Abstract
Soils altered with sewage sludge typically have higher amounts of a variety of heavy metals, making them of importance in terms of their possible influence on human health. In this review, we attempt to explore how sewage sludge is created, as well as its features in the presence of heavy metals. Sewage sludge is commonly utilized in agricultural airs or landfills. Soil, plants, and humans are all linked in some way in both circumstances. Heavy metals including Zn, Pb, Fe, and Cu are prevalent in the environment and play an important role in the sustainability and balance of ecosystem processes. However, because of their bioaccumulation, non-degradability, and abundance, these metals pollute the food chain and constitute a source of toxicity to humans and the overall ecological function, which is a major problem in the study of environmental science and geochemistry. The current study intends to consolidate all of the previously mentioned features of heavy metal distribution in nature and their implications. This study would be designed to persuade policymakers to intervene with a viable and rapid solution to the nuisance.
Keywords: Heavy metals, Sewage Sludge, Industry, Agriculture, Human health
Introduction
”Too many cars, too many factories, too much detergent, too many pesticides, multiplying contrails, inadequate sewage treatment plants, too little water, too much carbon dioxide - all can be traced easily to too many people.” The famous quote by Paul R. Ehrlich (Ehrlich 1969) could highlight the contextual viewpoint of the population inflation related to the management of sewage. Sludge generation and its management are the prime headaches for both developed and developing countries. To move forward with this discussion, we need to first realize what is Sewage Sludge (SS)? Sewage is described as a residual, semi-solid material created as a byproduct of industrial or municipal wastewater treatment (Kumar and Chopra, 2016a). Precisely, sewage sludge is created as a byproduct of the various treatment stages of residential home wastewater, and it may also comprise industrial and commercial effluents. (Williams, 2005).
Sewage sludge can be utilized to generate energy (through anaerobic digestion or thermal treatment), processed and applied to land as a fertilizer and soil conditioner, or even used to extract valuable chemicals (phosphorous recovery). A considerable number of wastewater treatment plants (WWTPs) compost dewatered sewage sludge with green wastes or other bulking agents under aerobic conditions, or dry it in heat drying facilities to 95 percent dry mass for use as fertilizer or fuel. Most industrialized countries place a premium on effective sewage sludge treatment to enhance the quality and safety of land usage. Biosolids are defined by the United States Environmental Protection Agency (US-EPA) as treated sewage sludge that fulfills the appropriate levels of pollutants or pathogens and is utilized as fertilizer for landscape application (USEPA, 2009). Wastewater sludge is a complex heterogeneous mixture of microorganisms, undigested organics such as cellulose, plant residues, oils, or fecal material, inorganic material, sand is a resource of organic matter, nitrogen, phosphorous, micronutrients, and even heavy metals, bio-fuel, hydrogen, syngas, bio-oil, bio-diesel, bio-plastics, bio-pesticides, proteins and enzymes (Tyagi and Lo, 2013). The exploration of the best recycling solutions for such precious chemicals is now one of the primary trends in the creation of sustainable human societies (LeBlanc et al., 2009).
However, when it comes to waste that is reintroduced into natural systems, cautious measures must be taken, especially when it comes to the limit values (quality criteria) for probable toxins and pollutants that are hazardous to human health and the environment.
The Sewage Sludge Regulation (86/278/EEC) (SSD), Europe’s oldest mandatory directive, was established to stimulate the use of sewage sludge in agriculture and to control its usage to minimize detrimental effects on the environment by restricting the probable transmission of heavy metals and infections. In general, the Directive had a favorable impact on enhancing source control methods to ensure high sludge quality, albeit it is today regarded out-of-date and has been flagged by the Commission as a candidate for change for roughly ten years (Environment, 2014). According to a European Commission report released in 2010, only 39% of sewage sludge in the EU gets recycled into agriculture owing to increased leaching of pollutants into water and soil, smells, and greenhouse gas emissions (CH4 and CO2). Sludge usage on land varies greatly across the EU, ranging from none (Nederland, Switzerland) to more than 50% (Norway, Great Britain, France). In November 2013, the German federal government reached an agreement that said, ”We would oppose the direct use of sewage sludge as a fertilizer on land and support the recycling of phosphorus and other nutrients” (Bergs, 2015). In other high-income nations, such as the United States, Canada, Australia, and New Zealand, treated biosolids are commonly applied to soils; however, incineration has been proposed as a possible option for ultimate sewage sludge disposal. Nonetheless, land application of treated sewage sludge is becoming a feasible alternative to landfilling in underdeveloped countries.
From the above facts, the obvious question would come to mind where the sludge is coming from? and what are its general characteristics? In the next section, we would focus our discussion on the same.
Source and Characteristics of Sewage Sludge (SS)
Primary, secondary, and chemical treatment procedures all produce sewage sludge. Primary sludge is the settleable material that accumulates at the bottom of the clarifier. Because it has not been decomposed, primary sludge is also known as raw sludge. Raw primary sludge from a normal household facility is unpleasant and has a high amount of water, both of which make handling problematic. The secondary treatment is intended to convert colloidal materials into settleable solids that may be removed. These solids are removed in the secondary clarifier once they have settled. Domestic sewage, industrial sewage, and storm sewage are the three forms of sewage. Domestic sewage transports wastewater from homes and flats; it is also known as sanitary sewage. Water from manufacturing or chemical operations is utilized in industrial sewage. Storm sewage, often known as stormwater, is drainage from precipitation gathered in a network of pipelines or open channels. Domestic sewage contains slightly more than 99.9% water by weight. The remainder, less than 0.1 percent, is made up of a wide range of dissolved and suspended contaminants. Depending on the nature of the industrial process, industrial effluent often comprises particular and easily recognizable chemical components. Storm sewage contains organic compounds, suspended and dissolved particles, and other things that it picks up as it passes through the ground. The features of sewage sludge or biosolids vary and comprise organic and inorganic compounds, harmful metals, and microorganisms. Because of its broad use in soil amendment, energy generation, nutrient delivery, and other applications, it is frequently regarded as a resource. The sewage sludge, which contains around 1% wastewater when it enters the sewage treatment plant for treatment, is digested anaerobically, resulting in the removal of the wastewater from the sludge. After mechanical drying, sludge has almost 80% moisture and 20% dry matter at the production vent. (Kumar et al., 2017). The details of the Physicochemical and biological constituents of sludge have been discussed below (Table 1).
Table 1: Physiochemical and Biological properties of sewage sludge