Within-week Turning Frequency May Not Strongly Impact In-vessel Food Waste Composting

Composting is one way to reduce landfill inputs, while simultaneously producing a soil amendment which can act as a fertilizer and moisture moderator. Composting can occur outside in piles or windrows or in a closed, in-vessel system such as an Earth Tub™. In-vessel systems are purported to provide better control of temperature and moisture conditions to facilitate the composting process. Earth Tub™ vessels have screw augers for mixing and aerating materials, which is important for efficient thermophilic composting. The goal of this research was to determine if daily mixing is required to effectively compost pre- and post-consumer food waste feedstock with a wood chip bulking agent. A turning frequency (treatment) of 3 days/week compared to 7 days/week was evaluated throughout two vessel filling and composting processes. Temperature, pH, electrical conductivity (EC), and moisture content were measured weekly during vessel filling followed by a final 30-day composting interval during which no new inputs were added to the vessels. Additionally, compost C:N ratio and water soluble carbon concentrations were measured at the conclusion of composting. Moisture ranged from 36% to 68% with the 7 day/week and 37% to 68% with the 3 day/week treatment. The pH ranged from 5.0–8.3 with 7 day/week and 4.5–7.8 with 3 day/week turning, while EC ranged from 1.5–8.2 mS/cm with 7 day/week and 1.0–7.2 mS/cm with 3 day/week turning. Moisture, pH, and EC all vary throughout time but, across the two mixing frequencies, moisture, pH, and EC were similar in the last weeks of composting. The 3 day/week treatment did reach 55°C for a period of 28 days during one run; however, the 7 day/week treatment only crossed the 55 °C threshold at only two sampling points during the same run. Composting in Earth Tubs™ does not appear to be strongly affected by a 3 day/week compared to 7 day/week turning frequency, reducing labor requirements, which may have implications for sustainable commitment of volunteer groups involved in composting. For in-vessel composting of food waste feedstock with wood chip bulking agents, optimization of parameters other than aeration frequency may be required to facilitate effective thermophilic compost processing and to produce a high quality final compost product.