Ammonia losses from the storage and application of raw and chemo-mechanically separated slurry

Abstract

The effect of combined chemical–mechanical pig slurry separation on ammonia (NH₃) emissions was investigated in a field scale study. We determined the amount of volatized ammonia emitted from the solid and liquid fractions of raw pig slurry during its storage and after its broadcast to an alfalfa (Medicago sativa L.) meadow under summer and winter conditions. The solid fraction accounted for the largest NH₃ losses during both storage and land application. Chemical–mechanical separation of raw pig slurry under winter conditions caused a slight (−2%) reduction in NH₃ emissions during manure management (storage + broadcast application), as opposed to an emission increase of as much as 17% for separation under summer conditions. Evidence from this study suggests that environmental benefits are possible if solid and liquid fractions are managed using state-of-the-art NH₃ mitigation options, such as covered manure storage and band spread slurry application.

Introduction

Pig farms in Italy produce approximately 17 million tonnes per year of liquid (slurry) manure (Colonna and Alfano, 2010) that is commonly recycled into crop production as fertilizer. Although a good source of plant nutrients, slurry requires careful management, including disposal, to reduce its potential negative impacts on environmental and human health. In areas of high livestock density, animal excreta may lead to stream and ground water eutrophication from increased nitrogen (N) and phosphorous (P) concentrations (EEA, 2005). Manure storage and land application can also emit significant ammonia (NH₃) and greenhouse gases (GHG), namely carbon dioxide (CO₂), methane (CH₄), and nitrous oxide (N₂O) (FAO, 2006).

The European Union Nitrate Directive (91/676/EC), in an effort to protect the environment, mandates that the animal manure nitrogen (N) spread rate in “nitrate vulnerable zones” not exceed 170 kg ha⁻¹ y⁻¹, which implies that land is limited and transport distances are greater than is actually the case. Therefore, improved slurry handling through its separation into two fractions, one liquid and one rich in total solids (TSs) and nutrients, presents a reasonable solution to the costs and management transfer issues of untreated slurry (Petersen and Sørensen, 2008). Several separation techniques have been developed to reduce the nutrient content of slurry (Hjorth et al., 2010). Traditionally, solid–liquid separation of pig slurry has been performed mechanically by sedimentation, filtration, centrifuge, or drainage. However, most of the nutrients (such as N and P) found in pig slurry are suspended in small (<0.5 mm) particles (Zhang and Westerman, 1997) and mechanical separation cannot remove them easily (Hill and Tollner, 1980). Slurry separation can be made more efficient by combining chemical (e.g., additives such as bentonite or polyacrylamide – PAM) with mechanical techniques (Sievers et al., 1994, Hjorth et al., 2010). Pereira et al. (2005) reported an approximate reduction of 50% in total solid (TS) content when raw slurry was separated by screw press; a 70% reduction was observed when PAM flocculant polymers were added/combined to the mechanical treatment. The addition of PAM polymers to raw pig slurry increased the separation efficiency of TS, total N (TN), and P as much as 92%, 47%, and 91%, respectively (Martinez-Almela and Barrera, 2005, Balsari et al., 2008a).

Combined treatment technologies like this have been met by Italian farmer approval and are rapidly spreading throughout the country. The effect of slurry separation on NH₃ and GHG emissions from the separated fractions compared to the raw slurry is controversial. Studies conducted by Amon et al. (2006), Dinuccio et al. (2008), and Fangueiro et al. (2008) found high NH₃, CO₂, and NO₂ emissions during solid fraction raw slurry storage. Moreover, Nyord et al. (2008) and Dinuccio et al. (2011) claimed that high nitrogen losses (as NH₃) might occur from the liquid fraction of separated slurry, especially during land application. Balsari et al. (2008b), however, found NH₃ emissions fell as much as 26% after field application of cattle slurry fractions. A year later, Balsari et al. (2009) observed that compared to raw pig slurry, cropland application of liquid and solid fractions reduced NH₃ emissions up to 48%. Little information is available on GHG and NH₃ emissions generated during handling (i.e., storage + land application) of the solid and liquid fractions obtained by chemical–mechanical separation of raw slurry.

This study evaluated the effect of a chemical–mechanical separator (SELCO Ecopurin^®) on NH₃ emissions from pig slurry under the most common Italian manure management practices—uncovered solid and liquid manure storage and surface application by broadcast. Ammonia emissions were determined from the raw slurry and its liquid and solid fractions during storage and surface application by broadcast to alfalfa (Medicago sativa L.) meadow during winter and summer conditions.