Biogas plant reduces odours from farm and manure spredding

Smell is the perception of chemical compounds in the air. When the concentration of odor-producing compounds exceeds the so-called odor threshold, a person senses the smell. Each chemical compound has its own characteristic odor, which can be pleasant or unpleasant. Odor-causing substances are mainly volatile organic compounds (VOCs), sulfur compounds and nitrogen compounds. All of these are released as organic matter decomposes.

Volatile organic compounds are a natural part of the natural cycle, and the degradation of organic matter does not occur without the formation of volatile compounds such as fatty acids. In composting and natural decomposition, organic compounds are released into the air. In biogas plants, VOCs are not released into the air for a two reasons: VOCs, such as fatty acids, are one intermediate stage in biogas production, and the microbes in the next stage use them as a food when producing methane. With sufficient delay, the digestion residue hardly smells, because all the easily degradable organic matter has already decomposed, so that new fatty acids can no longer form. On the other hand, fatty acid does not end up ”ready” in the digestate ponds because they have been consumed in a biogas reactor. Small amounts of fatty acids may be released inside the biogas reactor, but they do not end up in the open air, because the raw gas is treated with activated carbon and further burned for energy. When burned, fatty acids form odorless carbon dioxide and water.

Odorous sulfur compounds include hydrogen sulfide and organosulfur compounds such as dimethyl sulfide and methyl mercaptans. The amount of organosulphur compounds in the biogas reactor decreases because methane-producing microbes also feed on them. Hydrogen sulfide, on the other hand, is converted into elemental sulphur or sulfate with a small addition of oxygen. Elemental sulfur and sulfate are not volatile compounds, so they cannot smell either.

Ammonia is the most significant odor-causing nitrogen compound. The amount of ammoniacal nitrogen increases in the biogas process, but because the heated biogas reactor has been shut down, it cannot be released at the plant. However, the odour threshold of ammonia is clearly higher than, for example, fatty acids or sulphur compounds, so an increase in the amount of ammonia does not yet raise the smell of digestate to the level of raw manure. Therefore, ammonia evaporation should be avoided primarily to prevent nitrogen losses. The evaporation of ammonia is reduced, for example, by placement application.

In other words, farm biogas plants reduce odours in two different locations. On the farm, odours are reduced because slurry is practically pumped directly into the biogas reactor. Thus, most odor-forming compounds are converted into biogas before the compounds have had time to evaporate into the air. When surplus and spoiled feed is regularly fed into the biogas reactor, their decomposition has not had time to start strongly, which further reduces the amount of odours in the farm centre. The odours of digestate ponds and digestate dry fraction stores are lower than those of sludge and dry manure houses. The reduction of odours in facility centres may have an impact on, for example, environmental permits and, of course, working comfort.

According to a general interpretation of environmental law, odours from spreading manure in fields are, as a rule, subject to the so-called tolerance obligation. However, smaller odours reduced the potential irritation of summer holidaymakers and can bring individual fields back into the scope of application.

You can read more about odours in the publications of Häme University of Applied Sciences and the University of Jyväskylä, for example:

Kymäläinen M. ja Pakarinen O. (toim.) 2015. BIOKAASUTEKNOLOGIA Raaka-aineet, prosessointi ja lopputuotteiden hyödyntäminen. HAMKin e-julkaisuja 36/2015 https://www.theseus.fi/bitstream/handle/10024/104180/HAMK_Biokaasun_tuotanto_2015_ekirja.pdf

Lehtinen J. 2012 Odorous Volatile Organic Compounds in Waste and Wastewater Management. JYVÄSKYLÄ STUDIES IN BIOLOGICAL AND ENVIRONMENTAL SCIENCE 252 https://jyx.jyu.fi/bitstream/handle/123456789/40931/978-951-39-4931-0_2012.pdf