Heavy Metals in Wastewater Effluent: Causes, Effects, and Removal Technologies

Abstract

Heavy metal contamination is one of the fundamental ecological problems of new instances. These heavy metals are very tricky as they without delay have an effect on residing organisms and human health. These contaminations typically occur as a result of herbal and human activities. Technological development and business improvement have brought about an upward push in heavy metallic pollution inside the surroundings. Indiscriminate discharge of toxic wastewater into the encompassing surroundings regularly are reasons for severe environmental and health impacts. The heavy metal particles are nonbiodegradable and could popularly amass inside the dwelling organisms, accordingly are carcinogenic and teratogenic. Elimination of heavy metals has therefore become a problem of first-rate difficulty. Various technology and traditional techniques were utilized in heavy metallic elimination. This review paper is therefore geared toward assessing some of the reasons and results in addition to diverse strategies for the elimination of heavy metals in wastewater effluent.

1. Introduction

Water is a key resource for survival of all living organisms all over the world. With an increase in population and human activities such as urbanization, development of industrial parks, recreation facilities, and agricultural practices, there is reduced access to water resources as all these activities require water. These human-induced activities require the use of water resource. This in turn increases on water consumption and the water demand, thus putting a strain on the ever diminishing water resources. The various water uses often results in high levels of pollution that usually find their way to the environment through indiscriminate wastewater disposal. These pollutants include organic and inorganic compounds as well as heavy metals. Since the freshwater resource is continuously decreasing, there have been high advocacy for water reuse to try and address on the ever-increasing demands for freshwater resources. This further elevates the water contamination problem as some of these pollutants find their way to the environment and affects living organisms. For instance, reuse of wastewater in irrigation tends to increase the prevalence of health infections [1, 2, 3]. Heavy metals are components with atomic density that is greater than 5 g/cm³ and a relative atomic mass that ranges from 63.5 to 200.6 are viewed as the major contaminations for freshwater resources due to their direct effect on living organisms and human health [4, 5, 6]. They are nonbiodegradable and have been reported to cause environmental pollution worldwide due to their persistence in wastewater [4, 6].

Industrial and municipal effluents are considered to be the main sources of heavy metal pollution [7]. Increased industrial activities leads to deposition of large wastes and wastewater pollution with heavy metals content [8]. Discharge of heavy metal ions to the environment as a result of industrial development and springing of urban centers has posed a great problem globally [9]. This has made heavy metal ions to be one of the most problematic issue of environmental concern of the recent time [2, 10]. Therefore, there has continuously been an increased focus toward the disposal of heavy metals, since they are increasingly becoming an environmental threat because of their unfavorable impacts on human health [7, 9, 11]. Elimination of metal ions from wastewater effluent has also been an issue of great conversation because of their interaction with health risk for all living things, for their toxicity and nonbiodegradable nature [2, 7].

2. Causes and effects of heavy metals in wastewater effluent

Water pollution is a worldwide problem and needs to be controlled [12]. The main sources of heavy metals in wastewater are natural and man-made activities [6, 11]. The natural factors include weathering and soil erosion, precipitation runoffs, volcanic eruptions from active volcanos, particulate matter such as aerosols, while the human sources are quite numerous [4, 5, 11]. They include effluents resulting from industrial and municipal waste. The industrial effluents results from processes such as metal smelting and electroplating processes, leather tanning and textile, mineral extraction, and nuclear power, whereas municipal effluent are from residential and municipal discharges [4, 5, 13]. Municipal and industrial effluents contain organic matter, water contaminants, and heavy metals ions. Heavy metals mainly emanates from various industries [13]. These heavy metals are nonbiodegradable and require to be removed from the wastewater effluent before disposal since their presence in water deteriorates water quality [4, 11]. They are a major concern due to their toxicity effects, bioaccumulation tendencies, and threat to human life and the environment [8, 13]. Agricultural activities consisting of irrigation with wastewater effluent can also end in regular pollutants with heavy metals to soil and groundwater, consequently posing a threat to human health. Similarly, the accumulation of heavy metals usually by using vegetable plants is deemed to be of extreme environmental trouble in regions where wastewater irrigation has come to be significant [1, 2, 3, 14].

