Addition of seaweed and bentonite accelerates the two-stage composting of green waste

•Seaweed (SW) and bentonite (BT) were the additives in green waste (GW) composting.•SW and/or BT addition enhanced the degradation and the humification of GW composting.•Water retention, porosity, respiration rate, enzymes, and nutrients were optimized.•Combination of 35% SW and 4.5% BT reduced the two-stage composting time to 21 days.

Green waste (GW) is an important recyclable resource, and composting is an effective technology for the recycling of organic solid waste, including GW. This study investigated the changes in physical and chemical characteristics during the two-stage composting of GW with or without addition of seaweed (SW, Ulva ohnoi) (at 0, 35, and 55%) and bentonite (BT) (at 0.0, 2.5%, and 4.5%). During the bio-oxidative phase, the combined addition of SW and BT improved the physicochemical conditions, increased the respiration rate and enzyme activities, and decreased ammonia and nitrous oxide emissions. The combination of SW and BT also enhanced the quality of the final compost in terms of water-holding capacity, porosity, particle-size distribution, water soluble organic carbon/organic nitrogen ratio, humification, nutrient content, and phytotoxicity. The best quality compost, which matured in only 21 days, was obtained with 35% SW and 4.5% BT.