Comparison of techniques for ammonia emission mitigation during storage of livestock manure and assessment of their effect in the management chain

The reduction of ammonia (NH3) emissions associated with manure management requires identification and implementation of effective techniques. The objective of this study was to measure potential ammonia emissions from animal manure and evaluate emission reductions for five mitigation techniques (straw, sawdust, clay, oil and sulphuric acid). Although numerous studies have evaluated individual mitigation techniques, the variability of their effect with different types of slurries has not been fully investigated. Furthermore, the assessment of ammonia emissions from the subsequent land application of stored manure (or slurry) using different techniques would indicate the practical consequences of the entire slurry management chain. The effects of mitigation techniques were evaluated using a model to simulate field application of slurry. Three techniques were compared: broadcast spreading, band spreading and closed-slot injection. Simulations utilised data from experiments conducted at a controlled temperature on six slurries of three different types: pig, cattle and digestate. Ammonia emissions from the raw slurries (i.e., untreated slurry) were determined using the dynamic chamber technique and compared with those from the slurries treated using each of five mitigation techniques. A subsample of one 1 L of each slurry was transferred into 2 L plastic bottles. An airflow of 1 L min^–1 across the headspace was established and then emissions were measured over a period of 24 h. The air outlet was connected to two serial acids traps filled with 1% boric acid. The quantity of NH3 trapped was determined by titration. Acidification and oil addition were the most effective techniques, reducing ammonia emission from raw slurries by more than 95% and 80%, respectively. The mitigation effects of straw and sawdust were higher for cattle slurry and digestate than for pig slurry, while clay had an opposite effect. The overall assessment of ammonia emissions from storage and subsequent field application showed that acidification followed by closed-slot injection emitted at most 12% of the emissions from the reference system, while emissions from acidification followed by band spreading were between 14% and 22% of those from the reference system. The latter appears to be both more effective than broadcast spreading and technically more easily operated than a closed-slot injector.