The smell of garbage piles is not always the same. The odor generated by garbage is closely related to the composition of the garbage, the number of days it is stored, whether it is anaerobic fermentation or aerobic fermentation, and the climate. For example, fresh garbage, if there is a lot of kitchen waste (such as rotten vegetable leaves, fruit peels, etc.), will produce a slightly alcoholic and sweet odor in the early stages of fermentation (when stored outdoors), which is usually "fresh" (note: these odors are my personal experience, so the description is entirely based on my personal feelings); If aerobic fermentation is carried out, such as composting, which is mostly composed of organic components such as kitchen waste, the odor will change as the composting process progresses. Overall, it is a stuffy smell, similar to the smell of rotten pulp. If the concentration is too high, it will be uncomfortable to smell for a long time; In the later stage of composting, it will gradually transform into the smell of humus, similar to the soil odor smelled in damp forests. This odor will gradually decrease with the degree of humification and transform into a more pleasant odor (which can be called soil aroma), which is also the magic of composting. If garbage undergoes anaerobic fermentation, it is another situation. Anaerobic fermentation is also the most foul smelling situation. The foul odor around landfills in China is often caused by anaerobic fermentation of garbage (usually easily degradable organic components). The odor components are explained below.
During the anaerobic fermentation process of garbage, a large amount of landfill gas is generated, which usually consists of about 50% to 70% methane, about 30% to 50% carbon dioxide, and about 1% trace components. And the foul odor is caused by certain substances in these 1% trace components. These trace components include both organic and inorganic compounds, with a wide variety of types and extremely low concentrations. Landfills can detect over a hundred substances at concentrations of ppm or ppb. These substances can be classified into several categories: sulfur-containing compounds (hydrogen sulfide, thiols, thioethers, etc.), oxygen-containing compounds (alcohols, aldehydes, ketones, esters, etc.), alkanes, alkenes, halogenated hydrocarbons, terpenes, and aromatic hydrocarbons, as well as ammonia gas. Among them, sulfides and oxygen-containing compounds have relatively high concentrations due to their low odor threshold (as they are the main products in the early stages of anaerobic and facultative fermentation), often leading to severe odors. At present, there are only 8 substances specified in China's odor pollutant emission standards, including ammonia, trimethylamine, hydrogen sulfide, methyl mercaptan, methyl sulfide, dimethyl disulfide, carbon disulfide, and styrene. However, according to our research, all organic sulfides and oxygen-containing compounds such as aldehydes and ketones are important odor causing substances in the waste treatment process.
It is worth mentioning that many substances, such as ethyl acetate, ethanol, etc., have pleasant odors when present alone, but when mixed with other substances, they may become unbearable odors. The synergistic or weakening effects between odorous substances have always been a confusing topic, and there is currently no mature research that can elucidate the mechanisms involved. This is also a major challenge encountered in odor prediction and the establishment of monitoring models. Similarly, there is still a lack of systematic research on the mechanism of odor occurrence and concentration prediction in the process of domestic waste treatment and disposal (although we are currently working on it, the road is blocked and long). The trace components listed above also include a large amount of alkanes, alkenes, terpenes, halogenated hydrocarbons, aromatic hydrocarbons, etc. These substances have relatively high odor thresholds, and their contribution to odor is not high when simply calculating their threshold dilution ratio. However, the same question is whether they will produce odor when mixed with other substances? There is currently no relevant research on this point, but it is speculated that the addition of odor may occur when different odorous substances are mixed. And the human sense of smell is actually a very magical thing. The odors and degree of smell that different people smell are often difficult to describe or measure with a unified standard, especially since the process of smell also involves psychological activities. In detail, it can also involve human body structure, neural reactions, molecular chemistry, and so on, which are not within my research field, and it seems to have gone off topic... In fact, the conclusion is that the odorous gases generated during the garbage disposal process have complex components, and it is currently believed that the main odorous substances are sulfur-containing compounds and oxygen-containing compounds.
