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In the Dutch Hercules project feces and urine from pigs are collected separately and treated in two processes: feces are composted and urine is concentrated by water evaporation. Water unsaturated exhaust air from the pig house is used for evaporation in a packed bed scrubber. The urine is acidified with nitric acid to scrub ammonia from the air. In this way a concentrated N/K fertilizer is produced from urine and ammonia is removed from the pig house exhaust air. Experiments were conducted using a pilot scale packed bed scrubber in an experimental pig house. The ammonia scrubbing efficiency and the evaporation rate were determined during one pig fattening round of 112 days.

The average composition of the collected pig urine was 4.4 g nitrogen/kg, 6.6 g potassium/kg and 0.03 g phosphorus/kg. The volume of the urine was reduced by a factor 6-7 resulting in a concentrate composition of 91.3 g/kg total nitrogen (including nitrogen added as nitric acid), 46.5 g/kg potassium and 0.2 g/kg phosphorus. The mean evaporation rate was 28 kg/day at inlet air conditions of 74% relative humidity and a temperature of 19C. The inlet air of the scrubber was more humid (74% instead of 60%) than expected. This resulted in a evaporation rate that was below expectations. The ammonia scrubbing efficiency was determined 5 times and ranged from 68 to 95%. The efficiency decreased at higher salt concentrations of the urine.