mL of biomass for the three biomasses; ε is 0.7; the radius of the particles in the treatment system is 3.6 cm; the area of each treatment system is 30 cm².

The total process volume for that concentration was 2200 mL, giving K_f = 1.2 cm/min. In the initial experiments of 200 mg/L, Cr (VI) showed 1.1 cm/min K_f in the graph. In

Table 1. Diffusion constant (Kf) (cm/min).

Experiments	Diffusion Constant (Kf) (cm/min) EC	Diffusion Constant (Kf) (cm/min) EC1	Diffusion Constant (Kf) (cm/min) EC2
200 (mg/L)	0.518	1.1	1.2
400 (mg/L)	0.466	1.2	1.3
Average	0.51	1.1	1.2

, we find the diffusion constant K_f of all the constants.

The K_f of E crassipes has a rate in the diffusion process of 0.50 cm/min to absorb Cr (VI) chemically as a constant. In experiments carried out [28], diffusion constants of 0.19 cm/min in biomass to retain phenols, similar to the present process, were found. This rate constantly increased through mixing with EDTA to 1.1 cm/min and 1.2 due to the chemisorption power provided by EDTA.

The equation of Carreño (2) could be used to adjust to the needs of wastewater treatment systems contaminated with heavy metals; for example, the treatment required in the volume to be treated could be scaled, and the increase in the contact area could be established [38,39].

In addition, with this equation, the treatment needs of the final concentration of Cr (VI) could be adjusted, as some legislations establish that 1 mg/L of final chromium is necessary in their effluents.

The EC₁ biomass was taken as an example, where the breaking point was around 1600 mL, according to Figure 2, and together with the flow rate of 15 mL/min, with a breaking time of 94 min. With Equation (5), the density of the biomass occupied in the treatment system of 0.66 g/mL was established because 45 g of this biomass occupied a volume of 68 mL.

This biomass obtained a capacity of 16 mg/g.

$${\mathrm{qEC}}_1=\frac{94\times 15\times 0.6}{45}-\frac{94\times 15\times 0.06}{45}-\frac{0.66\times 68\times 0.6}{45}=$$

• q: Adsorption capacity
• Co: 0.4 mg/mL
• CF: 0.06 mg/mL
• M: 45 g
• Tb: break time 94 min
• Q: 15 Flow mL/min
• ε: 0.7
• V: Volume occupied 68 mL
• qEC₁: 16 mg/g

Table 2. Summary with the parameters.

Experiments with 400 mg/L	EC	EC1	EC2
Volume treat (mL)	900	1600	1800
Break time (Min)	65	120	130
Adsorption capacity (mg/g)	7	16	18

shows each of the parameters obtained through the removal efficiencies, Figure 1, Figure 2 and Figure 3 and Equation (2).

The adsorption capacity of the EC biomass was 7 mg/g, similar to the adsorption result in [27], with the biomass having a low capacity, but taking into account that this biomass does not have any contribution of chemicals.

The EC₁ composite obtained continuous adsorption capacities of 16 mg/g, a 120% better capacity with the presence of EDTA, and together with a treated volume of 1600 mL, treating 700 mL more water compared to EC alone and with a breakpoint of 120 min, The EC₂ biomass presented the highest adsorption capacity in continuous tests, with 18 mg/g. EDTA was increased, but not significantly, because this material has 5 g more of EDTA in the cellulose xanthate; the break time was 110 min and in the cellulose alkaline was 90 min [27]. In the experimient with the EC and chloride of iron, the break time was 125 min [36].

Thomas model. The graphic representation of the adjustment to this mathematical model is shown in Figure 5, where the Thomas constants of all the experimental processes of 400 mg/L initial Cr (VI) were obtained for all the biomasses evaluated.

The Thomas model is used to estimate the maximum adsorption capacity and predict the breakthrough curves, assuming second-order reversible reaction kinetics and the Langmuir isotherm [40,41]. The Langmuir isothermal model assumes that adsorption occurs at specific homogeneous sites in the E. crassipes biomass together with EDTA. The Langmuir model represents the experimental data of heavy metal adsorption in E. crassipes better than the other adsorption models [19,35]. These evidences indicate that the adsorption of heavy metals on E. crassipes adsorbents is a monolayer adsorption. Most of the kinetic adsorption data fit a second-order model; therefore, the main control mechanism in adsorption using E. crassipes as adsorbent is chemisorption; the binding between heavy metal molecules and functional surface groups of the E. crassipes biomass plays an important role during the adsorption process [19].

