Optimization of sewage sludge: an integrated approach to sustainable
compost production
Optimización de lodos
residuales: un enfoque integral para la producción sostenible de abono orgánico
José Gerardo León Chimbolema1
Published Instituto
Superior Tecnológico Edwards Deming. Quito - Ecuador Periodicity January - March Vol. 1, Num. 24, 2025 pp. 53-66 http://centrosuragraria.com/index.php/revista Dates of receipt Received: June 12, 2024 Approved: November 04, 2024 Correspondence author Creative Commons License Creative Commons License,
Attribution-NonCommercial-ShareAlike 4.0
International.https://creativecommons.org/licenses/by-nc-sa/4.0/deed.es
Abstract: The management of sewage sludge from wastewater
treatment plants is a significant environmental and economic challenge. This
study presents an integrated approach for the optimization of sewage sludge
through the application of pre-composting, aerobic composting and lumber
farming, with the objective of transforming it into high quality organic
fertilizer. Initial analyses indicated that the sludge has a high organic
matter content (53.5 %) and heavy metal levels within regulatory limits, but a
high pathogen and moisture load that requires specific treatments. The results
show that pre-composting reduced moisture by 20 % and prepared the sludge for
subsequent stages. During composting, temperatures up to 65°C were reached,
eliminating pathogens such as Salmonella spp. and helminths, in addition to
stabilizing the carbon/nitrogen ratio at 10:1. Finally, lumbriculture
increased the levels of essential nutrients such as nitrogen (4.2 %) and
phosphorus (2.5 %), guaranteeing an organic fertilizer with superior agronomic
properties and safe for use on agricultural crops.
Key words: sewage sludge, composting, lumber, organic
fertilizer, agricultural sustainability.
1 PhD in Chemistry, Master in Environmental
Protection, Research Professor, Polytechnic School of Chimborazogerardo.leon@espoch.edu.ec https://orcid.org/0000-0001-9202-8542
Palabras clave: Lodos residuales, compostaje, lumbricultura,
abono orgánico, sostenibilidad agrícola.
Introduction
The management of sewage sludge from wastewater treatment plants (WWTPs)
is a significant environmental and economic challenge globally. These
by-products, rich in organic matter and nutrients, but also carriers of
pathogens and pollutants, require innovative management strategies that
minimize their negative impact and maximize their utilization (Amador-Díaz et
al., 2015; Diocaretz & Vidal, 2010).
The exponential increase in sludge generation due to population growth
and urbanization has saturated traditional disposal practices such as landfills
and incineration (Espinoza Eche & Santos de la
Cruz, 2021; Diocaretz & Vidal, 2010). In this
context, the use of sludge for the production of organic fertilizers is
emerging as a sustainable solution that promotes the circular economy by
transforming a problematic waste into a valuable resource for agriculture
(García & Benarroch, n.d.; Saldaña
Escorcia & Castillo Gámez,
2021). However, effective implementation of these strategies requires
overcoming technical, regulatory and economic challenges (Diocaretz
& Vidal, 2010; Ospina López et al., 2016).
The case of the "Las Viñitas"
wastewater treatment plant in Ambato, Ecuador, highlights the need for local
solutions. This plant generates approximately 950 tons of pressed sludge per
month, most of which is not adequately managed. In this context, the research
developed by Clavijo Chato (2024) explored the use of
the lumbriculture technique as an alternative to
stabilize the sludge and obtain high quality organic fertilizers. The results
indicated that this method not only significantly reduced the load of pathogens
and contaminants, but also produced a safe compost suitable for agricultural
use, complying with the international standard NOM-004-SEMARNAT-2002
(Amador-Díaz et al., 2015; Clavijo, 2024; Carrasco, 2000).
Globally,
techniques such as composting, anaerobic digestion and pre-composting have
proven to be effective in stabilizing sludge and reducing its pollutant load 5
and 14. These strategies also promote agricultural sustainability by improving
soil properties and reducing dependence on chemical fertilizers (Garcia & Benarroch, n.d.; Diocaretz &
Vidal, 2010). In addition, recent studies have highlighted the economic
viability of these alternatives, showing that the production of organic
fertilizers from sludge can be competitive with traditional management models
(Carrasco, 2000; Ospina López et al., 2016).
