The objective of this study was to monitor the degradation and obtain the mineralization fraction of anaerobically digested sludge, also known as digestate, under field conditions, when applied to the surface or incorporated into the soil. Sludge was applied to a dystrophic Inceptisol at a dose of 500 kg ha–1 yr–1 of total nitrogen, where the monitoring period of the mineralization process lasted 131 days. Samples of the soil-residue mixture were collected for analysis of the total organic carbon (TOC) and easily oxidizable organic carbon (OOC), total, ammonia, nitrate and organic nitrogen (ON). The annual mineralization fractions of the digestate, estimated based on the difference between the initial and final contents of TOC, OOC and ON in samples of the material collected, were 99.5 and 100%, respectively, when incorporated with the soil or applied to the soil surface.
This study aimed to monitor degradation of compost from undigested sewage sludge disposed on soil surface or incorporated it. The sewage sludge was blended with wood sawdust during a 90-day composting to achieve a C/N ratio of 9. Each waste was dosed based on its total content of nitrogen and on an annual dose of 500 kg ha -1 , which is broadly recommended for forage fertilization. After incorporation and surface applied (five replicates each), waste mineralization was monitored for 131 days by samplings taken from both conditions for further analyses. The samples were measured for total organic carbon (TOC); easily oxidizable carbon (OOC); total nitrogen (TN); ammonia and nitric nitrogen, and from them the total organic nitrogen (ON); volatile solids; and water content. The findings pointed to a decrease in TOC, OOC and ON contents throughout monitoring, regardless of disposal method, however, most remarkable whether incorporated to the soil, computing mineralization rates above 87%.
Little is known about the degradation kinetics of sewage sludge, sanitized with Ca(OH)2, in soil. Thus, this work aimed to monitor the degradation of limed sludge, under field conditions, when applied on soil surface or incorporated into the soil. The limed sludge was applied in dystrophic Inceptisol at a dose of 500 kg ha -1 yr -1 of total nitrogen. The mineralization process in the soil was monitored for 131 days. Samples of organic material were collected for analysis of total and easily oxidizable organic carbon; total, ammonium, nitric and organic nitrogen; volatile solids; and water contents. The undigested secondary sewage sludge generated in wastewater aerobic treatment mineralizes faster when limed and incorporated, compared to being arranged on soil surface. The estimated annual mineralization fraction of the limed sludge was 100% and greater than 95%, when incorporated or arranged on soil surface, respectively. Such values are higher than those established in the Brazilian environmental legislation for undigested sewage sludge disposal on soil.
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