Figure 4. Simplified flow-chart for identifying sludge disposal
or recycling routes
Landfilling or incineration should only be considered from a sustainable
standpoint if the sludge quality renders beneficial usage impossible.
Several nations have previously enacted economic and legal tools to
encourage sludge recycling while tightening landfill restrictions,
significantly influencing decisions about the final disposal of
wastewater sludges.
Sludge Management Trends in Developed Nations
As a result of the expansion of sewage and treatment systems in many
affluent nations as well as sections of the developing globe, sludge
output is skyrocketing. More stricter rules in terms of higher biosolids
quality are increasingly being implemented in tandem with the growth in
sludge production, with the goal of minimizing negative sanitary and
environmental repercussions (Rolsky et al., 2020). Mechanical dewatering
systems have gained popularity due to their increased water removal
efficiency; additionally, there is growing interest in thermal drying,
sludge palletization, and other advanced biosolids quality-improving
processes such as composting, alkaline stabilization, and a number of
patented systems (Hyrycz et al., 2022). Because of rising transportation
costs and environmental regulations, landfills are gradually being
recognized as unviable. Disposal options for agricultural techniques
must be guided by strong technological principles in order to ensure an
ecologically safe and cost-effective means of increasing farmers’
revenue (Zhou et al., 2020). As quality and environmental regulations
become more stringent, the expense of such operations is on the rise.
Many factors influence sludge management strategies in different
nations, the most significant of which are population density, arable
land area, economic considerations, and societal acceptability.
Minimization of trash output is desired followed by recycling in the
European and North American countries. For example, by land application
if possible, however, landfilling is usually not a good solution in
these countries. In contrast, more sludge is landfilled or disposed of
on uncontrolled areas in developing nations like India. European and
North American industrialized countries have complex law frameworks for
sewage sludge control. Sewage sludge management in the EU is regulated
both at the international level (by directives) and at the national
level. In general, there are three types of legal regulations regarding
sewage sludge in EU member states:
- EU directives and other legal acts
- Laws of EU member states created to implement EU directives
- Standards and norms from non-EU countries.
Waste recycling, including sewage sludge, is governed by the Waste
Framework Directive (European Parliament and Council Directive
2008/98/EC on sewage). According to the directive, the first priority is
waste avoidance, followed by waste preparation for reuse, recycling, or
other types of recovery, and ultimately garbage disposal.
In general, the use of sewage sludge for agricultural purposes has
increased considerably across recent years in Europe (Placek et al.,
2017). The direct use of sewage sludge in agriculture and for land
restoration in the EU is controlled by Sludge Use in Agriculture
Directive 86/278/EEC. According to this guideline, the use of sewage
sludge shall not have an adverse effect on soil quality or yield. When
the concentration of heavy metals in sludge and soil exceeds the
directive’s limitations, the use of sewage sludge is forbidden. Stricter
regulations have been established for agricultural compost use. The use
of sludge for agricultural purposes is prohibited across several German
states too, and Germany is also regarded as a pioneer in phosphorus
recovery from sludge (Mininni and Dentel, 2013). The majority of created
sludge in the United States is utilized on agriculture (Christodoulou
and Stamatelatou, 2016). In the United States, agricultural sludge usage
is governed by the ”Standards for the Use or Disposal of Sewage Sludge,”
which were created in 1993 (40 CFR Part 503). Around the world, sewage
sludge is utilised in agriculture (37%), incineration (11%), and
landfill (40%), with the remaining 12% employed in forestry or land
restoration (Fytili and Zabaniotou, 2008).
Currently, the EU has greater regulations than the US, particularly in
terms of metal standards. The usage of sludge in these nations is
frequently restricted due to its nitrogen concentration, as the EU’s
permissible nitrogen application rate has been cut from 210 kgN/ha to
170 kgN/ha (Botter et al., 2020). Odor issues during sludge processing
and storage are by far the most important factor affecting public
acceptance. Serious alternatives are being investigated, as some
development in processing technologies has lately been achieved, as more
and more rules aimed at safe sludge in terms of metal content and
sanitary concerns have been enacted. An effective sludge management
technique, on the other hand, requires community participation, as well
as proper information and transparent findings from the environmental
monitoring program (Shaddel et al., 2019). The principal biosolids are
listed in Table 6 below.
Table 6 . Biosolid management trends in the United States and
Europe