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REVISTA CIENTÍFICA
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APPLICATION OF THE DISCRETE LEAST SQUARES METHOD TO MEASUREMENTS OF
CHEMICAL OXYGEN DEMAND IN TANNERY EFFLUENTS
APLICACIÓN DEL MÉTODO DE MÍNIMOS CUADRADOS DISCRETOS A MEDICIONES DE
DEMANDA DE OXÍGENO QUÍMICO EN EFLUENTES DE CURTEMBRES
Tipo de Publicación: Articulo Científico
Recibido: 30/03/2024
Aceptado: 09/05/2024
Publicado: 15/05/2024
Código Único AV: e305
Páginas: 1 (265-287)
DOI: https://doi.org/10.5281/zenodo.12190801
Autores:
Cristian Luis Inca Balseca
Ingeniero Automotriz
Magister en Métodos Matemáticos y Simulación
Numérica en Ingeniería
https://orcid.org/0000-0002-4795-8297
E-mail: cristianl.inca@espoch.edu.ec
Afiliación: Escuela Superior Politécnica de Chimborazo
País: Ecuador
María Gabriela Barrera Rea
Ingeniera en Sistemas Computacionales
Master en Ingeniería Matemática y Computación
https://orcid.org/0000-0001-9840-4668
E-mail: mbarrerar1@unemi.edu.ec
Afiliación: Universidad Estatal de Milagro
País: Ecuador
Franklin Marcelo Coronel Maji
Doctor en Matemática
Magister en Ciencias de la Educación. Aprendizaje de la
Matemática
https://orcid.org/0000-0002-0352-4382
E-mail: marcelo.coronel@espoch.edu.ec
Afiliación: Escuela Superior Politécnica de Chimborazo
País: Ecuador
Jorge Leonardo Magallanes Tomalá
Ingeniero en Petróleo
Master en Química Aplicada
https://orcid.org/0009-0005-8651-2086
E-mail: jorg_magallans@hotmail.com
Afiliación: Investigador Independiente
País: Ecuador
Abstract
The tanning process is the process of treatment and transformation of
animal skin into leather, where said process resides especially in the
addition to the skins of a series of tanning products such as chrome salts,
among others. In this sense, it can be said that, in the present investigation,
the physicochemical and microbiological behavior of the reactor was
studied during the leather process in the industries, which has been
increasing this activity in the behavior of the wastewater produced by the
producers. leather, taking into account that they need more efficient
systems for the treatment of effluents generated in this sector. This is how,
for the development of this research, a reactor was managed per load with
a useful volume of 2 liters that worked in aerobic circumstances with 24-
hour cycles. Likewise, for the development of this investigation, it was
possible to progressively increase the concentration of the effluent from
the tannery, taking into account the DQO and the count of heterotrophic
microorganisms, which were the main variables studied during the
investigation. Finally, biodegradable DQO removals achieved at the end
of the acclimatization process were 57.9% for DQOt and 76.8% for DQOs,
also presenting little significant amount of heterotrophic bacteria in the
effluent.
Keywords:
Reactor, acclimatization, tannery effluent, DQO
Resumen
El proceso de curtido es el proceso de tratamiento y transformación de la
piel animal en cuero, donde dicho proceso reside especialmente en la
adición a las pieles de una serie de productos curtientes como las sales de
cromo, entre otros. En este sentido, se puede decir que, en la presente
investigación, se estudió el comportamiento fisicoquímico y
microbiológico del reactor durante el proceso del cuero en las industrias,
lo que ha ido incrementando esta actividad en el comportamiento de las
aguas residuales producidas por los productores cuero, teniendo en cuenta
que necesitan sistemas más eficientes para el tratamiento de los efluentes
generados en este sector. Es así como para el desarrollo de esta
investigación se manejó un reactor por carga con un volumen útil de 2 litros
que trabajó en circunstancias aeróbicas con ciclos de 24 horas. Asimismo,
para el desarrollo de esta investigación se logró aumentar progresivamente
la concentración del efluente de la curtiduría, teniendo en cuenta el DQO
y el recuento de microorganismos heterótrofos, que fueron las principales
variables estudiadas durante la investigación. Finalmente, las remociones
de DQO biodegradables logradas al final del proceso de aclimatación
fueron del 57,9% para DQOt y del 76,8% para DQOs, presentando además
cantidades poco significativas de bacterias heterótrofas en el efluente.
