Increased aeration for improved large-scale composting of low-pH biowaste Förbättrad storskalig kompostering av bioavfall med lågt pH-värde genom ökat luftflöde

Biowaste composting at several plants in Scandinavia has been troubled by low pH in the collected waste as well as after the composting process. Our hypothesis was that increased aeration would give a higher and faster rise in pH during the composting process, and that this would give a higher decomposition rate. The objective was to test this hypothesis by experiments in full scale, with an emphasis on the role of temperature in the transition from acidic to neutral pH. Experiments were carried out at two large composting plants. At one plant the temperature and CO2-concentration were similar during the early processes regardless of aeration rate. At higher aeration rates the pH, decomposition rate and stability became higher. At the other plant higher aeration rates during the first two weeks resulted in much higher pH and intensive decomposition. That compost also received a large water addition to keep the compost moist, and the product from the process was stable. At both composting plants, increased aeration rates at the start of the process resulted in higher microbial activity and a rapid rise in pH. The increased aeration also gave a more stable end product. The main recommendation for process improvement is therefore to increase the aeration. Increased aeration at the start improved the decomposition, but increased aeration later in the process may also be important. Increased aeration lead to severe drying of the compost. Heat is produced by microbial activity, and is transported from the compost with the air, mainly by evaporation. Drying is thus a result of decomposition activity. If the aeration is reduced in order to avoid drying of the compost, the activity is also reduced and the product will not become stable. The recommendation thus is to keep the compost moist by adding water, not by reducing aeration.