Looking to the future, we need to recycle more, burn less, while staying safe. The science on land-based recycling of organic wastes, indicates that soils can be amazingly resilient and helpful for us. Indeed, they are already providing services that we do not appreciate and have only just started to understand.
In the developing circular economy, we face questions on how best to manage the societal organic waste products, that inevitably contains unwanted components, such as micro-plastics, medicinal residues, heavy metals, pathogens and e.g. antibiotic resistance.
Common sense would suggest a precautionary approach, entailing advanced technical solutions, often involving incineration. However, when considering agricultural land application of animal wastes with high contents of pathogens, that has been going on for centuries, there is a cause for pause. How is it possible that humanity has survived the enormous routine application of pathogens to the land that gives us our food?
In a recent study comparing risks associated with contemporary conventional animal manure and sewage sludge we assessed risk factors mentioned above, considering the impact on the soil environment. We also considered human health impacts of antibiotic resistance in soils, and transmission of medicinal residues and heavy metals through edible plants. The main conclusion of this study was that the risk associated with agricultural use of Danish sewage sludge is comparable to that of pig slurry, once the EU limits for Zn and Cu addition to pig feed have been fully implemented.
Since 2003 we have systematically applied waste materials annually, in a long-term experiment on Copenhagen University’s experimental farm. Some treatments were applied in both high and unrealistically high rates to test soil resilience to be able to assess cocktail effects of unwanted component in waste.
Despite exceeding legal application limits manyfold (>200 years), we have so far been unable to identify stress reactions in the soil. We have observed that addition of composted household waste and sewage sludge has resulted in several positive effects, such as increase in soil organic matter, increases in soil biota across the food web, with no negative effects on biodiversity. We found that effects on microbial antibiotic resistance appear to be short lived, and only small increases in soil heavy metal content and no increase in crop uptake of heavy metals.
While not all soils can be expected to show a similar resilience, and ability to process unwanted components, we believe that there is cause to celebrate that soils are quietly helping us. We understand that the relative strength of soils in this respect varies according to the mineralogy, the soil pH and the organic matter content, but we need to better understand the limits to their capacity as ‘helpers’, not least for soils that are less resilient.
Advanced technical waste incineration solutions are needed but are economically expensive and come with environmental costs that are not well understood. High temperatures cause an almost complete loss of carbon, nitrogen and sulfur, that are needed in agricultural systems, but also a decline in the quality of other nutrients, such as phosphorus. There is no current balancing of the risk avoidance with the costs mentioned above, economic costs, and human health costs related to greenhouse gas and fine particle emissions related to incineration.
Occasionally, warranted public health concerns catches the eyes of the press, as has recently happened in Denmark, where the finding of concerningly high PFAS content in free grazing cattle gave rise to a media storm. The reaction from the Danish Ministry of Environment has been to impose the strictest limits on acceptable concentrations in drinking water in the world – limits that are well below what is commonly found in rainwater, as well as very strict limits on what is acceptable in sewage sludge. When calculating the amounts of measurable PFAS in the sewage sludge applied to Danish farmland, I found that a conservative estimate yielded less than 2 kg total annually. By contrast the Danish national metabolism of PFAS has been estimated to be in the range of 20-40 tons. It my considered opinion that the regulation on sewage sludge, while expensive, will have no measurable effect on the environmental load of PFAS.
This is an example of a case where the precautionary principle has been applied, without considering the system effects, as has been a strong criticism from notabilities within sociology and law e.g. Anthony Giddens (UK) and Cass Sunstein (US). Authorities and politicians need help to balance risk versus sustainability, with as much caution as considered reasonable, considering the costs of caution. This is a complex problem, that requires input from different branches of science and should be the focus of concerted interdisciplinary research in the years to come, where the science on land ecosystem resilience will be one contributor.
Jakob Magid
Associate Professor, Department of Plant and Environmental Sciences, University of Copenhagen
Denmark
