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:
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