Photoacoustic-based sensor for real-time monitoring of methane and nitrous oxide in composting

Highlights

• Trace emission of CH₄ and N₂O in composting of agricultural waste.

• Real-time monitoring of greenhouse gases through photoacoustic spectroscopy.

• Spectrum convolution approach to quantify the concentration of greenhouse gases.

• O₂ concentration and temperature influence on CH₄ and N₂O in composting.

• Denitrification in composting process.

Abstract

The emission of greenhouse gases from disturbed biological systems has caused damage to the environment. Thus, sensitive, efficient and accurate techniques have been required to monitor these gases. In this article, the gas dynamics in the composting of an agricultural waste (horse manure and lettuce lefts) was studied. The progress of composting was accompanied by monitoring O₂ and CO₂ gases and temperature. The results showed that in the thermophilic phase the composting was predominantly aerobic. Photoacoustic spectroscopy was used throughout the process to monitor the dynamics of the greenhouse gases N₂O and CH₄. The technique was effective in identifying both gases, and when combined with the convolution approach using the absorption lines from the molecular spectroscopic database (HITRAN), it also allowed the quantification of them.

Introduction

Since the industrial revolution by 1750, the human activities have significantly contributed to the accumulation of the greenhouse gases (GHG) in the atmosphere [1]. Pollutant sources involve different practices such as energy production and its use, forest fires, agriculture, waste treatment processes, pest and disease control methods in agriculture, among others [2], [3]. The major GHG produced from these activities are water vapor (H₂O), nitrous oxide (N₂O), carbon dioxide (CO₂), methane (CH₄), and chlorofluorocarbons (CFCs) [2]. Another environmental issue to be concerned is the accentual increase of waste stimulated through the intense industrial development, and the quick urbanization in the 50s along with the world population growth. As consequence, the availability of disposal areas for municipal waste has gradually decreased.

Even though decompositions of animal and plant wastes are a natural decay process, they are regarded as pollutant because of foul smells and the emissions of GHG. Moreover, if care is not taken with the storage of these wastes, they create a conductive environment to the development of microorganisms that can generally be disease causing agents [4]. In addition, water pollution is one of the biggest problems caused by the livestock sector [5].

In 2015, an action plan entitled Agenda 2030 for Sustainable Development was created by the United Nations to encourage the poverty eradication through sustainable economic growth, education, social protection, while tackling climate change until 2030. Reduction of environmental impacts, improvement of the air quality, waste management, sustainable use of the biodiversity and ecosystems, and action to tackle climate change are among the goals in Agenda 2030 for a sustainable development.

Given the above, waste treatment methods and correlated techniques have become important and have been intensively studied [6], [7]. Among different approaches, composting stands out as simple, economic, and efficient method to treat organic matter (green waste), reducing its volume and stabilizing its composition through an end product (humus-like material) with useful destination in gardens, landscaping, horticulture, urban agriculture and organic farming [7]. Basically, composting is a biological decomposing process of green waste into a humus-like material. This process requires the presence of oxygen for oxidizing carbon and water. Typically, a pile containing a mixture of carbon, nitrogen (C/N ratio of about 30 or less) and water is prepared and regularly managed (e.g., turning or forced aeration) to maintain the supply of oxygen inside the pile. Water is added adequately into the mixture so that an air/water balance is ensured, thereby avoiding the formation of anaerobic sites. During the composting, the oxidation of carbon by microorganisms generates heat as long as the material is broken down. Thermophilic temperatures ranging from 59.5 to 71.0 °C are often achieved [8].

Inasmuch as Brazil holds a prominent position in the list of the largest worldwide exporters of agricultural commodities, the generation of waste has turned a major environmental liability for the country. Therefore, composting appears as a viable solution to this environmental drawback. Natural fertilizer generation is important as this input can be later reused in agricultural activities, making it a self-sustaining cycle. However, during the composting process GHG like CH₄, N₂O and CO₂ are generated. Besides that, formaldehyde (CH₂O) may also be present. This chemical specie is a by-product of CH₄ oxidation induced by CH₄-oxidizing bacteria (methanotrophs) [9]. For that very reason, new techniques are required to monitoring emissions of these chemical species in a relatively long-term period (days). Techniques which must be sensitive and precise enough to identify and quantify multicomponent mixtures of gases.

As described above, the main goal of composting is promoting the treatment of organic residues that become reusable material as organic fertilizer. However, composting produces the GHG CH₄ and N₂O [7], mainly in not well managed systems, i.e., under predominant anaerobic conditions. Therefore, it is necessary to study and monitor the production dynamics of these gases in real time during composting, in order to mitigate them by means of good practices in management and technology. To do so, sensitive, rapid response and efficient techniques are required. Photoacoustic spectroscopy (PAS), in turn, has been employed to monitor gaseous emissions directly from the environment and traces of some parts per billion by volume (ppbv) have been successfully detected. Studies employing PAS have attested its potentiality in various areas of scientific knowledge [10], [11], [12], [13], [14], [15]. Thus, this work introduces a PA-based sensor which helps in real-time monitoring the emissions of CH₄ and N₂O during composting.