**1. Introduction**

One of the crucial strategies for waste management both in Poland and around the world is the treatment of the organic fraction of municipal solid waste (OFMSW). Basic assumptions and objectives in the area of municipal solid waste (MSW) management, adopted both in the EU [1–3] and in national legal acts, aim to limit the use of landfilling as a means of reducing the organic fraction [4]. One of the solutions is the mechanical and biological treatment (MBT) of mixed municipal waste [5]. In Poland, at the end of 2016, there were 192 MBT plants with a total mechanical capacity of around 11 million tons of waste per year [6]. Mostly, the biological process of aerobic biostabilization (AB) has been adopted for OFMSW treatment [7]. However, optimization AB of OFMSW is difficult, due to its heterogeneity, thermal gradients, and some side effects in the bioreactors. Additionally, bioreactors treating many tons of OFMSW are poorly equipped with sensors for temperature, oxygen, or moisture, so that the plant operator has little control over most of the waste mass.

Gas flow patterns within the waste have a large influence on heat and mass transfers. Consequently, O2 supply, moisture and temperature distribution have a large impact on the end-product quality (kinetics of biodegradation; stage of stabilization; hygienization of the compost), as well as on the environmental impact of the treatment (gaseous emissions and odors) [8]. Within the AB process, gases in the pores are heated due to microbial activity [9],

**Citation:** Stegenta-D ˛abrowska, S.; Randerson, P.F.; Białowiec, A. Aerobic Biostabilization of the Organic Fraction of Municipal Solid Waste—Monitoring Hot and Cold Spots in the Reactor as a Novel Tool for Process Optimization. *Materials* **2022**, *15*, 3300. https://doi.org/ 10.3390/ma15093300

Academic Editors: Rossana Bellopede and Lorena Zichella

Received: 6 April 2022 Accepted: 2 May 2022 Published: 4 May 2022

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**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

which reduces air density inside the pile pores and increases partial pressure, creating a flux from the bottom to the top layers in naturally aerated piles [10]. However, the gas production rate differs with spatial gradients because of ineffective mixing (inhomogeneity) and compaction effects, resulting in concentration gradients which drive gas diffusion and transfer inside the pores [11]. The effect can be visible even in well-mixed waste, especially in municipal solids which are characterized by huge inhomogeneity. Anaerobic areas (socalled "hot spots") need to be diagnosed and eliminated because gaseous biostabilization products (CO, CO2, CH4) are substantial threats to both low treatment efficiency and human and environmental protection.

The technique of forced aeration has been adopted in MSW treatment, as it is more effective than natural (passive) ventilation for the aerobic metabolism of microorganisms, removal of water, and control of the temperature of the system [12]. It has been reported that the O2 content in the air space has no significant effect on the biological degradation efficiency until it falls below 5% in the composting matrix [13]. During the aeration process O2 content may rise above 15%, gradually decreasing again after the air blower stops, although O2 content may support aerobic bioactivity for an extended period. Furthermore, it has been demonstrated that intermittent aeration could reduce NH3 loss from the composting system compared to continuous aeration [14]. Our previous research showed that low O2 concentration could also favor CO production [15,16]. Gaseous emissions during biostabilization not only reduce the compost quality, but also cause atmospheric pollution [17]. Therefore, an improved AB process is urgently needed.

Even if forced aeration is more effective than natural ventilation, "hot spots" in OFMSW exist due to its heterogeneity [15], as well as aeration rate and reactor design. The proper design and operation of a biostabilization project requires an understanding of the dynamics of biostabilization [18]. In particular, the process depends on the abundance and activity of microorganisms, which are mainly affected by temperature, moisture, readily degradable organic content, O2 level and its diffusion in the matrix, and presence of inhibiting compounds. Without frequent turning in a static composting system, or in the absence of dynamic aeration, significant spatial differences in these parameters resulting from one-directional air flow will impact the spatio-temporal dynamics and hence the uniformity of the compost product [16,19]. An appropriate reactor, adapted to the characteristics of the waste, should not only maintain appropriate levels of O2 and temperature, but also allow for uniform distribution within the pile [20]. Hence, monitoring the spatial and temporal distribution of pore gas concentrations is an important method for evaluating and optimizing the aeration strategy and reactor design and operation [21].

Adoption of a flow aeration regime, together with knowledge of the spatial and temporal distribution of process gases, and temperature can enable optimization and control of parameters such as temperature, O2 or even CO. Monitoring the anaerobic "hot spots" during AB of OFMSW may be a useful tool in mitigating emissions of gaseous pollutants and optimizing the biostabilization processes.

The aim of this study was to investigate the spatial and temporal distribution of temperature and pore gas (O2, CO2, and CO) concentrations in relation to anaerobic "hot spots". Spatial and temporal variability of gas concentrations and temperatures were determined at full technical scale in a municipal biostabilization plant.
