Environmental Engineering Reference
In-Depth Information
feedstock materials for the production of quality digestate. Their main
limitation is that, depending on their origin and the way they are collected,
these feedstocks can contain varying amounts of non-digestible materials
such as pieces of plastic, rubber, glass, metal, stones, sand, excessively large
pieces of organic material, ligno-cellulosic materials (roots, wood and bark)
or other recalcitrant contaminants. The presence of physical impurities in
the AD biomass flow can cause problems such as perturbation of operation
stability and damage to pumps, pipes, stirrers, etc. If physical impurities are
present in the digested material, it will decrease its quality as fertilizer and its
public acceptance (e.g. visible plastic pieces from un-degradable household
collection bags). Depending on their nature, physical impurities in digestate
can also have harmful effects on the environment. In the case of organic
household waste, physical impurities are managed most effectively by source
separation and separate collection of the digestible fraction in paper or other
biodegradable bags. Non-digestible materials can also be removed with the
use of physical barriers (screens, sieves, stone traps and protection grills)
installed in the pre-storage tanks to prevent access into the AD system. If
particle sizes in the feedstock supplied are too large, they can be reduced by
chopping, maceration or treatment by other means prior to entering the AD
system.
Chemical impurities in AD feedstock
Digestible materials such as sewage sludge, mixed waste (bulk collected
waste) or domestic wastewaters can contain various amounts of unwanted
chemical matter (heavy metals and organic compounds), of which some are
POPs. Chemical pollutants can also be present in some industrial organic
wastes, household waste and even in food waste. Agriculturally derived
feedstock materials in most European countries, where strict legislation bans
the use of pesticides from the United Nations list of POPs, do not contain
such pollutants, although trace amounts of other pesticides, antibiotics and
chemicals used in agriculture can be present in agricultural feedstock. In
developing countries, where pesticides classified as POPs are still used in
agricultural practices (e.g. DTT and HCH), the occurrence of POPs in
agricultural feedstock for AD is likely to be much higher (United Nations
Environment Programme, 2010; Stockholm Convention, 2011). POPs,
including polycyclic aromatic hydrocarbons (PAHs), are recognized as
being directly toxic to biota and can progressively accumulate higher up the
food chain such that chronic exposure of lower organisms to much lower
concentrations can expose predatory organisms, including humans and
wildlife, to potentially harmful concentrations (European Environment
Agency, 2011).
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