Two-stage separation and acidification of pig slurry - Nutrient separation efficiency and agronomical implications.

Separation of slurries can facilitate the nutrient management on farms through nutrient partitioning between the liquid and the solid fraction. The distribution of nutrients in the slurry fractions depends largely on the type of separator used. The current study assessed the separation efficiency of a two-step separation treatment of pig slurry including in-series a screw press and a centrifuge followed by acidification (to pH 5.9) of the final liquid effluent. The system concentrated 73.8% of the slurry's Phosphorus (P) content, 52.6% of Total solids (TS) and 14.4% of total Nitrogen to the solid fraction. The apparent N recovery from ryegrass fertilized with the raw slurry and non-acidified liquid fractions was not decreased by the separation treatment. The acidified liquid fraction showed 28% and 9% higher apparent N recovery compared to the raw slurry and the non-acidified liquid effluent from the centrifuge respectively. The biochemical methane production potential (Bo) of the acidified liquid fraction was reduced by 50% and 25%, compared to the non-acidified counterpart and the raw slurry, respectively. The results highlight the potential of a double separation system coupled with acidification of the liquid fraction, to extract P into a solid fraction which can be transported outside the farm, and to increase N utilization from the liquid fraction when this is used as organic fertiliser on or nearby the farm. The study further highlights the potential to reduce CH4 emissions from slurry storage after mechanical separation and acidification of the liquid slurry fraction.

Thermal drying of the solid fraction from biogas digestate: Effects of acidification, temperature and ventilation on nitrogen content.

Drying of solids produced from digestate is prone to N losses through NH3 volatilisation. The applicability of acidification as an NH3 emission mitigation technique during the drying of solids from digestate was assessed in a drying experiment. Operating conditions comprised four drying temperatures (70-160°C), two air ventilation rates (natural, 420ml/min) and three pH levels (9.2, 6.5 and 5.5) of the solids, modified by the addition of concentrated sulphuric acid. Acidification of the solids from digestate significantly decreased the NH3 emission during drying, irrespective of the drying conditions. A parallel decrease in the organic nitrogen content and an increase in the ammonium content of the solids was observed after acidification of the solids. It was confirmed that acidification before thermal concentration of solids from digestate, minimised NH3 losses under a wide range of drying conditions.