Palabras Clave:
Reactor, aclimatación, efluente de curtiembre, DQO
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Introduction
The production process carried out in
tanneries involves transforming animal hides into
leather. This process is divided into four stages:
cleaning, tanning, retanning, and finishing. The
purpose of the tanning process is to seek the highest
quality material that is not in a state of
decomposition. These industries are characterized
by consuming large volumes of water, leading to the
generation of substantial and highly contaminated
effluents that are challenging to treat. This is
primarily due to the use of various chemical
reagents such as surfactants, organometallic dyes,
natural tanning agents, and chromium salts during
leather production (Bhardwaj et al., 2023).
In these industries, water is used as a raw
material and is considered a vital element in leather
production. The water used traverses the production
process and becomes laden with contaminants.
Consequently, the quantity of wastewater
originating from various industries, along with daily
and hourly fluctuations, contributes to significant
pollution (Kumar et al., 2023).
As a result, wastewater generated in the
tanning industry is primarily characterized by a high
organic load, elevated conductivity, and the
presence of heavy metals, such as chromium.
Studies conducted by various researchers
demonstrate that the effluents generated during the
production processes in tanneries generally exhibit
high concentrations of Chemical Oxygen Demand
(COD), ranging from 2,000 to 60,000 mg/L.
Between 79% and 83% of this COD corresponds to
the biodegradable fraction of the total COD, while
the remainder is non-biodegradable. Additionally,
these effluents contain high concentrations of Total
Kjeldahl Nitrogen (TKN), NH4+, suspended solids
(SS), and chromium (Oliveira et al., 2021).
Theoretical framework
Fundamentals of Leather Tanning Processes
The leather industry has been an integral part
of human civilization for centuries, providing a
versatile material with numerous applications. One
of the crucial processes involved in transforming
raw hides and skins into leather is tanning. Tanning
is a complex chemical process that stabilizes the
collagen structure of the hides, rendering them
resistant to putrefaction and imparting desired
properties such as flexibility, durability, and
appearance.
The tanning process can be broadly
categorized into two main types: chrome tanning
and vegetable tanning. Chrome tanning, which
involves the use of chromium salts, is the
predominant method employed in modern leather
production due to its efficiency and versatility
(Hasan et al., 2021). Vegetable tanning, on the other
hand, utilizes natural tannins derived from various
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plant sources, such as bark, leaves, and fruits, and is
known for producing high-quality leathers with
unique characteristics
The fundamental principles of leather tanning
lie in the chemistry of collagen, the primary
structural protein found in hides and skins. Collagen
is a fibrous protein with a triple-helix structure, and
its stabilization is essential for the production of
leather. During the tanning process, tanning agents
interact with the collagen fibers, forming cross-links
that prevent the collagen from undergoing
putrefaction and impart the desired properties to the
leather (Rajan et al., 2023).
In chrome tanning, the chromium salts,
typically basic chromium sulfate, react with the
carboxyl groups of the collagen molecules, forming
coordinative cross-links This cross-linking process
results in improved thermal stability, resistance to
biodegradation, and increased hydrothermal
stability of the leather However, the environmental
impact of chromium tanning has been a subject of
concern, leading to the development of alternative
tanning methods and strategies for chromium
recovery and recycling (Xu et al., 2017).
Vegetable tanning, on the other hand, employs
polyphenolic compounds known as tannins, which
are extracted from various plant sources. These
tannins form hydrogen bonds and hydrophobic
interactions with the collagen fibers, resulting in a
dense, rigid structure (Etuk and Ojekudo, 2015).
Vegetable-tanned leathers are known for their
unique characteristics, such as increased resistance
to heat, water, and abrasion, as well as their
distinctive color and aroma (Doulah, 2018).
Recent advancements in leather tanning have
focused on the development of eco-friendly and
sustainable practices. One such approach is the use
of alternative tanning agents, such as aluminum,
titanium, zirconium, and silica-based compounds.
(Milenkovic and Bojovic, 2014). These tanning
agents offer potential advantages in terms of
reduced environmental impact and improved leather
properties.
Furthermore, researchers have explored the
incorporation of natural and renewable materials,
such as plant extracts and biopolymers, into the
tanning process. These materials can serve as
tanning agents or auxiliaries, contributing to the
development of more sustainable and
environmentally friendly leather production
methods (Devi et al., 2023).
Overall, the fundamentals of leather tanning
processes lie in the understanding of collagen
chemistry and the interactions between tanning
agents and collagen fibers. As the industry continues
to evolve, the pursuit of eco-friendly and sustainable
practices, coupled with advancements in tanning
technologies, holds promise for the future of leather
production.
