On-site composting of waste hop biomass: the impact of covering piles on leachate quantity and compost quality

Covering hop waste composting piles with semipermeable membrane after the thermophilic phase until spring, when the compost was ready to use, reduced the volume of leachate and leached nutrients amounts significantly; there was a negligible amount of leachate and low amount of leached nutrients during winter and spring at all treatments. At treatments with additives (biochar, preparation effective microorganisms) and larger percent of particles of 2–5 cm, it was indicated that composting pile should probably also be covered in the first months of composting and be opened only when turning/mixing it; on the other hand, the amount of leachate was much lower in a pile without additives and particles of 2–10 cm by the time of covering (2.2- and 2.5-fold less respectively) and the amount of leached nutrients as well. All treatments produced compost without a bad smell, had a total nitrogen content greater than 2%, could be considered as mature (the C : N ratio was below 20) and stable, and the biomass hygienisation threshold was reached. However, there were some significant differences among them. The compost of the pile with added biochar and starting hop waste biomass particles of 2–5 cm was considered phytotoxic (germination index of radish was 31%); also, its other results were less promising in comparison to other treatments. The pile with no additive and the starting particles of waste hop biomass 2 to 10 cm, mixed properly related to regular temperature measurements in the thermophilic phase, reached the best results; the leachate amount and leaked nutrients amount were significantly the lowest, while the final compost contained significantly higher amount of nutrients and had the highest germination index.