4.2 Iria Regueiro – University of Copenhagen

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Development of combined Acidification and Separation of slurry: Effect on slurry fractions composition and gaseous emission, Iria Regueiro, University of Lisbon

There is an increasing concern about the impacts that agriculture has on the environment and on public health. Agriculture is responsible for a significant proportion of the global gaseous emissions caused by human activities, due to fertilizer use and livestock production. During the last decades, livestock production has been intensified in response to a greater demand for food from a more populated world. These intensified operations produce large amount of animal slurries which, has created the need for novel slurry management technologies and strategies.

One of the major goals of these technologies is to reduce the negative environmental impacts of these slurries whilst simultaneously recovering the nutrient and energy value of slurries. Two of these techniques, acidification and mechanical separation, are currently being implemented independently at the farm scale in different countries in Europe. Acidification of slurry is a treatment known to reduce environmentally hazardous ammonia (NH3) emissions whilst affecting physicochemical characteristics of slurries. However the main acidifying additive in use today is sulfuric acid, a strong acid that poses health hazards to livestock and farmers and results in damage to farm equipment and installations. Furthermore, acidification is often implemented before application to fields, whereas a large amount of  emissions occur during slurry storage. Mechanical separation, improves the management of slurry by concentrating nutrients into an easily transportable solid fraction and a liquid fraction rich is soluble compounds which can be used for irrigation on the farm. However, mechanical separation is typically regarded as an extra management cost rather than a profitable. Additionally, separation is known to increase the gaseous emissions from slurry.

There are drawbacks associated to the current acidification strategies used, and it is unknown how the combination of acidification and separation affect the environment and slurry nutrient value. Although these two technologies have been studied since early times as separate processes, little is known about the combination of these two techniques. Acidification affects many slurry characteristics which, in turn, could have an effect on the separation efficiency. The combination of acidification and separation is a promising issue of research which takes advantage of already existing technologies to, abate the gaseous emissions released from slurries and on the other hand, to improve the characteristics of the fractions obtained. This thesis presents an integrated study of the combined treatment of slurry by acidification and separation of slurry. This includes a systematic study of different acidification additives, target pH values and approaches for acidification as well as different separation techniques in different types of slurries. The composition of slurries in terms of dry matter, organic matter and nutrients (N and P) content and their soluble forms were analyzed and NH3 emissions were followed during storage. Although many different types of acidifying additives exist, sulfuric acid is the most commonly used.

We tested the ability of five different additives to reduce NH3 emissions in pig and dairy slurries over a storage period of 60 days. The major finding was that aluminium sulphate is a highly suitable alternative to sulphuric acid: it is safer to handle and inhibits foam formation while simultaneously reducing NH3 emissions, all at a similar cost to sulphuric acid. Next, three different acids were compared, for their potential to reduce NH3 emissions under two different acidification regimes: an initial and unique acidification to pH 3.5 and a continuous acidification to pH 5.5 using incremental additions of acid. Alum was the most effective in reducing the amount of NH3 emissions and, surprisingly, worked equally well in both acidification regimes. The combination of acidification and separation affects the characteristics of the resulting solid and liquid fractions, including the nutrient content and potential of N mineralization (PNM). In this thesis it is showed that the nutrient composition and PNM of acidified slurry fractions differ depending on the separation technique used, and that acidification is a suitable method to avoid NH3 volatilization when the screw press is used. This work also shows that acidification affects particle size distribution, nutrient distribution among particles and separation efficiency. Different additives showed different results depending on the type of slurry considered. The use of sulfuric acid increases the amount of smaller particles while the use of alum increases the amount of larger ones. The proportion of solid fraction was notably higher when alum was applied before separation. Moreover, alum also increased the P separation efficiency with an almost complete removal (90%) to the solid fraction in dairy slurry.

Finally, the effect of acidification and separation on gaseous emissions during the storage of pig and co-digested slurry fractions was studied. In both cases acidification using alum significantly reduces NH3 and greenhouse gases (GHGs) emissions during a 70 days storage period. According to the results presented here, the combination of acidification and separation improves the slurry management process: It increases the amount of solid fractions, preserves N in slurries by reducing NH3 emissions during separation, increases the P separation efficiency to the solid fraction, and reduces the total NH3 and GHG emissions during storage. The combined approach of acidification and separation presented here decreases drastically harmful gaseous emissions whilst increasing the nutrient value of slurries. Therefore, the combined technology provides farmers with an economically sound and environmentally safe approach to slurry re-use.

  • Regueiro, I., Coutinho, J.O., Balsari, P., Popovic, O., & Fangueiro, D. 2015. Acidification of Pig Slurry Before Separation to Improve Slurry Management on Farms. Environmental Technology 1-22 available from: http://dx.doi.org/10.1080/09593330.2015.1135992
  • I. Regueiro1, Magdalena Pociask, J. Coutinho, D. Fangueiro 2016. Animal slurry acidification affects slurry particle size distribution and improves separation efficiency. Accepted manuscript. Journal of Environmental Quality.
  • Alternatives for slurry acidification to decrease NH3 emissions”. Manuscript submitted to Journal of Environmental Sciences.
  • Acidification of raw and co-digested pig slurries before mechanical separation reduces gaseous emission during storage of solid and liquid fractions. Manuscript submitted to Agriculture, Ecosystems and Environment.

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