Biorenewable Fuel and Fertiliser: Realising the Potential
Does the Science Address the Risks?
Professor Stephen R Smith, Department of Civil and Environmental Engineering,
Imperial College London, SW7 2AZ, UK.
Tel +44 (0)20 75946051; Email: s.r.smith@imperial.ac.uk
Biorenewable fertilisers can often contain larger concentrations of heavy metals, organic
contaminants and pathogenic microorganisms compared to the soil environment. Therefore
understanding the fate and significance of these during treatment processes and in the soil
is essential for the long-term security and sustainability of land applying biorenewable
fertiliser materials.
Extensive amounts of research have been completed on all three groups of contaminants.
Source controls and improved environmental practices have also generally had a marked
impact reducing the concentrations of chemical contaminants in biorenewable fertilisers and
this has been a significant step towards underpinning the long-term sustainability of land
application. Nevertheless concerns remain about the potential long-term consequences of
applying chemical contaminants to soil in biorenewable fertilisers. For instance, impacts of
heavy metals have been demonstrated, but these are usually associated with the application
of highly contaminated materials to soil. A review of scientific evidence shows that low metal
materials have intrinsically low bioavailability and could be applied indefinitely to land without
long-term environmental consequences. Indeed, given the reductions in metal
concentrations apparent, for instance, in wastewater biosolids, there is now a case that
regulatory controls on certain elements, such as Cr, Ni, Pb and Hg are no longer required to
protect the environment when biosolids are recycled to farmland.
In contrast to the relatively small group of heavy metals of potential concern for
biorenewable fertilisers, there are a much larger number of organic contaminants to
consider. However, the increasing volume of research completed on organic contaminants
shows that the risk to the human foodchain and environment from these substances in
biorenewable fertilisers is small. This is because intrinsic barriers to the transfer of organic
chemicals applied to soil operate that protect human health and the environment. These
include volatilisation, biodegradation and soil sorption processes. Furthermore, source and
emission controls have resulted in a profound reduction in the concentrations of the main
contaminants of concern, such as the classical persistent organic pollutants (eg PCBs, PAHs
and PCDD/Fs), and other substances such as the endocrine disruptor, nonylphenol.
Biological treatment processes also play an important role in reducing the concentrations of
organic contaminants in biorenewable fertilisers and the most effective techniques include
both aerobic and anaerobic microbial consortia, high temperatures and associated microbial
activity with extended retention times.
Risks to health from pathogenic diseases potentially contained in biorenewable fertilisers are
controlled by single or multi-barriers. A single barrier is provided by a treatment process that
reduces pathogen numbers to background or undetectable levels. Multi-barriers require
treatment to reduce, but not necessarily eliminate pathogens, followed by land use
restrictions to allow the natural attenuation of residual pathogenic organisms to take place in
the environment.