Pensamiento crítico, acercamiento semántico

TOMLINSON, S.J.1_{, SCHMIDT-HANSEN, A.}2_{, SUTTON, M.}1_{, TANG, Y.S.}1_{, DRAGOSITS, U.}1

1 _{Centre for Ecology & Hydrology, Edinburgh, UK; 2 University of Edinburgh, Edinburgh, UK}

INTRODUCTION

Anaerobic Digestion (AD) is an organic treatment technology for biological materials to produce energy and heat from biogas and nitrogen-rich (N) fertiliser by-products known as digestates. The European Commission (EC, 2016) estimated biogas production to be in the region of 15 Mt of oil equivalent at the start of 2015, with over three quarters of the activity taking place in Germany, the United Kingdom (UK) and Italy. Digestate production through AD can act as a closed loop recycling system for mineral nutrients such as N, its application to fields can restore soil organic matter and production can be a sustainable practice (Tiwary et al., 2015). However, due to the higher fraction of ammoniacal N in digestates and their (usually) elevated pH, there is a higher potential for N losses

through ammonia (NH3) volatilisation, compared with undigested materials.

NH3 emissions in Europe, which are responsible for the eutrophication and acidification of ecosystems, are

dominated by the agricultural sector. In the UK, the AD industry has experienced rapid growth from around 32 plants in 2009 (processing circa 1 Mt of materials) to 400 plants at the end of 2016 (processing over 10 Mt of materials), with sectoral growth in the UK aided through government schemes. Given the rapid growth and large

potential for NH3 emissions, research has been carried out to quantify UK emissions from AD in detail, especially

with regard to the impact of AD sources on the UK’s National Emissions Ceilings. Emissions estimates are required to inform future policy surrounding the AD sector and are vital for developing effective mitigation strategies. MATERIAL AND METHODS

Information on AD sector activity in the UK was compiled regarding site throughput, materials processed and locations (Tomlinson et al., 2017). The data were processed to remove information not considered relevant to

NH3 emissions calculations due to their very large volumes and very low N content – primarily brewery/distillery

effluents. NH3 emissions were calculated for activity at the AD plant itself (site-based processes) and also for the

subsequent landspreading of digestates using emission factors (EFs) as shown in Table 1 (Bell et al., 2016; Nicholson et al., 2017).

Table 1. Emission Factors for Ammonia Emissions Estimates. Emission Factors (EFs) used (with ranges) in various stages of anaerobic digestion (AD) process to calculate ammonia emissions estimates.

Anaerobic Digestion: Stage and Activity EF Unit Range

Site-based Pre-digestion storage 0.005 kg NH3 t-1 0.003 – 0.008

Digestion of materials 0.004 feedstock 0.002 – 0.006 Post-digestion storage 0.059 fresh weight 0 – 0.24 Landspreading Food only digestates 0.83 kg NH3 t-1 -

Crop/slurry/mixed digestates 2.13 digestate 1.82 – 2.43

NH3 emissions were subsequently spatially distributed in the UK onto suitable land use types (arable and improved

grassland) within a varying distance of each AD plant, using a function of the quantity of digestate produced. RESULTS AND DISCUSSION

Figure 1 shows the estimated total NH3 emissions from AD in the UK from 1990 to 2016. NH3 emissions estimates

for 2016 are 9.8 kt (range 8.3 kt – 12.3 kt), roughly double the estimate for 2013. It is estimated that around 90%

of NH3 emissions from the AD sector are from the spreading of digestates to land, due to the higher ammoniacal

N content. The strength of emissions varies, due to, for example, some plants specializing in processing large amounts of high-N food waste materials.

Figure 1. Estimated Emissions of Ammonia from Anaerobic Digestion in the UK. Estimated emissions of ammonia (NH3) from

anaerobic digestion (AD) in the United Kingdom from 1990 to 2016 in kilotons (both site-based process emissions and digestate field spreading emissions). Low and high estimates indicated by error bars (grey).

There are a number of uncertainties that may effect NH3 estimations, including robust quantification of the

methods by which digestate is applied to land (e.g. shallow injection vs. band spreading), the wide ranging effects that co-digestion of differing materials may have on N-content and pH, and what type of soils and land cover digestate is applied to. Furthermore, mitigation strategies such as the acidification of digestates (to lower the

volatilization rate of NH3) and better soil incorporation techniques may have a large reduction effect in NH3

emissions to the atmosphere. Work is being undertaken to explore the effect some of these mitigation strategies may have on current and future estimates.

CONCLUSION

Emissions of NH3 from AD in the UK are a growing trend, with potentially a further 5 Mt of materials going to

digesters in 2018, in addition to the current 10 MT. This may be an issue for the UK’s ability to meet it national emissions ceilings, but could be addressed with targeted mitigation strategies. The UK Clean Growth Strategy

states a desire for the UK to develop its bioeconomy but this must be done with forethought to future NH3

emissions. REFERENCES

Bell, M. W., Tang, Y. S., Dragosits, U., Flechard, C. R., Ward, P. and Braban, C. F. (2016). Ammonia emissions from an anaerobic digestion plant estimated using atmospheric measurements and dispersion modelling, Waste

Management, 56, 113–124.

European Commission, 2016. Optimal use of biogas from waste streams. An assessment of the potential of biogas from digestion in the EU beyond 2020, viewed 26 Jan 2018,

Nicholson, F., Bhogal, A., Cardenas, L., Chadwick, D., Misselbrook, T., Rollett, A., Taylor, M., Thorman, R. and Williams, J. (2017). Nitrogen losses to the environment following food-based digestate and compost applications to agricultural land, Environmental Pollution, 228, 504-516. doi: 10.1016/j.envpol.2017.05.023

Tiwary, A., Williams, I. D., Pant, D. C. and Kishore, V. V. N. (2015). Assessment and mitigation of the environmental burdens to air from land applied food-based digestate, Environmental Pollution, 203, 62–270. doi:

10.1016/j.envpol.2015.02.001

Tomlinson, S.J., Carnell, E.J., Tang, Y.S., Sutton, M.A. and Dragosits, U., 2017, Ammonia emissions from UK non- agricultural sources in 2016: contribution to the National Atmospheric Emission Inventory. Edinburgh, NERC/Centre for Ecology & Hydrology, 23pp.