Possibilities for improvement of plant nutrient management in biogas plants in Latvia

Abstract

Pollution resulting from agricultural activities, including biogas plant operation, accounts for majority of phosphorus and nitrogen compounds that reach the Baltic Sea after being discharged into rivers. The number of agricultural biogas plants in Latvia reached its maximum number of 50 in 2017, and decreased to 43 plants in 2021, due to the reduction of state aid for mandatory procurement of electricity. To assess the potential risks of environmental contamination with plant nutrients and to identify the possible nutrient losses 22 biogas plants were inspected, and 98 samples of raw materials and fermentation residues were collected and analysed in a certified laboratory. In the surveyed biogas plants, the input biomass consisted of silage (31%), various types of manure (58%), food industry residues (8%), and sewage sludge 3%. Silage analyses show that 39% of samples had dry matter content less than 30%, indicating an increased risk of silage effluent runoff. The carbon-nitrogen (C: N) ratio in most of input biomass and digestate was below the optimum value range (1:25 1:35), indicating the risk of inhibition of the anaerobic fermentation and risk of gaseous ammonia emissions from both biogas fermenters and digestate storages. Comparing the content of plant nutrients in the separated and dried fractions of digestate, an increased risk of nitrogen loss due to the evaporation of volatile nitrogen compounds during the drying process was determined. Improvements of plant nutrient management may include addition of raw materials having high C:N ratio, e.g., straw, other lignocellulosic materials in input substrate, covering the silage bunkers and digestate storages with a plastic layer with increased gases impermeability.