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Overview of Leather Production Stages
The leather industry is a complex and
multifaceted sector that involves various stages to
transform raw hides and skins into finished leather
products. Each stage plays a crucial role in
determining the quality, properties, and
characteristics of the final product. This theoretical
framework aims to provide an overview of the
primary stages involved in leather production,
highlighting their significance and the latest
advancements in the field.
The production of leather begins with the
procurement of raw hides and skins, which are
byproducts of the meat industry or obtained from
animals specifically raised for their hides (Tian
2020). These raw materials undergo several
preparatory stages, including trimming, curing, and
soaking, to remove unwanted materials and prepare
the hides for subsequent processes.
The next stage, known as liming, involves the
immersion of the hides in a lime-based solution,
which facilitates the removal of hair, epidermis, and
other unwanted components (Zhang et al., 2022).
This process also initiates the swelling and opening
up of the collagen structure, preparing the hides for
further processing.
Following liming, the deliming and bating
stages are carried out to remove residual lime and
facilitate the breakdown of non-fibrous proteins,
respectively (Moeeni et al., 2017). These steps
ensure proper preparation for the subsequent
tanning process, which is the pivotal stage in leather
production.
Tanning is the process of stabilizing the
collagen structure of the hides, rendering them
resistant to putrefaction and imparting desired
properties (Fashae et al., 2019). Various tanning
methods exist, with chrome tanning and vegetable
tanning being the most prevalent. Chrome tanning
involves the use of chromium salts, while vegetable
tanning employs natural tannins derived from plant
sources (Kramar and Alchakov, 2023).
After tanning, the leather undergoes a series
of post-tanning operations, including slamming,
setting, and dyeing, to improve its appearance,
physical properties, and color (Kanagaraj et al.,
2020) These processes are essential for achieving
the desired aesthetic and functional characteristics
of the leather.
Chemical and Environmental Implications of
Tanning
The tanning process, which is an essential
component of leather production, has significant
chemical and environmental implications that
require careful consideration. Despite the
importance of the leather industry, the use of
hazardous chemicals and the generation of
pollutants during tanning operations pose potential
risks to human health and the environment. This
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theoretical framework aims to explore the chemical
aspects of tanning and their associated
environmental impacts, as well as the strategies and
approaches being developed to mitigate these
challenges.
One of the primary concerns in the tanning
industry is the use of chromium salts, particularly
basic chromium sulfate, in the chrome tanning
process. Chromium (III) compounds are widely
employed due to their effectiveness in stabilizing
collagen fibers and imparting desirable properties to
the leather (Nur et al., 2020). However, the potential
conversion of chromium (III) to the more toxic and
carcinogenic chromium (VI) form under certain
conditions has raised concerns regarding its
environmental impact and occupational exposure
(Oruko et al., 2021).
The disposal of tanning effluents containing
chromium and other pollutants poses a significant
challenge. These effluents often contain high levels
of biochemical oxygen demand (BOD), chemical
oxygen demand (COD), total dissolved solids
(TDS), and suspended solids, which can have
detrimental effects on aquatic ecosystems if not
properly treated. Furthermore, the presence of toxic
substances, such as sulfides, amines, and phenolic
compounds, further exacerbates the environmental
impact (Tisha et al., 2020).
To address these issues, various strategies
have been explored to minimize the environmental
footprint of tanning processes. One approach is the
recovery and recycling of chromium from tanning
effluents, which not only reduces the amount of
chromium discharged into the environment but also
facilitates the reuse of this valuable resource (Hira
et al., 2022). Techniques such as membrane
filtration, ion exchange, and chemical precipitation
have been employed for chromium recovery, with
ongoing research aimed at improving their
efficiency and cost-effectiveness (Niamat et al.,
2023).
Characterization of Organic and Inorganic
Contaminants
The presence of organic and inorganic
contaminants in various environmental matrices,
such as water, soil, and air, poses significant
challenges to human health and ecological systems.
Accurate characterization of these contaminants is
crucial for understanding their sources, behavior,
and potential impacts, as well as developing
effective mitigation strategies. This theoretical
framework aims to explore the analytical techniques
and methodologies employed in the characterization
of organic and inorganic contaminants, highlighting
the latest advancements and challenges in this field.
Organic contaminants encompass a diverse
range of compounds, including persistent organic
pollutants (POPs), polycyclic aromatic
hydrocarbons (PAHs), pesticides, pharmaceuticals,
and personal care products (PPCPs), among others
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(Yadav et al., 2020). These compounds can
originate from various sources, such as industrial
activities, agricultural practices, and domestic
activities, and can have adverse effects on human
health and the environment due to their persistence,
bioaccumulation potential, and toxicity.
