Daniel Carlos Machado: «Sewage sludge can improve soil fertility without harming the environment if applied correctly»
Daniel Carlos Machado is a visiting researcher at Campus Terra at the University of Santiago de Compostela, where he is participating in the Doctoral Programme in Agriculture and the Environment for Development, as part of the UXAFORES group and under the supervision of Professor Esperanza Álvarez Rodríguez. With a PhD in Agronomy from Brazil, specialising in Soil Sciences, his research career has focused since its inception on sustainability, waste management, and the reuse of by-products as a strategy for more efficient, environmentally friendly agriculture.
In this interview, Daniel Carlos takes us inside his research journey, exploring how applying sewage sludge to soil can unlock its fertility and recycle crucial nutrients—while also confronting the environmental challenges that come with this practice.
His work aims to determine the long-term effects of sewage sludge applications, especially their impact on heavy metal dynamics, soil health, and the safety of the soil-plant system. The goal is to generate key knowledge for defining appropriate doses that maximise agricultural benefits while minimising environmental and health risks.
The researcher highly values his experience at Campus Terra, both scientifically and personally. He highlights the quality of the laboratory resources, the support received, and the close connection between research and the territory as distinguishing features of the Campus model.
He sees his time at Campus Terra as a game-changer, expanding his technical horizons and deepening his commitment to science that fosters the circular economy, shares knowledge, and builds a foundation for sustainable solutions.
-Your academic career shows a progressive specialisation towards sustainability and soil science. How would you describe the evolution of your research interests from environmental engineering to your current focus on waste reuse and the application of sewage sludge?
-During my degree, I was always interested in subjects focusing on renewable resources and solid waste management, as we live in a consumerist society that generates a large amount of waste, and its improper disposal causes damage to the environment, directly or indirectly affecting environmental and human health. Since then, I have always been interested in finding alternatives for the proper disposal of this waste, favouring its reuse and promoting the circular economy.
-Waste recovery through its application to soil is considered a key strategy within the circular economy. What scientific or technical gaps are you specifically seeking to address with your research on sewage sludge?
-Sewage sludge is a waste product that has numerous benefits, such as providing nutrients, including nitrogen, phosphorus, potassium and organic matter, which are essential for crops, as well as improving soil fertility and contributing to productivity. Currently, numerous studies show the benefits of using sewage sludge in agriculture; however, most focus on short-term applications, and few analyse their long-term dynamics.
Given that sewage sludge may contain non-essential elements such as lead, cadmium, arsenic and chromium, as well as some essential elements that are present in quantities greater than those required by plants and may therefore be toxic (Mn, Cu, Zn, Ni), long-term studies are of great importance in verifying how the soil-plant system evolves after several applications. In Brazil, my study area has received annual applications of sewage sludge (5, 10 e 20 Mg ha-1) for 28 years, with the aim of identifying possible effects of these applications. Some of our research focuses on:
- Non-essential elements, such as heavy metals and metalloids, can accumulate in the soil at toxic levels, negatively impacting its structure and reducing its fertility, productivity, growth and overall plant development. In addition, these elements can be transferred and accumulate in plant leaves, potentially entering the food chain and reaching humans, posing a health risk. These elements are not eliminated by the body and accumulate in different organs, potentially causing serious problems.
- Possible changes in the microbial community: since the repeated application of sewage sludge, rich in organic matter and potential contaminants, can favour the selection of specific groups of microorganisms, causing imbalances in the soil and affecting the cycle of some nutrients.
- Soil acidification may also occur, as sewage sludge is rich in organic matter and nitrogen; therefore, the mineralisation of organic matter, nitrification and oxidation can release H⁺ ions, which can acidify the soil over time.
Long-term research is therefore essential to monitor these parameters and determine the optimal doses that maximise the benefits of sludge use, mitigate potential environmental risks, and ensure soil health for future generations.
