(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