Acidification of pig slurry to pH 5.5 is used as a measure to reduce ammonia emission from pits and storages. The slurry is acidified with sulphuric acid in a process tank and pumped back to the slurry pits or to a storage tank. We investigated the effect of acidification on microbial activity. Oxygen consumption rate, methanogenesis and sulphate reduction were all reduced by more than 98% in the stored acidified slurry compared to untreated slurry. Despite higher sulphate concentration, the microbial metabolism was greatly compromised or absent in the acidified slurry. This could be explained by the high concentration of protonized short-chained volatile fatty acids in the acidified slurry (approximately 25 mM, compared to untreated slurry <0.1 mM), which act as an uncoupling agent of the cell membrane potential and thereby arrest microbial metabolism. In total the consequences of slurry acidification are greatly reduced production rates and loss of sulphide and methane, and eliminated loss of ammonia. On the other hand, increased volatilization and loss of smelly fatty acids is to be expected.

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