Effects of screen size and aeration on volatile fatty acids and Escherichia coli during storage of dairy manure slurry.

Owing to intensive dairy farming, large amounts of animal waste accumulate in small areas, leading to various environmental problems. Solid separation technology and subsequent treatment of animal manure can facilitate manure handling, including composting of the solid fraction and land application of the liquid fraction. Removal of solids from manure slurry is expected to reduce the source of odorants; however, the specific compounds effectively mitigated remain unclear. In this study, we evaluated the effects of separation using screens with three different mesh sizes (10, 2, and 0.5 mm) on the production of volatile fatty acids (VFAs), representative odorants of dairy manure slurry, and on the persistence of Escherichia coli in raw and aerated liquid fractions stored for 45 days. We found that separation effectively reduced VFAs, particularly low-odor-threshold compounds (n-butyric and n-valeric acids). However, separation was not effective in reducing branched fatty acids (i-butyric and i-valeric acids), probably because these compounds originate from fine particles (<0.5 mm) containing protein. Separation significantly reduced E. coli counts during storage. The liquid fraction obtained using a 0.5 mm screen exhibited E. coli counts below detection limit in both raw and aerated treatments. This study provides evidence supporting the use of fine screen sizes to significantly reduce VFAs, which impact the olfactory sense and prevent spreading E. coli into the wider environment through slurry application.