Nitrogen Conservation in Simulated Food Waste Aerobic Composting Process with Different Mg and P Salt Mixtures

To assess the effects of three types of Mg and P salt mixtures (potassium phosphate [K3PO4]/magnesium sulfate [MgSO4], potassium dihydrogen phosphate [K2HPO4]/MgSO4, KH2PO4/MgSO4) on the conservation of N and the biodegradation of organic materials in an aerobic food waste composting process, batch experiments were undertaken in four reactors (each with an effective volume of 30 L). The synthetic food waste was composted of potatoes, rice, carrots, leaves, meat, soybeans, and seed soil, and the ratio of C and N was 17:1. Runs R-1-R-3 were conducted with the addition of K3PO4/MgSO4, K2HPO4/MgSO4, and KH2PO4/MgSO4 mixtures, respectively; run R-0 was a blank performed without the addition of Mg and P salts. After composting for 25 days, the degrees of degradation of the organic materials in runs R-0-R-3 were 53.87, 62.58, 59.14, and 49.13%, respectively. X-ray diffraction indicated that struvite crystals were formed in runs R-1-R-3 but not in run R-0; the gaseous ammonia nitrogen (NH3-N) losses in runs R-0-R-3 were 21.2, 32.8, 12.6, and 3.5% of the initial total N, respectively. Of the tested Mg/P salt mixtures, the K2HPO4/MgSO4 system provided the best combination of conservation of N and biodegradation of organic materials in this food waste composting process.