**3. Types of sewage contaminants**

#### **3.1 Organic sewage contaminants**

Various sources, such as urban runoff, domestic wastewater, industrial discharges, and wet and dry atmospheric deposition, can contribute organic contaminants to wastewater. Sewage sludge typically contains large quantities of many kinds of organic contaminants because they tend to sorb on the suspended solids in wastewater. Sewage sludge contains more than 300 chemicals, representing several different chemical groups, according to the type of sewage that the household produces, whether it's municipal or industrial. The range of their concentrations is between pg. kg-1 and g kg-1. Phthalic acid esters (PAEs), organochlorinated pesticides, monocyclic aromatics, polychlorinated biphenyls (PCBs), phenols, polycyclic aromatic hydrocarbons (PAHs), chlorobenzenes (CBs), amines, nitrosamines, and polychlorinated dibenzo-p-dioxins and furans (PCDD/Fs) are the contaminants that are most frequently found [5].

#### **3.2 Inorganic sewage contaminants**

The dangerous inorganic chemicals found in sewage sludge are generally heavy metals, having concentrations ranging from cadmium 1–3410, arsenic 3–230, copper 80–2300, zinc 101–49,000, nickel 2–179, lead 13–465 to chromium 10–990,000. Environmental risks are not eliminated by thermal procedures, which often transport, and pollutants can build up in both solid and liquid states. Using sewage that has not been treated, which is a source of numerous hazardous and lethal substances, it may be particularly harmful [6].

### **4. Role of heavy metals in sewage pollution**

There have been many harmful substances introduced into the agro-ecosystem, but heavy metals are particularly problematic because they do not degrade and are bio toxic. In addition to lowering crop yield and quality, heavy metals in soil can pose a hazard to the ecosystem's security and public health [7]. For the most part, because of their toxicity, bioaccumulation, and environmental endurance, heavy metals are well-known inorganic contaminants. Along with other contaminants, these inorganic ones slowly migrate to the nearby water and soil. Waterways can become contaminated by heavy metals in several ways. In places with such contamination, severe cases of diseases caused by heavy metals in living systems have been documented. In some countries, wastewater can be discharged into lakes, rivers, and other bodies of water. Such practices contaminate freshwater, harming the environment. Most the earth's natural species are severely affected by heavy metal contamination. The earth's crust contains heavy metals, but due to human activity, there are geochemical and biochemical imbalances involving naturally occurring compounds [8]. Because they may cause cancer and mutagenesis, heavy metal ions from the paper, leather, and textile industries


#### **Table 2.**

*Sewage sludge is one of the sources of heavy metals in water and soil.*

are a severe issue. Some contaminants are poisonous, damaging, and carcinogenic to ecosystems and people. Impurities in some heavy metals are extremely hazardous. Arsenic has long been recognized as a deadly substance. The very poisonous heavy metals chromium, lead, copper, mercury, cadmium, zinc, and nickel, among others, can have detrimental toxic effects. Additionally, the following nitrates, fluoride, selenides, chlorides, phosphates, and chromates have a high-level dangerous effect (**Table 2**) [1].
