Environmental Engineering Reference
In-Depth Information
presence of relevant microbial populations inside the digester, which are
able to metabolize the respective compounds at specified environmental
conditions for these microbial populations. The organic compounds must be
accessible to the microorganisms and the composition and the amount of
feedstock mixture must be maintained constant throughout the process. The
microorganisms need an adaptation period to the substrate.
There are 50 million chemicals entered in the Chemical Abstracts Registry
and this number is continuously increasing. Of these, 143,000 chemicals are
registered with the European Chemicals Agency for industrial use (Clarke
and Smith, 2011). The UN list of POPs is constantly updated as new
compounds are studied and their long-term effects are documented (United
Nations Environment Programme, 2010; Stockholm Convention, 2011). A
recent EU report (European Commission/JRC-IPTS, 2011) emphasizes that
there are important knowledge gaps concerning the scientific base for setting
the limit values for chemical pollutants in waste-derived materials as well as
insufficient toxicological and eco-toxicological risk assessments. The issue of
application of sewage sludge on land therefore remains open and ongoing
research is expected to clarify sustainable biosolids management.
In practice, it is impossible to perform screening of a broad spectrum of
chemical pollutants at reasonable costs. It is therefore widely accepted that
quality assurance of digestate can only be provided through a responsible
selection of the AD feedstock materials used, combined with permanent
control of their quality.
12.2.7 Legal frameworks for control of chemical pollutants
The levels of chemical pollutants in digestate produced from high-quality
agricultural feedstock are in general much lower than the limit values
prescribed by legislations (Al Seadi and Lukehurst, 2012). Tables 12.2 and
12.3 show examples of limit values for heavy metals and organic pollutants
in waste-derived products applied as fertilizers; they are also applicable to
digestate used for agricultural purposes.
12.2.8 Pathogens and other unwanted biological impurities
The AD process is very effective in terms of inactivation of most pathogenic
matter such as bacteria, viruses, intestinal parasites, weed and crop seeds,
and crop diseases. Nevertheless, exclusion of feedstock contaminated with
high-risk biological material is an important measure in digestate quality
control (hence the positive lists in some countries) and applies to all
feedstock types, including manure and other agricultural feedstock. No
biomass should be supplied from farms where there are serious animal
health problems. Strict pathogen control in waste products used as fertilizers
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