The characterization of organic contaminants
often relies on advanced analytical techniques, such
as gas chromatography-mass spectrometry (GC-
MS) and liquid chromatography-mass spectrometry
(LC-MS). These hyphenated techniques provide
high sensitivity, selectivity, and structural
information, enabling the identification and
quantification of trace levels of organic compounds
in complex matrices. Additionally, sample
preparation methods, such as solid-phase extraction
(SPE) and QuEChERS (Quick, Easy, Cheap,
Effective, Rugged, and Safe), play a crucial role in
concentrating and isolating the target analytes from
the sample matrix, improving analytical
performance (Han et al., 2016).
Inorganic contaminants, on the other hand,
encompass a wide range of elements and
compounds, including heavy metals, metalloids,
and radionuclides. These contaminants can
originate from natural sources, such as geological
formations and volcanic activities, as well as
anthropogenic sources, including industrial
processes, mining activities, and the use of
fertilizers and pesticides. Exposure to inorganic
contaminants can have detrimental effects on
human health and the environment, as many of these
substances are toxic, persistent, and can
bioaccumulate in the food chain (Zhang et al.,
2021).
The characterization of inorganic
contaminants typically involves the use of advanced
instrumental techniques, such as inductively
coupled plasma-mass spectrometry (ICP-MS),
atomic absorption spectroscopy (AAS), and X-ray
fluorescence (XRF). These techniques offer high
sensitivity, multi-element analysis capabilities, and
the ability to determine speciation and isotopic
composition of inorganic contaminants. Sample
preparation methods, such as acid digestion,
extraction, and preconcentration, are often required
to ensure accurate and reliable analysis of inorganic
contaminants in various matrices (Al-Jabari et al.,
2021).
Principles of Aerobic and Anaerobic Digestion
Waste management and resource recovery are
critical challenges in modern societies, and
biological processes such as aerobic and anaerobic
digestion play a crucial role in addressing these
issues. These processes involve the breakdown of
organic matter by microbial communities, leading to
the production of valuable products and the
stabilization of waste materials. This theoretical
framework aims to explore the fundamental
principles governing aerobic and anaerobic
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digestion, highlighting their applications, benefits,
and recent advancements in the field.
Aerobic digestion, also known as composting,
is a process in which organic matter is decomposed
by aerobic microorganisms in the presence of
oxygen. This process involves a complex
consortium of microorganisms, including bacteria,
fungi, and actinomycetes, which work together to
break down the organic matter through a series of
metabolic pathways. The main stages of aerobic
digestion include the mesophilic phase, where
readily biodegradable compounds are consumed,
and the thermophilic phase, where more recalcitrant
materials are degraded at higher temperatures
(Sharma and Vuppu, 2023).
The aerobic digestion process is influenced by
various factors, such as the composition of the
organic matter, moisture content, aeration rate, pH,
temperature, and the presence of inhibitory
compounds. Proper management of these factors is
crucial for optimizing the process and ensuring
efficient decomposition of the organic matter. The
end products of aerobic digestion include a
stabilized organic material called compost, which
can be used as a soil amendment, and carbon
dioxide, which is released into the atmosphere
(Auad et al., 2020).
In contrast, anaerobic digestion is a process
that occurs in the absence of oxygen, where organic
matter is broken down by a consortium of anaerobic
microorganisms, primarily bacteria and archaea.
This process involves a series of complex
biochemical reactions, including hydrolysis,
acidogenesis, acetogenesis, and methanogenesis,
resulting in the production of biogas, a mixture of
methane and carbon dioxide, as the primary end
product (China et al., 2020).
Anaerobic digestion is widely used for the
treatment of various organic waste streams, such as
municipal solid waste, agricultural residues,
industrial wastewater, and sewage sludge. The
process offers several advantages, including the
production of renewable energy in the form of
biogas, reduction of waste volume and greenhouse
gas emissions, and the generation of a nutrient-rich
digestate that can be used as a fertilizer or soil
amendment (Fraga et al., 2020).
Theoretical Basis of COD Measurement
The measurement of Chemical Oxygen
Demand (COD) is a widely used analytical
technique in various fields, including environmental
monitoring, wastewater treatment, and industrial
process control. COD serves as an important
parameter for assessing the organic matter content
and the degree of pollution in water and wastewater
samples. This theoretical framework aims to explore
the fundamental principles and methodologies
underlying COD measurement, highlighting recent
advancements and challenges in this area.
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The COD value represents the amount of
oxygen required to chemically oxidize the organic
matter present in a sample under specific conditions.
