Liquid Manure And Digestate Composting

While much of the discussion about nutrient-rich runoff degradation of water quality is focused on phosphorus (see “Phosphorus and Compost Use Dynamics,” December 2016), there are significant areas in the U.S. where nitrate contamination of groundwater remains an issue. These nutrient-sensitive areas may pose some constraints to the direct land application of animal manures, sewage sludges and digestates from anaerobic digestion (AD) plants. A team of researchers at the University of Udine in Italy has examined the fate of nitrogenous compounds as part of an investigation into means of treating animal manures and digestates to meet the European Union’s 1991 Nitrates Directive, which limits land application of nitrogen nutrients to 170 kg/hectare/year (about 152 lbs/acre) in nitrate-vulnerable areas. This work was carried out at full-scale in several facilities, including four composting plants treating manure and digestate in liquid phase. In particular, tests were performed at a 1,000-head swine farm in Bosco Chiesanuova in northern Italy. The composting system at the farm is designed to handle 8,000 cubic meters (2.1 million gallons) per year of liquid swine manure. The composting system, the CLF Modil (Biogest Service, Italy), is based on the agitated bed technology with significant modifications. The composting reactor at Bosco Chiesanuova is a horizontal trough, 252 ft. (77 m) long, 33 ft. (10 m) wide and 4.3 ft. (1.3 m) deep (Figure 1). (Minimum width of the trough is 20 ft. to a maximum 65 ft., with length up to 393 ft. for a single unit, treating roughly 1,300 to 18,500 gallons/day.) In order to process the input, liquid absorbents are used, such as straw, wood chips, sawdust, corn stalks and similar carbon-rich materials. Eventually the solid fraction from liquid/solid separation can be utilized. In this research project, only straw and sawdust were used as amendment.