Environmental Engineering Reference
In-Depth Information
12.2.6 Sewage sludge
The impact of land application of sewage sludge from waste treatment
plants can be controversial. Sewage sludge is a highly complex digestible
material, with a methane potential similar to that of cattle and pig slurry. Its
composition varies significantly with geographical area, consumption
patterns and local environmental and waste treatment policies (Smith,
2009). Recycling sewage sludge to agricultural land is regarded in some
countries as a pragmatic approach to managing the sludge generated from
urban wastewater and the cheapest disposal method for wastewater
treatment plants. The European Commission permits monitored and well-
regulated use of sewage sludge on agricultural soils as a fertilizer, provided
that it does not pose any threat to the environment or to animal and human
health (Smith, 2009). The acceptance of this practice varies significantly
among European countries and has declined significantly in many places,
leading to banning of the use of sludge in agriculture (Smith, 2009) in
countries such as Switzerland, the Netherlands and Austria.
The same divergence of opinions can be found in the scientific literature
concerning sewage sludge. Smith (2009) states that 'recycling sewage sludge
on farmland is not constrained by concentrations of OCs' (organic
contaminants) and indicates that 'the chemical quality of sludge is
continually improving and concentrations of potentially harmful and
persistent organic compounds have declined to background values'.
Stevens et al. (2003) report that data on the concentrations of certain
classes of persistent organic compounds in digested sewage sludge are
scarce. They also indicate that, in line with data from other countries, very
high concentrations of PAHs and short- and medium-chained polychlori-
nated alkanes, exceeding many times the EU limit values for such
compounds, are found in slurry from wastewater plants in the UK.
Nevertheless, according to Clarke and Smith (2011), most of the existing
risk assessments demonstrate that use of digested sewage sludge as fertilizer
does not place human health at risk, from the point of view of the organic
contaminants studied so far, but also note that 'continued vigilance in
assessing the significance and implications of ''emerging'' OCs in sludge is
necessary to support and ensure the long-term sustainability and security of
the beneficial agricultural route for biosolids management'. According to
Clarke and Smith (2011), two classes of POPs require research priority -
perfluorinated compounds (PFCs) and polychlorinated alkanes (PCAs) -
which, due to their unique chemistry that facilitates a degree of water
solubility, are likely to cause exposure through all pathways.
Mogensen et al. (1999) indicate that the AD process can have a certain
effect on the decomposition of some POPs belonging to PAHs, if some
special process conditions are fulfilled. The special conditions refer to the
Search WWH ::
Custom Search