Biofiltrazione di effluenti gassosi da un processo di compostaggio: confronto tra differenti mezzi di riempimento

During the last decades, the sector of air pollution control has encountered important technological advances. The biological filtration of the exhaust air coming from various civil and industrial processes has followed the same path. Nevertheless, the selection of the most appropriate filtering media is still challenging in biofltration systems and this choice is a key factor to ensure satisfying removal efficiencies in biofilters. Although organic filtering media are characterized by low costs, broad availability, microbial diversity and a relatively high nutrient content, they suffer from a shorter lifetime than inorganic materials. The present study has the purpose of evaluating the application of different filtering media for their adoption in biofiltration systems, with the aim of reducing the concentration of volatile organic compounds (VOCs), hydrogen sulphide (H2S) and odors. To this purpose, a pilot-scale plant was created to simulate the gaseous effluent generated from a composting process. The pilot-scale plant consists in a biocell with an inner volume of 1 m3, a scrubber for the abatement of ammonia, and three biofiltration columns filled with 1) wood chips, 2) lava rock (50%) and peat (50%) combination and 3) peat only. The biocell was filled with mixed organic waste, composed of the organic fraction of municipal solid waste and green waste. The biocell was fed with an airflow rate of 6 Nm3 h-1 by alternating aeration/no aeration phases. The duration of each phase was set to 30 min. Air samples were taken during four days distributed along the first phase of the composting process, during which the waste undergoes strong oxiditation by the microorganisms. The air samples were analyzed through dynamic olfactometry, photoionization and an electronic nose analyzer, to evaluate the removal efficiency of the biofilters in terms of abatement of the concentrations of odors, VOCs and H2S, respectively. To control the process, key parameters were continuously monitored, such as the temperature within the waste in the biocell, the temperature of the air at the inlet of the biofilters, the pH of the leachate extracted from the biocell and the pressure drop between the entrance and the exit of the three biofilters. The waste temperature was characterized by a constant increase during the first week of the experiment, and achieved a peak temperature of about 63°C. The temperature was permanently > 55°C for ten consecutive days, allowing for proper sterilization of the waste. A decreasing temperature trend was observed starting from the 13th day. The trend of air temperature at the inlet of the biofilters resembles the trend of the waste temperature, although the thermal excursion is lower as well as the temperature values, due to the cooling effect generated by the scrubber and by the transit along the pipelines. For this reason, the inlet temperature varied in the range 27÷36°C during the experiment. After an acidic phase at the beginning of the experiment, with values comprised between 5 and 6, the pH of the leachate showed an increase until reaching a peak value of 8,81. After the achievement of the peak value, the pH started decreasing and reached values slightly higher than neutrality. The pressure drop, moreover, was < 300 Pa during the entire process for all the media used, with initial higher values for lava rock-peat and peat only columns, probably related to higher water content after the inoculation phase. The analyses on the air samples collected during the experiment showed that lava rock-peat combination seems to be the most interesting solution for the abatement of odors, VOCs and H2S generated from composting processes in biocells, especially at the beginning of the process, when the pollutant load is higher. The biofilter filled with lava rock and peat allowed obtaining average removal efficiencies of 96%, 95% e 77%, respectively for the abatement of odors, VOCs and H2S. Wood chips and peat only showed a weaker performance, respectively with average removal efficiencies of 90%, 88% e 62% and 95%, 89% e 69%. The results obtained in this paper show that lava rock and peat combination can be considered as a promising option for the biofiltration sector.