Traditionally, the COD measurement involves the
use of a strong oxidizing agent, such as potassium
dichromate, in an acidic environment, to oxidize the
organic compounds in the sample (Naima et al.,
2015). The reduction of the oxidizing agent is then
measured, and the COD value is calculated based on
the amount of oxidant consumed during the
reaction.
The theoretical basis of the COD
measurement lies in the principles of redox
chemistry and the stoichiometry of the oxidation
reactions. The choice of the oxidizing agent and the
reaction conditions are crucial for ensuring accurate
and reproducible results. Potassium dichromate is
widely used as the oxidizing agent due to its strong
oxidizing capability and its ability to oxidize a wide
range of organic compounds, including those that
are resistant to biological oxidation (Riguetto et al.,
2020).
However, the traditional dichromate-based
COD method has several limitations, including the
use of hazardous chemicals, the generation of toxic
waste, and the inability to oxidize certain organic
compounds completely (Covington and Wise.
2020). To address these challenges, alternative
methods for COD measurement have been
developed, such as the manganese (III) oxidation
method and the photocatalytic oxidation method
(Khambhaty, 2020).
The manganese (III) oxidation method is
based on the use of manganese (III) as the oxidizing
agent, which is less hazardous than dichromate and
can effectively oxidize a wide range of organic
compounds (Sahu et al., 2022). This method has
gained popularity due to its environmental
friendliness and the potential for automation.
Methodology
This study employed a multifaceted approach
to evaluate the efficiency of a discrete least squares
method in assessing the treatment of tannery
effluents, with a specific focus on the reduction of
Chemical Oxygen Demand (COD). The
methodology is structured into several key
components as outlined below.
Reactor Setup and Operation
A cylindrical batch reactor constructed from
transparent acrylic (polymethyl methacrylate) with
a 3-liter capacity and a working volume of 2 liters
was utilized. The reactor, measuring 50 cm in height
and 10 cm in diameter, was equipped with three
ports: an upper port for wastewater introduction, a
middle port for effluent discharge, and a lower port
for cleaning purposes.
The system was automated, utilizing digital
timers to control the activation of electronic
components within the treatment system.
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Wastewater loading was facilitated through a ¼"
solenoid valve, enabling gravity-fed transfer from a
feed container to the reactor. Effluent discharge was
managed using a peristaltic pump, and aeration was
provided by a fine bubble diffuser connected to a 3
PSI compressor, ensuring a minimum oxygen
concentration of 2 mg/L within the reactor.
Acclimatization Process
The reactor was initially fed with synthetic
water, mimicking the composition of tannery
effluents, and granular biomass obtained from a
laboratory-scale biological reactor. This phase
aimed to acclimatize the microbial community to
the specific contaminants present in tannery
wastewater. Operational cycles of 1440 minutes (24
hours) were established, consisting of filling, oxic
reaction, sedimentation, and discharge stages.
Analytical Procedures
The study's core analytical component
focused on the measurement of COD to evaluate the
organic matter concentration in the effluent before
and after treatment. The discrete least squares
method was applied to enhance the accuracy of
COD measurements, allowing for the precise
quantification of biodegradable and non-
biodegradable fractions. Samples were taken at two-
hour intervals, with additional measurements of
temperature and pH conducted every 15 minutes to
monitor the reactor's environmental conditions.
Wastewater Characterization
Tannery wastewater characterization involved
assessing physical, chemical, and biological
parameters, including pH, alkalinity, total and
soluble COD, Total Kjeldahl Nitrogen (TKN),
NH4+, and heavy metals content. This
comprehensive analysis provided a baseline for
evaluating the treatment process's efficiency.
Enhanced Acclimatization
Following the initial acclimatization with
synthetic water, the reactor was gradually
introduced to real tannery effluent, starting with a
mixture of 80% synthetic water and 20% tannery
effluent. This step aimed to adapt the microbial
community to the more complex and variable
composition of actual wastewater, thereby
improving the treatment process's robustness and
efficiency.
Data Analysis
Data collected throughout the study were
analyzed using the discrete least squares method to
determine the efficiency of COD removal and the
overall performance of the treatment process.
Statistical analysis was conducted to evaluate the
significance of observed changes in COD levels,
microbial activity, and other relevant parameters.
Results and discussion
Characteristics of the Biological Reactor
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In this study, a batch reactor with a cylindrical
shape and a 3-liter capacity was used. The reactor
was constructed from transparent acrylic
(polymethyl methacrylate) with an effective volume
of 2 liters. It had dimensions of 50 centimeters in
height and 10 centimeters in diameter. The reactor
featured three ports, which facilitated its operation.
