Dose effects of Mg and PO4 sources on the composting of swine manure

A series of experiments were performed to study the effects of magnesium (Mg) and phosphate (PO4) addition on the decomposition of organic matter and struvite formation during the composting of swine manure. In these experiments, different amounts of magnesium chloride (MgCl2) with or without PO4 salts were added to the samples, and the optimum molar ratio of Mg and PO4 to the total nitrogen (TN) in swine manure was studied. The temperature profiles revealed that no organic matter was decomposed when 0.1 M or more Mg and PO4 salts were added and that when 0.05 M ratio of Mg and PO4 salts was added, the addition inhibited the composting process. However, the thermophilic condition was readily established if only Mg salt was added. Further, the addition of only Mg salt easily established an optimum thermophilic condition even at a higher level. The highest amount of NH3 was emitted when no Mg or PO4 salts was added, and the rate of NH3 emission was inversely proportional to the salt level. The orthophosphate/total phosphorus ratio (OP/TP) proportionally increased with the PO4 amount added before composting but decreased after composting. In contrast, when only Mg was added, the OP/TP ratio proportionally decreased before composting and increased after composting. However, when more than 0.075 molar ratio of Mg was added, the OP/TP ratio decreased after composting; this result was identical to that obtained when both Mg and PO4 salts were added. The decrease in the OP/TP ratio before or after composting could be due to the formation of magnesium ammonium phosphate (MAP) crystals. The study revealed that adding less than 0.05 M of both Mg and PO4 salt or more than approximately 0.07 molar ratio of Mg with regard to the TN content was acceptable for the proper decomposition of organic matters and the continuous formation of struvite during the composting of swine manure.