Reducing greenhouse gas emissions from pig slurry by acidification with organic and inorganic acids

Abstract

Methane (CH4) emission from pig slurry is a large contributor to the climate footprint of livestock production. Acidification of excreta from livestock animals with sulfuric acid, reduce CH4 emission and is practiced at many Danish farms. Possible interaction effects with other acidic agents or management practices (e.g. frequent slurry removal and residual slurry acidification) have not been fully investigated. Here we assessed the effect of pig slurry acidification with a range of organic and inorganic acids with respect to their CH4 inhibitor potential in several batch experiments (BS). After careful selection of promising CH4 inhibitors, three continuous headspace experiments (CHS) were carried out to simulate management of manure in pig houses. In BS experiments, more than <99% CH4 reduction was observed with HNO3 treatment to pH 5.5. Treatments with HNO3, H2SO4, and H3PO4 reduced CH4 production more than acetic acid and other organic acids when acidified to the same initial pH of 5.5. Synergistic effects were not observed when mixing inorganic and organic acids as otherwise proposed in the literature, which was attributed to the high amount of acetic acid in the slurry to start with. In the CHS experiments, HNO3 treatment reduced CH4 more than H2SO4, but increased nitrous oxide (N2O) emission, particularly when the acidification target pH was above 6, suggesting considerable denitrification activity. Due to increased N2O emission from HNO3 treatments, HNO3 reduced total CO2-eq by 67%, whereas H2SO4 reduced CO2-eq by 91.5% compared to untreated slurry. In experiments with daily slurry addition, weekly slurry removal, and residual acidification, HNO3 and H2SO4 treatments reduced CO2-eq by 27% and 48%, respectively (not significant). More cycles of residual acidification are recommended in future research. The study provides solid evidence that HNO3 treatment is not suitable for reducing CO2-eq and H2SO4 should be the preferred acidic agent for slurry acidification.