This
article proposes a comprehensive approach for the optimization of sewage
sludge, integrating the findings of the research in "Las Viñitas" and international experiences. Technical,
regulatory and economic aspects are addressed to establish a sustainable
compost production model that not only mitigates environmental impacts, but
also promotes local agricultural and economic development. In doing so, it
seeks to demonstrate how these strategies can be replicable and scalable,
transforming an environmental problem into an opportunity for sustainability
and circular economy (Clavijo, 2024; Carrasco, 2000; García & Benarroch, n.d.; Diocaretz &
Vidal, 2010)
Methodology
The
study was conducted at the "Las Viñitas"
Wastewater Treatment Plant (WWTP), located in Ambato, Ecuador, which processes
approximately 950 tons of pressed sludge per month. An experimental design was
carried out that included the initial characterization of the sludge, the
implementation of sustainable stabilization alternatives and the evaluation of
the quality of the final product. The selected sludge was subjected to an
exhaustive analysis to determine its physicochemical, biological and
microbiological properties prior to treatment. This analysis included
measurements of pH, carbon/nitrogen (C/N) ratio, organic matter content,
moisture, microbial load and presence of heavy metals, following the standards
of NOM-004-SEMARNAT-2002.
The
treatment and stabilization process was carried out in
three main stages. First, pre-composting was carried out to reduce the initial
microbial load by controlled fermentation. Subsequently, the pretreated sludge
was subjected to aerobic composting, stabilizing the organic matter and
improving the physicochemical properties of the product. Finally, the technique
of lumbriculture using Eisenia foetida
was used, which allowed the sludge to be transformed into an enriched organic
fertilizer with excellent agronomic properties. During these stages, key
parameters such as temperature, pH, moisture content and the evolution of the
C/N ratio were monitored, ensuring the efficiency of the processes and the
quality of the final product.
The
fertilizer obtained was subjected to a detailed evaluation to validate its
safety and applicability in agriculture. This analysis included the measurement
of essential nutrients such as nitrogen, phosphorus and potassium, as well as
the verification of regulatory compliance and preliminary tests of efficiency
as a fertilizer in crops. In addition, a cost-benefit analysis was conducted to
determine the economic viability of the techniques employed. This analysis
included the calculation of implementation and operating costs, such as inputs,
machinery and infrastructure, and the estimation of economic benefits,
including the reduction of costs associated with the final disposal of sludge
and the commercial value of the fertilizer produced.
This
comprehensive methodology focused on the development of a technically and
economically viable model to transform sewage sludge into high quality organic
fertilizer, offering a sustainable solution for the management of this waste
and its integration into agriculture.
Results
The
initial characterization of the wastewater sludge from the "Las Viñitas" Wastewater Treatment Plant (WWTP) was
fundamental to determine its potential as a raw material for the production of
organic fertilizer. The analyses carried out revealed that the sludge contained
a high percentage of organic matter, reaching 53.5%, which makes it ideal for
stabilization processes such as composting and lumbriculture.
In addition, the initial carbon/nitrogen (C/N) ratio was located at 15:1, an
acceptable range for biological treatments, although indicating the need for
adjustments during composting to optimize the decomposition of organic matter.
(Amador-Díaz et al., 2015) (Saldaña Escorcia & Castillo Gámez,
2021) The pH of the sludge remained between 7.5 and 8.2, slightly alkaline,
which favors microbial activity in the initial stages of treatment. On the
other hand, the moisture content was 72 %, which required an initial reduction
to improve the handling and efficiency of the stabilization processes, a
typical parameter in sludge from urban plants (Clavijo, 2024; Diocaretz & Vidal, 2010). From a microbiological point of view, the
sludge presented a high pathogenic load, including Salmonella spp. and viable
helminth eggs, exceeding the limits established for direct application in
agricultural soils. This finding underscores the need to apply rigorous
sanitizing treatments to ensure the safety of the final product, since the
presence of pathogens represents a significant risk to public health if they
are not adequately eliminated. The elimination of this pathogen load is an
essential requirement for sludge to be safely used in agriculture, which
reinforces the importance of sanitization steps, such as thermophilic
composting (Saldaña Escorcia
& Castillo Gámez, 2021). The analysis of heavy
metals, such as cadmium (Cd), lead (Pb), nickel (Ni) and chromium (Cr), showed
levels that were below the limits established by NOM-004-SEMARNAT-2002 and
other applicable international regulations, such as U.S. EPA guidelines and European
regulations (García & Benarroch, n.d.; Saldaña Escorcia & Castillo Gámez, 2021). These results indicate that the sludge
generated at "Las Viñitas" is suitable for
transformation into organic fertilizer, provided that adequate processes are
followed to stabilize the organic matter and eliminate the pathogens present.