Table 3. Parameters of the Thomas Model of biomasses.

	qm (mg/g)	Thomas (Kt) (mL/mg∗min)	R2	Q (mg/g)
EC	7.5	0.045	0.93	7
EC1	17	0.055	0.99	16.4
EC2	19	0.066	0.97	17.8

shows each of the parameters of the Thomas model, where there were representative fits for the different biomasses evaluated.

In Table 3, the adsorption constant of the Thomas model (K_t) can be observed; it corresponds to the adsorption rate of Cr (VI) on the biomass [42,43]. This value was the lowest in EC biomass, with 0.045 (mL/mg·min), and the highest in the experiment was E. crassipes biomass, with 3 g/L with 0.065 (mL/mg·min).

In the adsorption of Cr (VI) by rice biomass, the Thomas constant is 0.1 (mL/mg·min) [27]. In the adsorption of Cr (VI) by nanocrystalline chlorapatite biomass in the Thomas constant, 0.013 (mL/mg·min) [43].

It can be concluded that the EC₂ treatment system has a higher adsorption capacity than the other biomasses, but it uses twice the EDTA reagent. Therefore, to continue with the treatment process on an industrial scale, the EC₁ biomass will be taken as a reference due to its high adsorption capacity of 17 mg/g and less use of EDTA. In the

Table 4. References with the capacities of adsorptions.

Reference	Biomass	Contaminate Treated	Capacity (mg/g)
Present article	EC1	Cr (VI)	16
Present article	EC2	Cr (VI)	18
[27]	E. Crassipes	Cr (VI)	7
[36]	EC + Fe	Cr (VI)	17
[37]	Cellulose xanthate	Cr (VI)	16
[44]	A. barbadensis Miller	Ni (II)	14
[45]	Brown algae	Al (III)	12
[46]	Green synthesized nanocrystalline chlorapatite	Cr (VI)	20
[47]	Pine cone shell	Pb (II)	22
[48]	Cassava	Cr (VI)	14

is show the references with the capacities of adsorptions.

In studies carried out on a pilot scale and in continuous processes, similar results were evidenced in the removal of heavy metals with biomass; in E. Crassipes, it obtained an adsorption capacity of 7 mg/g in [27]. The chemical modification in the E. Crassipes plant has promising results, such is the case of the modification with iron chloride, obtaining a capacity of 18 mg/g and also in obtaining cellulose xantate with this same biomass obtaining 20 mg/g [37], concluding that the biomass of E. Crassipes adapts to chemical modifications to increase adsorption capacities.

With algae, 14 mg/g capacities were obtained for Cr (VI) [44], with biochar alginate having the best performance for this heavy metal with 30 mg/g. The algae, like the E. Crassipes biomass, adapts to the modifications chemicals and obtains bio-char, as in the case of [45,46,47,48].

Costs of treatment systems. Another important aspect to evaluate the process is the costs of adsorbent materials; consequently, this section shows the estimate cost involved in obtaining it. The characterization of the costs of the treatment systems through the biomass evaluated was elaborated through unit production costs of 1 kg. The drying, crushing, and logistics to obtain E. crassipes cost around USD 3 per kg of this biomass [36].

EDTA used in the EC₁ biomass costs USD 1 because it has around 100 g of this reagent. The EC₂ composite material has a value of USD 5 because it has a total of 200 g of EDTA.

Table 5. Costs related to treatment systems.

Cost	EC	EC1	EC2
Capacity total (g Cr /kg material) Cost (USD) 1 kg material	7 3	16 4	17 5
g Cr /(USD)	2.2	4	3.4

shows the related costs together with the previously seen capacities and their relationship.

One way to reflect the viability of a project is to relate the cost-benefit trelation to obtain a monetary indicator [44]; it involves, either explicitly or implicitly, the cost versus the total possible benefits. To select the best or most profitable option, the benefit in this project is represented through the adsorption capacity of Cr (VI) by each of the evaluated biomasses. The representative cost vs. adsorption capacity for each biomass is shown in Table 4, where the adsorption capacity was taken and related to the production cost of 1 kg of biomass or composite material, yielding the capacity of adsorption per dollar spent. The EC₁ biomass, having an adsorption capacity indicator of 4 g of Cr (VI) for each dollar, is the highest due to the low costs of E crassipes and high efficiencies reported for this biomass.

The second indicator is for EC₂ biomass, with 3.4 g/USD due to the effectiveness of this biomass and the low costs of EC. Despite being the cheapest, EC biomass has an indicator of 2.2 g/USD due to its low adsorption capacity compared to other biomasses. For this reason, this treatment system was built with the biomass of EC₁.