The upper port was located 34 cm from the bottom
of the reactor and was used for the introduction of
wastewater, while there were two lower ports. One
of these was positioned 8 cm from the bottom and
was used for discharging the treated effluent, and
the other, located at the very bottom of the reactor,
was used for cleaning.
The reactor operated in an automated fashion
using digital timers to activate and deactivate the
electronic components in the treatment system. The
influent was introduced into the reactor by
activating a ¼" solenoid valve, allowing gravity-fed
loading from the feed container to the batch reactor.
The discharge of treated wastewater was achieved
using a peristaltic pump, enabling the effluent to exit
the system and enter a receiving container. The
aeration was provided through a fine bubble diffuser
installed at the bottom of the reactor, connected to a
3 PSI compressor with a power rating of 2.5
watts/hour and a flow rate of 2,500 cc/min (Elite 801
model). During the aerobic phase, this system
maintained a minimum oxygen concentration of 2
mg/L in the reactor (Figure 1).
Figure 1. Sp Schematic of the batch reactor used in the
research.
Characteristics of Granular Biomass
The reactor was fed with 2 liters of synthetic
water (Table N. º 1) and 25 g/L of granular biomass.
The biomass was obtained from another biological
reactor used at the laboratory scale, which processed
synthetic effluent with similar characteristics to that
of the tannery (Zhu et al., 2020).
Table 1. Composition of Synthetic Water
Reagent
Quantity per 1 L
NH4Cl
0,25 g
K2HPO4
0,045 g
CaCO3
0,030 g
MgSO4.7H2O
0,025 g
FeSO4.7H2O
0.020 g
NaCH3COO
4,5 g
In this context, to characterize the granular
biomass at the beginning of the acclimatization
process, biomass density was determined using the
Archimedes' principle. Additionally, the average
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size of the granules was measured by randomly
selecting 100 granules and determining their
diameter with a graduated instrument. This process
was conducted in triplicate to assess the physical
characteristics of the biomass.
A natural selection process of the existing
microorganisms was allowed through the system's
operational cycles. This facilitated the growth and
establishment of floc-forming microorganisms
while eliminating filamentous bacteria that could
cause delays in sedimentation times (Xu et al.,
2022). The reactor operated with cycles of 1440
minutes (24 hours), each cycle divided into stages
with specific durations as follows: filling for 15
minutes, oxic reaction for 1417 minutes,
sedimentation for 2 minutes, and discharge for 6
minutes (Xu et al., 2022).
Chemical Oxygen Demand (COD) Profile vs Time
Chemical Oxygen Demand (COD) represents
the amount of oxygen required to oxidize organic
components in a specific type of water under
specific conditions of oxidant, temperature, and
time. In this context, COD is a measurement that
quantifies the amount of readily oxidizable
dissolved or suspended substances by chemical
methods in a liquid sample. It is used to gauge the
level of contamination emitted in milligrams of
diatomic oxygen per liter (mgO2/L).
The COD (Chemical Oxygen Demand)
method is often used to measure contaminants in
natural and wastewater and to assess the strength of
waste, such as municipal and industrial wastewater.
The traditional method used to obtain the COD
value is known as the Standard Method, in which
potassium dichromate serves as the oxidizing agent.
Essentially, this method involves subjecting the
samples to heat treatment for about two hours in a
Hach digester, following the addition of a known
excess of the oxidant. The primary issue with this
method lies in the low efficiency of the reaction
mixture's heating method, resulting in excessively
long reaction times (Zhang et al., 2016).
Although this method primarily aims to
measure the concentration of organic matter, it is
susceptible to interference from the presence of
inorganic substances that can be oxidized (sulfides,
sulfites, iodides), which also affect the
measurements. This method is applicable to
freshwater (rivers, lakes, or aquifers), wastewater,
stormwater, or water from any other source that may
contain a significant amount of organic matter
(Ahmed et al., 2021).
In this context, it was necessary to conduct
pH, temperature, COD total (total chemical oxygen
demand), and COD soluble (soluble chemical
oxygen demand) profiles over the duration of the
cycle to initiate the enhanced acclimatization
process, considering the use of synthetic water.
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These profiles were developed in three stages, or in
other words, three repetitions, with the average of
measurements obtained during the first stage of the
research. Consequently, the process began by
working with synthetic water in the reactor and 24-
hour cycle durations. Subsequently, samples were
taken for the COD study, with a sampling frequency
of 2 hours, as well as every 15 minutes for
temperature and pH measurements. After
completing a 24-hour cycle in the reactor and
conducting all the tests and analyses, the data is
show in the Figure 2
Figure 2. Chemical Oxygen Demand (COD) Profile vs Time
Based on the results shown in Figure 1 of the
Chemical Oxygen Demand (COD) Profile vs Time,
it can be observed that both COD values decreased
gradually as the cycle progressed. This provides
evidence that the microorganisms in this biomass
analysis were absorbing the organic content of the
synthetic water (Figure 3).
