Impact of Air-Flow Rate and Biochar Addition on the Oxygen Concentration in Waste and Emitted Gases During Biostabilization of Undersized Fraction from Municipal Solid Waste

Abstract

Mechanical biological treatment of waste is still one of the most popular methods for mixed municipal waste treatment. The result of mechanical processing of waste is sorting out: the undersized fraction from municipal solid waste (UFMSW) with granulation below 80 mm, rich in biodegradable organic waste (mainly including food waste, paper, wood, etc.). UFMSW is treated in biological processes in order to reduce the negative environmental effect of this waste. Unfortunately, the processing is not neutral to the environment. The correct course of the aerobic biostabilization process depends on the activity of microorganisms, the intensity of aeration, and the oxygen content in the processed waste. The aim of this paper was to analyze the effect of air-flow rate and biochar addition on the oxygen concentration in waste and in emitted gases during the intensive phase of UFMSW biostabilization. The study was performed under laboratory conditions. Six different variants of the process (without biochar addition and using 1.5; 3; 5; 10 and 20% of biochar addition) were applied. Subsequent replicates were conducted using an averaged air-flow rate of 0.1, 0.2, and 0.4 m 3. d -1. kg dm.org -1 . As a result of the conducted experiments, it was found that both the air-flow rate and the addition of biochar have a significant effect on the oxygen concentration in the treatment waste, as well as its content in the outlet air. Using the highest air-flow rate resulted in the oxygen content not decreasing below 14%, both in the free spaces between the waste and in the emitted gases, while the addition of biochar significantly reduced the oxygen concentration. In the case of lower air-flow rate values, the oxygen content decreased even below 5%. It was found that a high addition of biochar (10 and 20% by weight) at the lowest air-flow rate resulted in the occurrence of anaerobic zones in waste in the first days of the intensive process (between days 2 and 6 of the process), as well as absence of oxygen in the outlet air (between days 2 and 4 of the process). Despite this, no methane (biogas) emissions were found in any of the conducted experiments.

Impact of Biochar Addition and Air-Flow Rate on Ammonia and Carbon Dioxide Concentration in the Emitted Gases from Aerobic Biostabilization of Waste

“…The physical and chemical characteristics of the raw materials and input mixtures are presented in Malinowski [ 42 ]. According to data ( Table 1 ), the DM of UFMSW was 56.8 wt% and it increased with an increase in the dose of the biochar added to the waste.…”

“…The waste samples under study consisted of UFMSW exiting the mechanical treatment process (MSW treatment with a rotary screen, 80 mm opening size) at an MBT facility (MIKI Recykling Ltd.) in Kraków (Poland). The biochar used in the research is a material available commercially on the market and was obtained from coniferous woodchips as a result of pyrolysis in the thermalization energy recovery module at 550 °C for 30 min [ 29 , 42 ]. As Lehmann [ 43 ] and Jin et al [ 44 ] have reported, this temperature allows for obtaining biochar with the most favorable properties, i.e., high carbon content, cation exchange capacity, and a specific surface area, which are important in the biological methods of waste treatment and biochar application to soils [ 45 , 46 , 47 , 48 ].…”

“…The time of analysis in each run covered the first three weeks of the process. The initial temperature of the input mixtures was 14.4 ± 2.1 °C [ 42 ]. Information about the test conditions, the initial mass of waste placed in the bioreactor, and the methods of mixing the biochar with UFMSW were provided by Malinowski [ 42 ].…”

“…The aeration intensity was controlled depending on the waste temperature in the bioreactors using the automatic control system equipped with a temperature feedback regulator. The controlling system reacted to prevent the temperatures in the bioreactors from exceeding 65 °C [ 42 ]. The applied air-flow rates were chosen based on the results of Neugebauer et al [ 50 ], Yuan et al [ 51 ], and Tom et al [ 52 ].…”

“…The authors of this article focused on the effect of biochar addition and various air-flow rates on CO 2 and NH 3 concentrations in the process gases that were directed into the biofilter. Changes in oxygen concentration, bulk density, and air-filled porosity are described by Malinowski [ 42 ]. The concentration of individual gases was measured using two probes: one placed in the upper part of the waste bed and the other in the conduit directing the gases out of the bioreactor to the biofilter.…”