(Source: Andreoli et al., 2007)
Figure 4 . Relationship chart of sewage, treatment technology,
generated sludge and the disposal options
A successful sludge management technique should not be limited to the
solid waste created by the process, but should interact with and
influence the whole wastewater treatment system, including the entire
sludge cycle from generation to disposal (Johnson & Affam, 2019). All
factors of wastewater quality, treatment technology, sludge production
size, environmental laws and local regulations, and soil and area
agriculture must be examined before any assessment of sludge processing
possibilities can be made.
One of the most crucial problems concerns future policy orientations for
sludge management. Are they biological, ecologically beneficial, and
allow for the recycling of nutrients into the environment? The issue is
the scarcity of arable land and the poor quality of sludge. Measures and
methods to enhance sludge quality, such as heavy metals leaching from
this waste material, are critical. Furthermore, circular economy
technologies, such as phosphorus recovery from sludge, offer a viable
alternative to traditional management strategies. Composting sewage
sludge is one option for direct agricultural usage. This approach
ensures sludge stability, pathogen killing, and a decrease in sludge
bulk and water content. Aside from sludge disposal, one significant goal
of composting is the ability to reuse sludge in the economy and the
environment. Thermal treatment of sewage sludge based on incineration
and co-incineration are suitable procedures that are becoming more
prevalent in EU nations, according to data. It’s also popular in Japan.
Thermal treatments are expensive, and in certain countries, there is
little societal approval for burning garbage. As a result, this form of
sludge disposal is one of the most contentious, and its use is
influenced by a variety of local conditions.
Applications of Sewage Sludge
Organic matter has been regarded a significant soil fertilizer for
thousands of years, and organic wastes from human activities were
employed as fertilizers by Chinese, Japanese, and Indians in ancient
times (Kiehl, 1985; Poudyal, 2020). The most common approach of sludge
management is land use or land application, which was first conceived in
the form of sewage disposal throughout the nineteenth and twentieth
centuries. However, as technology has progressed, sewage is now
subjected to specific procedures such as primary, secondary, and even
tertiary treatment in order to remove particles in the form of sewage
sludge, which is subsequently disposed of.
Land Application: Beneficial Uses and its impact on
environment
Sludge consists of nutrients that are important to plants, and their
availability is dependent on the quality of the influent sewage and the
wastewater and sludge treatment techniques utilized (Tarpani, &
Azapagic, 2018). Except in the sludge treated by alkaline stabilization,
nitrogen and phosphorus are abundant, whereas calcium and magnesium are
few. Sludge has low levels of potassium and phosphorus. Moreover, the
use of sludge on land, principally in agriculture, compared to
incineration or sanitary landfill, has lower costs. The use of sludge in
the agricultural sector varies greatly among different European nations
(Lamastra, et al., 2018). While using sewage sludge to add nutrients and
organic matter to the soil may be helpful, it also poses a danger owing
to the presence of pollutants such as heavy metals, organic compounds,
and infections. This, in turn, has harmful consequences on humans,
plants, and animals, both terrestrial and aquatic (Balkrishna et al,
2022). Researchers, on the other hand have attempted to make use of
sewage treatment plant sludge by composting it and turning it to
agricultural fertilizers. This study describes the conversion of sludge
to compost and its usage as a component in the production of three
agricultural input products by substituting compost with farmyard
manure. The physicochemical characteristics and metal content of the
revised goods were found to be within the norms prescribed (our ref).
Sludge nutrient concentrations may not meet all plant demands,
necessitating the use of organic or inorganic fertilizers to meet the
crop’s nutritional requirements (Pitas et al., 2020). Because crop
plants are the most sensitive to nitrogen, it is the element in the
sludge with the highest economic value. The organic component of the
sludge contains the majority of the nitrogen, ranging from 70% to 90%
depending on the type and age of the sludge (Quoc et al., 2021).
Residues, microorganism cells generated during wastewater treatment, and
phosphate-containing detergents and soaps are all sources of phosphorus
in sludge. The majority of the chemical fertilization of P is fixed by
the soil, and only a small amount is actually assimilated by the plants
for their growth and reproduction.
Sludge can also be used as a soil conditioner, which increases pH,
decreases hazardous levels of Al and Mn, supplies Ca and Mg, enhances
nutrient absorption, and stimulates microbial activity (Fei et al.,
2019). Organic fertilizer promotes water infiltration and retention, as
well as soil stability and erosion resistance. Sludge organic content,
when applied to silty clay to sandy clay soil types, causes a favorable
change in texture, allowing for greater air and water circulation and
hence better crop response. When added to coarse soils like sandy soil,
it causes particle agglomeration, which allows for improved water
retention. Sludge boosts the soil’s cationic exchange capacity,
functioning as a storage for nutritional materials while also increasing
pH buffering and microbial activity.
According to Andreoli et al., (2007), sludge treatment on land improves
the structure and fertility of wastelands. It produces a cost-effective
element with the use of chemical fertilisers, which might be reduced if
sludge is used for medium-term physical, chemical, and biological
changes in the soil. Table 10 shows a number of physical and chemical
impacts from biosolids application on land, as well as the implications
for natural resource conservation.
Table 7 . Effects of sludge land application on the soil erosion
and environment