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