Phase 2. Industrial scale with EC₁ biomass: Equation (6) was used for the sizing of a treatment system of 80 L of volume and with concentrations of 1000 mg/L of Cr (VI). EC₁ will be used for treatment due to its high removal in the pilot system and low concentration. The objective of this treatment system is to comply with the discharge standard (Resolution 631 of 2015) of 1 mg/L of this heavy metal.

$$\mathrm{Ln}\frac{1000}{1}=-1.1\frac{\mathrm{As}}{80000}1250$$

where,

• Final concentration = 1 mg/L
• Initial concentration = 1000 mg/L
• K_f = 1.1 cm/min
• V = water to be treated 80,000 mL
• T = treatment time 1250 min

$$\mathrm{As}=\frac{6.90\times 80000}{1.1\times 1250}=400 {\mathrm{cm}}^2$$

The surface area that the bioreactor must have to treat 80 L of water contaminated with 1000 mg/L Cr (VI) is 400 cm²

Through Equation (7), we proceeded to determine what volume this treatment system could occupy.

$$\mathrm{As}=\frac{3\mathrm{Vb}\left(1-\mathsf{\epsilon }\right)}{\mathrm{rp}}$$

$$400=\frac{3\mathrm{Vb}\left(1-0.7\right)}{3.3}$$

where,

• $\mathsf{\epsilon }$ = 0.7
• $\mathrm{rp}$ = 3.3 mL
• $\mathrm{Vb}=1466$ mL

Through this equation, the biomass volume and the diameter to be used for the treatment was adjusted; the biomass density of EC₁ is 0.67 g/L. The biomass to be used is about 1000 g to treat 80 L of water and remove 1000 mg/L to 1 mg/L of Cr (VI).

A scale pilot treatment system was built; on this occasion, 3-L PET plastic containers were used. Three compartments were built, each one as indicated by the 111 g model to guarantee around 1000 g of EC₁ biomass. Two similar treatment systems were built. The mixture consisted of going through the biomass in the treatment system under the concentration of 15 g/L of this reagent in 900 mL of distilled water, guaranteing the proportionality used in the EC₁. In the Figure 5 representations in scale Industry.

Figure 5. Representations in scale Industry.

Figure 5. Representations in scale Industry.

The flow was guaranteed by dripping over the upper capsule, preserving the system flow rate of 25 mL/min; also, the pH was kept constant in the feed flow rate. The bed density was constant, and at temperature and pressure conditions of 20 °C and 1 bar, respectively, the system had manual flow control.

Where the operating flow was distributed on an industrial scale for the treatment of 80 L, where this flow was divided into three sections doing a treatment in parallel and later in series. Treatment: The 80 L of contaminated water passed through the system, collecting samples every 9 L. Figure 6 shows treatment system removals.

In the first samples collected, concentrations were below the standard limit (1 mg/L of Cr (VI) (Resolution 631 of 2015), removing more than 99.9% of the chromium present in the water in the first samples collected. In samples 8 and 9, a Cr (VI) content was found at the established limit by the standard; for sample 10 and with 80 L treated, the limit was exceeded by 1.4 mg/L; finally, the 1000 g of biomass were eliminated as hazardous waste at a cost of US 2. The costs of this system of treatment are around USD 10, which is quite cheap compared to conventional ones.

4. Conclusions

The aim of this project is the design and development of an industrial wastewater treatment system using plant biomass and EDTA on an industrial scale. For this reason, mathematical models were used to design a suitable pilot treatment system to meet the needs of Cr (VI) discharges. To achieve the optimal design, three flow treatment systems were built in series with PET plastic containers, and, in order to calibrate and validate the models, in which the required treatment was determined, the volume to be treated, thus setting the increase in area of contact, together with the mathematical model of Thomas, it was determined that the mixture of EC₁ is the appropriate one to scale up to a larger pilot system. With these data and established parameters, a fixed-bed column adsorption system was designed to treat 80 L of water from a tannery, with an initial 1000 mg/L of Cr (VI), with the aim of removing more than 99% of the present chrome in this guide. The model provided the parameters, and thus a treatment system was developed with the characteristics of 1000 g of E crassipes biomass mixed with EDTA (with a 10:1 proportions), in a system with parallel and serial flow also with PET containers but this time with a 3-L capacity, meeting the objective of treating around 81 L of water. The costs of this system of treatment are around USD 10, which is quite cheap compared to conventional ones.