The
initial characterization allowed a comparison with studies carried out in other
countries, such as Chile, Argentina and Spain, where sewage sludge has similar
compositions in terms of organic matter, moisture and nutrients. However, the
levels of organic matter in the sludge from "Las Viñitas"
were higher, which represents an advantage for its use in the production of
organic fertilizers. This reinforces the potential of the sludge from this
plant as a valuable source of nutrients for the soil, provided that effective
treatment technologies are implemented (Clavijo, 2024).
The
results of the initial characterization confirmed that the sludge from
"Las Viñitas" possesses physical and
chemical properties that make it highly suitable for transformation into
organic fertilizer. Its high content of organic matter and essential nutrients,
combined with low levels of heavy metals, makes it a valuable resource for
agriculture. However, the high pathogenic load requires the application of
specific processes to ensure their safety (Saldaña Escorcia & Castillo Gámez,
2021; García & Benarroch, n.d.). These findings
provided a solid basis for designing treatment strategies that maximize the
value of sludge and minimize the risks associated with its agricultural use,
highlighting its feasibility to promote sustainable practices in waste management
and soil fertility improvement (Amador-Díaz et al., 2015; Carrasco, 2000) .
The
sewage sludge optimization process included three main treatment techniques:
pre-composting, composting and lumbering. Each of these stages was selected for
its ability to stabilize organic matter, reduce microbial load and improve the
agronomic properties of the final product, achieving significant results in the
transformation of sludge into high quality organic fertilizer (Saldaña Escorcia & Castillo Gámez, 2021).
Pre-composting
was carried out as an initial stabilization stage to reduce the moisture and
microbial load of the sludge. During this process, the temperature reached
values of up to 55°C, which favored the partial elimination of pathogens and
reduced the odors generated by the initial anaerobic decomposition (Carrasco,
2000). This method reduced the moisture content by 20%, creating optimal
conditions for the subsequent stages of treatment. Likewise, pre-composting
promoted a slight reduction in sludge weight (approximately 10 %), facilitating
its handling in subsequent stages (García & Benarroch,
n.d.). In this stage, the pH remained in a slightly alkaline range (7.5-8.0),
favoring the initial microbial activity. The evolution of this parameter during
the process can be observed in Graph 1, which shows how the pH is progressively
adjusted at each stage of the treatment (Diocaretz
& Vidal, 2010).
Graph
1. pH
monitoring in relation to time, alternative one.
Aerobic
composting, as a second stage, was key for the advanced stabilization of
organic matter and the sanitization of the sludge. During this process,
temperatures of up to 65°C were reached, sufficient to guarantee the complete
elimination of Salmonella spp. and viable helminth eggs, complying with
international agricultural safety standards (Carrasco, 2000). Composting also
significantly reduced the weight of the sludge, achieving a total decrease of
37.5%. In addition, the process stabilized the carbon/nitrogen (C/N) ratio,
bringing it to values of 10:1, an indicator that the material was ready for use
as fertilizer. During this stage, the pH showed a slight decrease due to the
formation of organic acids, gradually stabilizing towards neutral values, as
also reflected in the Graph (Clavijo, 2024; Saldaña Escorcia & Castillo Gámez,
2021). Lumbriculture, as the final treatment stage,
used Eisenia foetida to process the precomposted sludge, significantly improving the properties
of the final product. This method not only increased the organic matter content
in the compost (65 %), but also enriched the levels of nitrogen (4.2 %) and
available phosphorus (2.5 %), increasing its agronomic value. The earthworm
activity favored an additional decomposition of organic matter, generating a
product with a homogeneous texture and a low concentration of contaminants,
guaranteeing its safety for use in agricultural crops. The pH of the final
product remained in a neutral range, optimal for application in soils, consolidating
the stability of the compost produced (Carrasco, 2000).
Overall,
the combination of these three techniques proved to be highly efficient for
sewage sludge stabilization. Each stage played a complementary role in the
transformation of the material, ensuring the elimination of microbiological
hazards, the improvement of the physical and chemical properties of the
compost, and the reduction of the weight and volume of the initial sludge. The
results confirm that the integration of these technologies is not only
effective, but also sustainable and replicable in other similar contexts.
Continuous pH monitoring (Figure 1) reinforces the importance of controlling
key parameters during treatment, ensuring a safe, stable and high
quality end product.
The
efficiency of the techniques employed was demonstrated at each stage of the
process, converting a problematic waste into a valuable resource for
agriculture. These findings reinforce the importance of implementing
comprehensive and sustainable solutions in sewage sludge management, aligned
with the principles of circular economy and environmental sustainability (Saldaña Escorcia & Castillo Gámez, 2021).
