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