Relationship between the activity aM of a
measurement ion in a solution and the potential
measured between the reference electrode and the
measurement electrode. Temperature influences the
Nernst potential, often referred to as the slope in pH
measurement. [30] In this regard, the pH value is
probably the most commonly measured parameter
in analytical chemistry.
Figure 3. Temperature and pH Profile in the Reactor Using
Synthetic Water During the Initial Hours of the Cycle
It affects product characteristics, chemical and
biochemical reactions, and physiological processes,
among other things. Constant environmental
conditions are often required to obtain precise
measurement results (Pradeep et al., 2021).
Therefore, the results from Figure 3 confirm
that the existing biomass was processing organic
matter to grow, reproduce, and carry out metabolic
processes. The biomass indicated that it was active
at the beginning of the acclimatization process,
under optimal conditions, initiating the feeding of
the reactor with the synthetic water.
Tannery Wastewater
Wastewater consists of the effluents produced
by human activities in their daily routines, which are
collected in sewage systems or discharged directly
into the environment (Hansen et al., 2021).
Wastewater is characterized by its physical,
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chemical, and biological composition. Quantitative
analysis methods are used for the precise
determination of wastewater's chemical
composition, while qualitative analyses provide
insights into its physical and biological
characteristics (Zhao et al., 2022).
Regarding physical characteristics, Odor is an
important parameter for characterization, as it is a
result of gases released during the decomposition of
organic matter. Wastewater has a distinctive odor
due to the presence of hydrogen sulfide, a product
of the reduction of sulfates to sulfites by
microorganisms (Fan et al., 2020).
Temperature is another distinguishing feature
since wastewater typically has higher temperatures
than uncontaminated water, primarily due to
increased biochemical activity by microorganisms
(Ji et al., 2021). Density is commonly defined as
mass per unit volume, expressed in Kg/m3 and
g/cm3. Alternatively, the specific weight of
wastewater is obtained based on the known
coefficient between the density of water and the
density of wastewater (Min et al., 2021).
Turbidity measures the quality of water
discharged by assessing the relationship between
colloid and residual material in suspension (Mateo
et al., 2021). Solid content is represented by visible
and colloidal particles present in wastewater,
including organic matter like carbohydrates,
cellulose, fiber particles, chitin, and other elements
(Eray et al., 2020). Total Solids (TS) are residues
left after the sample has been evaporated and dried
at around 105°C for a period of twenty-four hours
under dry heat (Jasim, 2020). Color in wastewater is
due to the presence of suspended solids, with a
greenish color indicating the presence of colloidal
and dissolved substances (Samsami et al., 2020).
Particle Size Distribution:
The size of wastewater particles varies in
magnitude, including substances dissolved (< 0.08
µm), colloidal particles (0.08 to 1.0 µm),
supracolloidal particles (1 to 100 µm), and settleable
particles > 100 µm (Jiang et al., 2020). pH
represents the acidity or alkalinity of water,
depending on the proportion of hydrogen ions, with
pH values ranging from 0 to 14, where pH = 7 is
neutral. This parameter is significant because it
indicates the level of acidification in wastewater.
Regarding chemical characteristics, Inorganic
substances include
Nitrates, originating from the decomposition
of plant and animal materials or nitrogen
compounds, transformed into organic matter by
microorganisms in the presence of oxygen (Hu et
al., 2020). Sulfates are soluble and result from the
bacterial oxidation of sulfides. Their concentration
typically ranges from 20 to 50 mg/l in rivers (Xia et
al., 2020).
Chromium occurs naturally but becomes a
contaminating metal in wastewater. It forms
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aminated and cyanurated complexes in water, with
stability complexes with other chemical compounds
such as chlorides, sulfates, ammonium salts,
cyanides, and nitrates. Chromium is highly toxic to
organisms (Kerur et al., 2021).
Iron is present in wastewater due to steel and
other material production, typically trivalent in
surface waters. These chemicals cause serious
health issues, such as dermatitis (Aragaw et al.,
2021). Chlorides result from mineral deposits
dissolution, originating from various industrial or
domestic sources. They can also indicate unwanted
microbiological contamination (Li et al., 2022).