The
organic fertilizer obtained after sewage sludge treatment proved to meet high
quality standards, ensuring its applicability in agriculture and its
environmental safety. The physicochemical and microbiological analyses
performed confirmed that the final product is a valuable resource, aligned with
international regulations such as NOM-004-SEMARNAT-2002 (Amador-Díaz et al.,
2015; Carrasco, 2000).
In
terms of physicochemical properties, the compost showed an organic matter
content of 65%, a significant increase over the initial sludge. This enrichment
is the result of the combination of techniques such as composting and
lumbering, which favored the stabilization and transformation of organic matter
into more plant-available forms. In addition, nitrogen, phosphorus and
potassium levels reached values of 4.2 %, 2.5 % and 0.8 %, respectively,
positioning the compost as an efficient fertilizer for agricultural crops. The
final carbon/nitrogen (C/N) ratio stabilized at 10:1, indicating a mature
product with optimal characteristics to improve soil fertility (Saldaña Escorcia & Castillo Gámez, 2021).
From
a microbiological perspective, the compost produced met the required safety
standards. The analyses confirmed the total absence of pathogens such as
Salmonella spp. and helminth eggs, resulting in a product classified as Class A
according to NOM-004-SEMARNAT-2002. This result guarantees that the fertilizer
can be applied directly to agricultural soil without risk to human health or
the environment (García & Benarroch, n.d.).
In
addition, the compost was noted for its ability to improve soil physical
properties. Preliminary tests on crops showed a 25% increase in agricultural
yields compared to untreated soils, reflecting its effectiveness as an organic
amendment. Local farmers who participated in the pilot tests reported
improvements in water retention and soil structure, reinforcing the value of
compost as an integral solution for sustainable agriculture (Saldaña Escorcia & Castillo Gámez, 2021).
In
regulatory terms, the final product exceeded the criteria established for its
use in agricultural soils, showing levels of heavy metals such as cadmium, lead
and nickel well below the maximum limits allowed by international regulations.
This supports its unrestricted application in various types of crops, including
those intended for human consumption.
The
organic fertilizer produced from the waste sludge of the "Las Viñitas" WWTP represents a high
quality input for agriculture. Its enriched composition, absence of
pathogens and low heavy metal content confirm its suitability as a sustainable
organic fertilizer. This result highlights the effectiveness of the treatment
techniques employed and reinforces the feasibility of integrating sewage sludge
into a circular economy model, promoting more sustainable and safer
agricultural practices.
The
economic feasibility analysis of the sewage sludge treatment process
demonstrated that the transformation of this by-product into organic fertilizer
is not only environmentally sustainable, but also economically viable. The
implementation of pre-composting, composting and lumbering techniques resulted
in a competitive model against the traditional costs of sludge disposal and
commercial chemical fertilizers (Amador-Díaz et al., 2015).
First,
the costs associated with the final disposal of sludge in landfills or by
incineration, which represent a significant recurrent expense for the treatment
plants, were almost entirely eliminated. The reuse of sludge for composting
reduced these costs and also generated a product with commercial value. This
benefit was reflected in a 30% decrease in operating costs compared to
traditional sludge management methods (Diocaretz
& Vidal, 2010).
Implementation
costs included the necessary infrastructure for the treatment stages, such as
areas for pre-composting and composting, as well as specific units for lumber
production. Although the initial investment was moderate, its amortization was
projected in a short period due to the savings generated and the income derived
from the commercialization of the compost. The estimated sales price for the
compost produced was competitive with chemical fertilizers, offering a more
economical and sustainable alternative for local farmers.
The
cost-benefit analysis also included the added value of the final product. The
enriched compost obtained showed superior agronomic characteristics, such as a
high content of organic matter and essential nutrients, which increases its
potential demand in the agricultural market. In addition, farmers participating
in the pilot tests highlighted its effectiveness, which reinforces its
commercial attractiveness (Carrasco, 2000).
From
a circular economy perspective, the model also contributes to local value
creation by taking a waste previously considered a problem and transforming it
into a useful resource. This not only reduces dependence on imported chemical
fertilizers, but also encourages more sustainable and accessible agricultural
practices for small and medium-sized producers.
The
economic feasibility of sewage sludge optimization using the proposed
techniques is sound. Organic compost production costs are significantly lower
than those of commercial fertilizers, and the income from its sale quickly
compensates the initial investment. This model not only represents a
breakthrough in sustainable waste management, but also positions itself as an
economic opportunity to promote sustainability in agriculture and strengthen
the local economy.
