When carried out within safe parameters, land application is a low-cost
option with favorable environmental effects. However, effective planning
based on trustworthy information on wastewater flow and characteristics,
suitable agricultural regions at reasonable distances, management
capability to meet farmer demand, and good environmental monitoring are
all necessary. Sludge supply must offer a decent output for farmers
while also protecting public health and the environment.
Risks associated with Sludge land Application
There are in fact some drawbacks to using land application as an
alternative sludge disposal practice, which includes soil contamination
by metals and pathogenic organisms, and ground and surface water
contamination by phosphorus and nitrogen (Liew et al., 2022). A more
concerted effort can be seen around the world right now aiming for a
lower chemical as well as biological contaminants in the sludge through
improved wastewater technologies steadily leading towards better sewer
acceptance criteria. As we have discussed the contaminants in the
sludge, in this section focus in on associated risks with land
application of sludge.
There are certain disadvantages to adopting land application as an
alternate sludge disposal method, including soil contamination by metals
and pathogenic organisms, as well as phosphorus and nitrogen pollution
of ground and surface water. A more concentrated effort is currently
underway across the world to reduce chemical and biological pollutants
in sludge through improved wastewater technology, which is gradually
leading to better sewer acceptance requirements. The main risk
associated with the land application of sludge are as follows:
Metals content
When heavy metals are present in concentrations that exceed established
limits, they can be harmful to human and plant health (Martin &
Griswold, 2009). Because everything is concentrated in the sludge, heavy
metals are frequently high in sludge (Veeken, A. H. M., & Hamelers,
1999). There should be an eye to limit all the toxic chemicals to limit
the hazards. Arsenic (As), cadmium (Cd), copper (Cu), chromium (Cr),
mercury (Hg), nickel (Ni), molybdenum (Mo), lead (Pb), selenium (Se),
zinc (Zn), and cobalt (Co) are the principal elements of concern (Agoro
et al., 2020). However, as previously stated, concentrations fluctuate
depending on the source of wastewater and the treatment process used. It
is a major constraint to sludge land use in various nations, including
India.
Pathogens
During the treatment process, several disease-causing organisms, such as
bacteria, viruses, protozoans, and helminths, tend to concentrate in the
sludge and pose a serious threat to human and animal health because they
can be transmitted through food, surface water, run-off water, and
vectors such as insects, rodents, and birds. To reduce this danger,
sludge must be subjected to a pathogen elimination procedure prior to
its intended usage. The pathogen elimination method used has an impact
on biosolids management, processing, and application (Lopes et al.,
2020). Sludge properties are improved by alkaline stabilization, such as
the addition of lime, making it more acceptable for use in soil
applications (Mendez et al., 2002). Similarly, composting can be done
until the sludge is completely stabilized, at which point it can be used
as a soil conditioner once the organic matter has been transformed to
humus.
Organic pollutants
The majority of organic contaminants are volatized during biological
treatment, while some may reach the sludge processing line and
contaminate the finished sludge. Pesticides, aromatic and phenolic
hydrocarbons, polybrominated biphenyls (PBB) and polychlorinated
biphenyls (PCB), which are very harmful and persistent organic
micro-pollutants, are examples of such pollutants (Katsoyiannis &
Samara, 2005). They may breakdown by solar light (photo-oxidation) and
undergo volatilization or biodegradation when applied to land with
sewage sludge, which may significantly affect their structure or
toxicity properties. Some organic micro-pollutants are transported by
capillary capillaries and reach the plant’s aerial portions when
absorbed directly by plant roots.
Sewage Sludge Disposal Alternatives: Pros and Cons
Dry biosolids can be spread to the ground using the same machinery as is
used to apply animal manure. Pathogen-free biosolids can range in
consistency from loose to solid depending on moisture content.
Heat-dried granular biosolids may be handled, distributed, and
land-applied with traditional agricultural equipment, such as spreaders
for lime, moist lime, fertilizer, poultry litter, bedding, compost,
gypsum, sand, salt, cement, fly ash, and any other bulk material. Sludge
soil incorporation can also be done using traditional farming equipment,
in which digested or undigested sludge is put directly below the soil
surface using tractors equipped with specific ploughers that split the
ground surface and inject the sludge beneath it (Kekacs et al., 2015). A
summarized view of all the disposal alternatives have been presented in
Table 8 with their advantages and disadvantages:
Table 8 : Advantages and disadvantages of the main methods of
sludge disposal