Odorous impact from composting plant: biofilter versus RTO

The problem of odor emissions from mechanical – biological treatment (MBT) plants, and mainly from composting plants, is becoming a relevant parameter for the implementation and management of these plants. The more frequently used systems for cleaning the process air before releasing it into the environment are based on biofilters, scrubbers and thermal oxidizers (RTO). Only few countries, apart setting a concentration limit for the odor emissions from MBT, ask for the use of mathematical models to simulate the impact of those over a defined area. In Italy, a regulation at national level is absent, but there are indications in some regional standards and guidelines. The aim of this paper is to determine and discuss the odor impact from a composting plant, using a mathematical model (Aermod). In the basic configuration taken into account, this plant is equipped with a biofilter for the process air treatment, as commonly adopted in the sector. Two design variations were studied in discuss enhanced design criteria for decreasing the odor impact. Concerning biofiltration, two solutions were compared: biofilter at ground level and biofilter on the roof of the plant, treating the air flow from the receiving and biological oxidation phases; the maturation and storage of the finished product are located under an open shed. In order to give additional information on the needs of the impact optimization, a third scenario was considered: an RTO system treating the air flow from all closed zones (reception, biological oxidation, maturation, final storage) whose air is assumed to be convoyed directly to the stack of RTO. The obtained results led to the conclusion that the most common solution is the worst one in terms of impact on the territory. This is due to the fact that the dilution effects are strongly reduced by adopting a biofilter. Besides this, the maturation and storage phases of the final product under open shed were noted to have a crucial contribution to the total impact; their input is greater in percentage increasing the distance from the plant. Taking into account the results of the quantification of the odor impact, plant configurations with closed maturation and closed storage areas should be promoted in the sector. However, a conservatory design approach should maximize the role of a stack diluting the gaseous emissions even if this solution is not common for composting plants. This stack could service an RTO burner.