Biosolids - a solution or a problem?

What are biosolids?

Biosolids, the name given to the solid output from sewage treatment plants, are seen by some as a solution to a nutrient problem, and by others as environmentally unsafe.

Historically used as a fertiliser and more recently dumped at sea, there is a view that biosolids are a missing part of the closed nutrient cycles in agriculture.

What’s the problem with using biosolids?

Organic farmers are not permitted to use biosolids due to concerns about organic contaminants, pathogenic micro-organisms and heavy metals to the soil, and more recently microplastics, with resulting long-term consequences.

Less contamination from heavy industry … but is it enough?

The RELACs project, which looks at contentious inputs into organic farming systems, has been looking at this issue from a scientific point of view via a series of webinars in April 2021. This pan-European research has shown that heavy metal concerns are less important than they were when heavy industry was a major source of contamination.

However, aerobic digestion and sewerage plants don’t breakdown many of the pharmaceutical products or manufactured chemicals present in biosolids.

Although further research showed little evidence of soil biota damage following application (due to natural soil biology playing a role in denaturing many of these products) there are still concerns about the creation of antibiotic resistance strains if biosolid treatment was of poor quality, like that from over-capacity treatment plants.

Is converting to a recycled energy source the answer to reducing contaminants?

One possible solution is the extraction of minerals such as heavy metals or the important agricultural nutrients such as P (phosphate) and K (potash) and possibly nitrates (for example Struvite MAP) leaving a product which can be burnt as a recycled energy source. But even amongst the scientists there is disagreement.

The scientific position at the webinar conference was split - for some, an expensive extraction process was deemed unnecessary because they considered it safe. For others, creating a resource which can be used in a more controlled manner, or transported in a dry form, was a positive environmental step.

What about the infrastructure required?

Replacing mined fertiliser with a recycled product does make sense, but there’s a question of whether the economics of construction of plants to carry out this process, or the increased emissions associated, is responsible or sustainable.

Would the public accept biosolids on food crops?

Another issue not covered by the research is the public perception of biosolids, and its application for growing crops for us to eat. Livestock manures are generally accepted by most of the public, but would they have the same view of biosolids?

There are already concerns about the regulation of biosolid application in the UK, even within the constraints of the biosolids matrix which requires soil analysis and restrictions on further cropping.  

These issues are very valid, particularly if allowing increased use results in a reduction in regulation.

And what about the effect of micro-plastics and even the rubber from car tyres when road runoff ends up in the treatment process?

Which leads us back to the question of what we should do with a by-product of human existence which we are not keen on talking about!

About RELACS

RELACS (Replacement of Contentious Inputs in Organic Farming Systems) is pan-European project focussing on developing cost-efficient and environmentally safe tools and technologies to phase out the use of inputs considered contentious in organic farming systems.

Find out more about our work with RELACS.