Calcium, a metal, is present in wastewater as
it forms soluble salts with bicarbonate, sulfate,
fluoride, and chloride ions, associated with
mineralization levels (Liu et al., 2023). Zinc is rare
in surface and groundwater, existing in inorganic,
ionic, and colloidal forms. In significant quantities,
zinc causes water turbidity, indicating
contamination from batteries and engine oils
resulting from landfill leakage (Turkmen et al.,
2021)
In light of the previously mentioned
characteristics of wastewater, the tannery
wastewater handled in the acclimatization process
was represented by collecting a sample from the
lagoon, where a significant amount of effluents from
the organization's production process is stored. In
this context, acclimatization, in a physiological
sense, involves an organism adapting to changes in
its environment. The duration of this period varies
according to the species and the circumstances of
the change (Tiwari et al., 2021).
It's worth noting that acclimatization can
apply to any environmental change, with one of the
most studied being acclimatization to temperature
changes. Animals take approximately 5 to 10 days
to adjust their physiology to new conditions
following a sudden temperature change (Ji et al.,
2021). The results of wastewater characterization
are shown in Table 2, with the maximum discharge
limits to water bodies established by Ecuadorian
legal regulations (MAATE, 2015) also reported.
Table 2. Characterization of the tannery's raw wastewater
Parameters
Reagent
Quantity per 1
L
pH
9,28-+0,28
6 a 9
Alkalinity
20.850 +-
597,22
---
DQOt
5.584,74 +-
680,36
350
DQOs
3900,7-942,85
---
NT
264,40+-39,10
40
N-NH4+
80,83+-13,22
---
N-NH3-+N-NO2-
4+-0
10
Within this context, it can be said that the
comparison between the characterization and the
limits set by legal regulations revealed that the
industrial effluent requires treatment, given that the
parameters were far from the maximum allowable
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values for discharge into bodies of water, especially
pH, COD, and NT. Furthermore, to apply biological
treatment, it is necessary to have a group of
microorganisms capable of degrading organic
matter and being tolerant to other typical
contaminants in industrial effluent, such as
chromium, sulfides, chlorides, refractory material,
among others (Kapoor et al., 2021).
Among the characteristics of tannery effluent,
it is important to note that the pH is at the upper limit
set for a biological process to take place. Similarly,
the effluent is characterized by high levels of
organic matter and nitrogen. Thus, the study of the
acclimatization process was justified, allowing for
the treatment of microorganisms prepared to work
under these conditions (Chan et al., 2022).
Enhanced Acclimatization Process
After observing the activity of
microorganisms through COD profiles using
synthetic water, the enhanced acclimatization
process continued by feeding the reactor with a
mixture of synthetic water and tannery effluent. In
this regard, the laboratory-scale reactor was
operated for approximately two months, which is
the duration of the biomass acclimatization,
ensuring a smooth process without any issues
(Figure 4).
Figure 4. Behavior of CODtotal (Total Chemical Oxygen
Demand) during the enhanced biomass acclimatization
process
Conclusions
The leather tanning industry has traditionally
been considered a polluting industry with a
significant environmental impact. Often overlooked
is the fact that it involves a process that utilizes a
highly putrescible byproduct with slow
biodegradation. The tanning process can be carried
out in many ways, depending on the specific
requirements for the final use of the leather, the
animal source, and the specific characteristics
imparted to the leather to enhance its properties and
commercial value. Tanning is typically done in
batch processes, with a high-water consumption,
leading to the generation of polluting gases,
contaminated wastewater, and solid waste, with
wastewater being the most polluting component.
The success of wastewater treatment depends
on the management of the organization, which can
be regulated by authorities. This marks the
beginning of a process where wastewater with
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residues must be processed and redistributed for
disposal and utilization. Within this context,
wastewater characterization uses quality
parameters, considering physical, chemical, and
biological aspects. These parameters allow for the
quantification of the degree of contamination
present in a wastewater sample. When the
concentration of a particular parameter in
wastewater is high, whether physical, chemical, or
biological, it determines the appropriate treatment
according to its intended use.
In summary, the protocol of enhanced
acclimatization by feeding the reactor with a
mixture of synthetic water and tannery effluent
proved to be efficient, with a time frame of 60 days.
During this period, a granular biomass was
obtained, featuring a group of microorganisms
suitable for their metabolic processes and
sedimentation in less than two minutes, even with
inhibitory compounds and refractory organic matter
that characterize such effluents. Furthermore,
biodegradable COD removal at the end of the
acclimatization process reached 57.9% for
CODtotal and 76.8% for CODs. There was also a
limited presence of heterotrophic bacteria in the
effluent. Environmental Impact Assessment
indicated that the research conducted would not
have specific negative impacts, thereby ensuring
environmental viability and a positive impact on
environmental quality.
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