Environmental impact of fertilization with biogas digestates : studies of inorganic and organic contaminants

Abstract

Sustainable and safe energy and food production are important issues affected by climate change. Biogas production is among the technologies with a potential to reduce negative effects on the environment that increased energy production and consumption may have while making use of organic waste materials. Use of biogas to cover energy needs is increasing in Europe, but it is also followed by the production of an organic residue called biogas digestate. Biogas digestates have a potential as sustainable fertilizers in agricultural production due to a high content of plant-available nutrients. However, in Norway many/some farmers have concerns about the use of digestates due to limited confidence mostly in their chemical safety and thus possible consequences for future crop production. The most common feedstock for biogas production in Norway are sewage sludge and food waste from households and industry. The main goal of this thesis is to contribute to an increase in knowledge regarding environmental aspects of the use of biogas digestates as organic fertilizers in cereal production. The thesis addresses the safety of digestates with focus on potentially harmful compounds and their fate when applied on agricultural land in a field experiment (Papers 2, 3 and 4) and leaching studies (Papers 1 and 2). Both inorganic and organic pollutant groups were put in focus. In Paper 3, the focus was on trace elements in digestates and their soil content and variability. In addition, we have also investigated a potential for uptake in grains when applying digestates at a fertilizing rate of 100 kg plant-available N ha-1. The use of digestates as fertilizers was compared to cattle manure, mineral fertilizer and a control without fertilizer addition, with respect to the presence and distribution of the trace metals Cd, Cu, Zn, Ni, Cr, Mn and Mo, as well as Al in the soil and plant. The results of the study showed that despite an increase in total concentrations, there was no sign of increased plant uptake or increased mobility of these elements that would be considered to have an adverse effect to soil/water environment. As the digestates applied in the field experiment had high concentrations of aluminium (Al) and chromium (Cr), Paper 2 concentrates on the fate of these two elements both in the field and in leaching studies simulating heavy rainfall events. Results showed an increase in total concentrations for both metals on plots fertilised with digestates, but also a significant increase of water-soluble Al. Modelling free metal concentrations under field conditions using WHAM 7.0 software, suggested that an environmental risk due to high Al mobility has to be considered. Our study also showed that the use of digestates with high Al and Cr content was still comparable to the use of animal manure with respect to leaching. Paper 1 focuses on copper (Cu), zinc (Zn) and nickel (Ni) leaching potential from digestates under simulated heavy rainfall conditions shortly after application. Besides using the two commercial digestates, we have also included three experimental digestates that were produced from animal manure alone, or different mixtures of animal manure and whey with or without fish industry waste. The mobility and release of Cu, Ni and Zn were investigated in three differently textured soils (sand, silt, loam). The effect of digestates was compared to that of fresh manure, mineral fertilizer and an untreated control. In loam and silt, the addition of commercial digestates with higher original Ni, Cu and Zn content showed less environmental impact in terms of leaching than that treated with experimental digestates with lower original metal content and less than when animal manure or mineral fertilizer was applied. The final paper included in this dissertation is focused on organic pollutants, more precisely on polar organic pollutants and their presence in biogas digestates as well as their fate/concentration in the soil during the growing season (Paper 4). This study investigated the occurrence and fate of 21 micropollutants (POM), divided into perfluoroalkyl acids (PFAAs) as the most persistent organic contaminants and non-perfluoroalkyl compounds, mostly selected based on the screening studies and available methodological support. Overall, the concentration of these compounds in the two food waste based digestates was low compared to studies on other organic fertilisers. Use of biogas digestates at the selected fertilizer rate to grow cereal crops did not contribute significantly to the POM concentrations in soil. Measured concentrations of all POMs in soils after application of biogas digestates were consistent with theoretical estimates based on the concentrations determined in biogas digestates. Among the organic pollutants determined, only two (PFOA and 1-Methyl-1H-benzotriazole) were taken up into grains independent of fertilizer treatments, but concentrations were below 15 ppb. In conclusion, the effects of applying biogas digestate as organic fertilizer were comparable to the more common use of animal manure, with respect to trace element mobility, and grain uptake. Although some digestates contained high amounts of metals, such as Cr or Ni, all the soils have shown a capacity to hinder metal leaching. Polar organic pollutants have been detected in both of the used digestates, but also in animal manure, and were, with two exceptions, not taken up into edible plant parts.

Bærekraftig og sikker mat- og energiproduksjon, er de to viktige områder som er påvirket av klimaendringene. Biogassproduksjon er en av de teknologiprosessene som har et potensiale til å redusere de negative effektene økt energi- og matforbruk har på miljøet fordi den bruker organisk avfall som en ressurs. Bruken av biogass for å imøtekomme dagens energibehov er økende i Europa, men med den produseres samtidig en økende mengde organisk avfall, en såkalt biorest. Denne bioresten har et potensiale som bærekraftig gjødselmiddel i agronomisk planteproduksjon fordi den har et høyt innhold av plantetilgjengelige næringsstoffer. I Norge er imidlertid fremdeles en del bønder skeptiske til bruk av biorest som gjødsel. Skepsisen har sitt opphav i manglende kunnskap om kjemisk og mikrobielt innhold i biorest og dermed hvordan de vil påvirke planteproduksjonen i framtiden. De vanligste råstoffene for biogassproduksjon i Norge er kloakk-slam og organisk avfall fra husholdninger og industri. Hovedmålet med denne avhandlingen er å bidra med økt kunnskap om miljøaspektene knyttet til bruk av biorest som organisk gjødsel i kornproduksjon. I avhandlingen blir sikkerhetsaspektet belyst gjennom et fokus på potensielt skadelige innholdsstoffer og deres skjebne når biorest blir tilført jordbruksjord i feltforsøk (Artikkel 2, 3 og 4) og i utvaskingsforsøk (Artikkel 1 og 2).