The
results obtained confirm the effectiveness of the techniques implemented for
the optimization of sewage sludge, highlighting its potential to transform an
environmental problem into a resource of high agricultural value. The
incorporation of pre-composting, aerobic composting and lumbriculture
allowed not only to stabilize the sludge, but also to enrich its agronomic
properties and guarantee its microbiological safety. This integrated approach
aligns the processes with the principles of environmental sustainability and
circular economy (Rosa Mosquera-Losada et al., 2007).
In
the composting process, the effectiveness of maintaining optimal temperatures
(up to 65°C) was highlighted, achieving the elimination of pathogens and
stabilization of organic matter. Additional studies on aerothermal composting
indicate that these temperatures favor the proliferation of thermophilic
bacteria, which sanitized the piles and ensured a safe product. Also,
adjustments in the carbon/nitrogen ratio and moisture reduction during
composting reflect efficient waste transformation, results consistent with
previous research on improving soil fertility through the use of stabilized
compost (Angélica Ormeño & Ovalle, 2007).
The
lubrification stage, using Eisenia foetida, was
crucial to increase the final quality of the compost. The biological
transformation process using earthworms proved to be efficient in reducing
contaminants and improving the levels of essential nutrients such as nitrogen
and phosphorus. This method, widely recognized as a sustainable biotechnology,
has been highlighted for its ability to produce high quality fertilizers with
immediate benefits for agricultural crops (Ramírez Joyo, 2017).
From
an environmental perspective, the results support that this sludge management
model contributes significantly to the mitigation of negative impacts. The
reduction of greenhouse gas emissions and the improvement of the physical,
chemical and biological properties of soils are key factors that reinforce the
positive impact of this system. In addition, recent literature highlights how
the integration of processes such as lumbriculture
and composting allows the effective recovery of nutrients, closing material
cycles and minimizing the dependence on chemical fertilizers.
From
an economic point of view, this model is highly competitive. The transformation
of sludge into compost not only eliminates costs associated with final
disposal, but also generates income from the commercialization of the product.
The economic viability of the process is similar to the advantages observed in
research on organic fertilizers, where there is evidence of increased
agricultural profitability and less dependence on external inputs.
Finally, the combination of pre-composting, composting and lumbriculture represents a comprehensive and sustainable
solution for sewage sludge management. The findings highlight the potential of
this methodology to be replicated in different contexts, especially in regions
where waste management remains a challenge. Future studies could focus on
optimizing the conditions of each stage of the process and evaluating its
long-term impact on crop productivity and soil quality (Albarracín
Sánchez et al., 2018).
Conclusions
The results of this research confirm
the technical, economic and environmental feasibility of sewage sludge
optimization through the integration of pre-composting, aerobic composting and lumbriculture. This integrated approach effectively
addresses the problems related to the disposal of these by-products, while
promoting agricultural sustainability and the circular economy.
First, physicochemical and
microbiological analyses showed that the sludge from the "Las Viñitas" WWTP has suitable characteristics for
transformation into organic fertilizer, such as a high organic matter content
(53.5%), an adjustable carbon/nitrogen ratio and heavy metal levels within
regulatory limits. These initial properties, together with the efficiency of
the treatment techniques implemented, made it possible to obtain a safe final
product with excellent agronomic properties, complying with international
standards such as NOM-004-SEMARNAT-2002.
Second, the compost produced was
presented as a competitive fertilizer, enriched with essential nutrients such
as nitrogen, phosphorus and potassium. This product not only improves soil
quality and agricultural yields, but is also suitable for direct use on crops
due to the complete elimination of pathogens. Therefore, the compost represents
a sustainable and high quality alternative for
farmers, reducing dependence on chemical fertilizers and associated impacts.
In addition, the economic viability
of the model was evidenced in the reduction of costs related to sludge
disposal, as well as in the potential income derived from the commercialization
of the compost. This strategy allows treatment plants to transform an
environmental liability into an economic asset, in line with modern sustainable
waste management practices.
From an environmental perspective,
this model contributes significantly to mitigating negative impacts by reducing
greenhouse gas emissions and promoting the reuse of nutrients. The
implementation of this system is an important step towards sustainability in
waste management, closing material cycles and strengthening the local economy.
In summary, this research
demonstrates that sewage sludge optimization is a viable and scalable solution
to address environmental and agricultural problems. These findings raise the
possibility of replicating this approach in other treatment plants and local
contexts, with the potential to integrate new technologies and further optimize
the process. Finally, future studies could focus on assessing the long-term
impact of this model on soil quality, agricultural productivity and climate
change mitigation.
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