-This project analyses the adsorption and desorption of heavy metals in the soil system. What methodological challenges does this type of analysis present, and what experimental approach are you applying to overcome them?
-Heavy metals accumulate in the soil due to various natural or anthropogenic factors, and fertilisation with sewage sludge can be a route of entry. However, the environmental impact of heavy metals depends on several factors, including their removal from the soil solution by adsorption onto soil colloids or their release from soil particles by desorption. The occurrence of these processes is related to certain soil characteristics, such as pH, clay content, organic matter, and the presence of aluminium and iron oxides and non-crystalline aluminosilicates.
Therefore, understanding the processes that determine metal competition and adsorption is essential, as it allows us to predict the behaviour of these metals in the system.
One technique used to determine the soil's capacity to act as a sink for these contaminants and prevent their entry into water and the food chain is batch adsorption-desorption studies. These consist of adding increasing concentrations of each of these heavy metals to soils and testing the capacity of those soils to retain them strongly, in an almost irreversible manner, and remove them from solution. This allows us to identify which soils are most vulnerable and which strongly adsorb these heavy metals, preventing them from entering the food chain.
-Your work addresses two critical dimensions: improving soil fertility and ensuring environmental safety. How do you plan to balance both objectives in your conclusions and recommendations?
-In my study, we aim to determine whether sewage sludge can serve as an alternative fertiliser without harming the environment. Our objectives include identifying the optimal application rate to meet crop nutrient needs, reducing dependence on chemical fertilisers, promoting sustainable agriculture, and avoiding adverse effects on soil health, such as contamination by non-essential elements.
-What academic, scientific or personal factors influenced your decision to join the Doctoral Programme in Agriculture and the Environment for Development at Campus Terra?
-Firstly, I chose this programme because it is aligned with my programme in Brazil and addresses issues such as natural resource management and food security, which coincide with my line of research. Finding a programme that not only addresses these issues but also organises its mission around them, recognising food production, land management and population security as components of a single system, was an opportunity to delve deeper into an international context. In addition, it has qualified professors and a laboratory equipped to carry out my analyses. Specifically, I joined the UXAFORES research team under the direction of Dr Esperanza Álvarez Rodríguez.
-After this period in Lugo, how would you rate the research experience, the support you received and the academic ecosystem of Campus Terra?
-From the first email I sent to my professor, I was very well received, and she always treated me with kindness. When I arrived here, I was very well received. I don't speak Spanish very well, and they always helped me and were patient. Wherever I go on Campus Terra, I am treated well, and people are always willing to help. I am learning a lot here, both professionally and personally. It is very interesting to see the team's unity and support at work.
-From an international perspective, what elements of Campus Terra's scientific or educational approach do you consider to be particularly distinctive compared to Brazil?
-One of the distinctive features of Campus Terra is its access to laboratory resources. These laboratories are well equipped, making it easy to carry out many analyses. This allows for a constant and complete workflow, as all stages, from sample preparation to analysis, can be carried out in the same environment. Another distinguishing feature is the rural community's access to the laboratories, which receive numerous samples for analysis, making it easier for them to access the results and thus determine the best way to prepare the soil, avoiding environmental impacts. In this way, knowledge and science are transferred to the community, which brings numerous benefits.
-What do you hope this stage of your doctoral training will contribute to your future research career, and what lines of work would you like to develop after completing your thesis?
-At the USC Terra Campus, I am acquiring extensive laboratory knowledge by working with various analytical methods, which provides me with technical and multidisciplinary training. At a later stage, I will analyse the data and discuss the results, which will help me further my training. After completing my thesis, I intend to continue working on projects related to waste reuse, especially because smaller municipalities often lack the resources to manage it properly.
In Brazil, the most common method of managing this waste is through sanitary landfills. Although this option is suitable as a final destination, it requires a significant investment and a large construction footprint. In addition, this disposal method results in the loss of potentially usable nutrients, undermining sustainable agriculture and failing to align with Sustainable Development Goals 2, 11, and 12.