Metal ions including lead, nickel, cadmium, and chromium are additives from the earth crust and are usually related to high degrees of toxicity [10, 15]. These metals, however, have various uses in consumer products and fundamental engineering works, paper and pulp industries, leather tanning, plastic stabilizers, photographic substances, fertilizers, pigments, batteries, metal plating, mining, jua-kali, radiator production, smelting, and alloy industries [11, 13, 15]. Pollution occurs when wastewater resulting from these industrial processes find their way in to the environment [16]. Since these metal ions are nonbiodegradable, they can undergo transformation that can have health, environmental, and economic impacts [11]. Untreated and poorly treated heavy metal contaminated wastewater often results in health and environmental impacts [4]. If these heavy metals are ingested, they generally tend to build up within the essential organs of dwelling organisms and vegetation [4, 5, 8, 11]. These can result in damage of the human nervous system, liver, brain cells, and bone marrow. Similarly, they can block some of the functional groups of essential enzymes thus disrupting the metabolic processes of the body [4, 5, 8]. Because heavy metals tend to accumulate in the living organism, they are carcinogenic and teratogenic [6]. Cadmium and lead, even as trace levels, are known to have toxicological effects and have adverse effects on human health and other living organisms [11]. Exposure to a few metals, including mercury and lead, may motive development of autoimmunity, in which a person’s immune device attacks its very own cells. This can cause joint illnesses inclusive of rheumatoid arthritis, and sicknesses of the kidneys, circulatory gadgets, worried system, and unfavorable of the fetal mind. At higher doses, heavy metals can cause irreversible mind damage [4, 5]. On the other hand, zinc and copper ions are rather not harmful to human health and animals at low concentrations as they may be required for various sports for residing organisms. However, exposure to high concentrations are extremely harmful [11].

3. Heavy metals removal technologies

Numerous traditional methods and technologies have often been used for the elimination of heavy metals ions from wastewater. They include chemical precipitation, ion change, opposite osmosis, oxidation-discount, solvent extraction, adsorption, electrochemical remedy, evaporation recuperation, membrane era, biological remedy, and coagulation, among other methods [7, 8, 9, 10, 11, 12, 13]. However, some of these methods are rather too expensive or very complex for treatment of heavy metals [15]. The important downside of these technologies is the manufacturing of toxic sludge whose disposal is intricate and now not green [9, 12]. For instance, metal extraction of less than 100 ppm tend to be very costly [9]. Bioremediation, on the other hand, is a new technology which employs the use of enzymes, micro-organisms, eco-friendly plants, and agricultural by-products [4, 9].

Adsorption has proven to be economical and is vastly accepted in the environmental treatment processes globally [8, 10, 17]. Adsorption has shown many advantages over physicochemical methods [15]. It is always considered better among other methods due to convenience, easy operation, and simplicity of design as well as low cost of the initial materials used as adsorbents [6, 8, 10, 18]. Adsorption of heavy metals on conventional adsorbents such as activated carbon is one of the methods that have been widely studied. However, activated carbon has a cost implication that is not sustainable [16]. Activated carbon shows a high adsorption capacity, but its expensive nature presents a major drawback for practical applications especially in industrial effluent treatment [7, 8, 13]. Activated carbon requires chelating agents to enhance their performance [7, 8, 16]. The activating carbon is commonly used as industrial sorbents. Other materials include silica gel and alumina since they present a great surface area per unit weight [10, 18].

These commercially available adsorbents are expensive and therefore, there has always been the need to study alternative substitute which has high adsorption efficiency and is low in cost [16, 18]. This need for safe and economical method has necessitated research interest toward production of low-cost alternatives to commercially available activating carbon [8, 13, 17]. These research works have employed the use of agricultural wastes, clay, cooked tea dust, zeolites, cashew nut shells, fly ash activated carbon, sawdust, peat, sewage sludge, fruit wastes, diatomite, and rice husk [8, 15]. Agricultural by-products such as rice husk, fly ash, sugarcane bagasse, palm oil shell, and coconut husk have yielded some promising results in the adsorption of heavy metals from wastewater effluent [9, 13, 19]. Agricultural resources has therefore been found to be an alternative due to their low cost (cost efficiency) and abundance [13, 16, 20]. Most investigations on agricultural by-products such as fruit wastes have asserted that they can act as a good sorbent for hazardous material and heavy metals [7, 9, 12].

4. Conclusion

This paper review was aimed at discussing heavy metals in wastewater effluent, with a great focus on some of the causes, effects and removal technologies in wastewater effluents. It revealed the main causes of heavy metal pollution as a result of natural and human activities. However, metal ion contaminations have been catalyzed by human activities such as technological advancements and industrial developments. Affluent and inadequately treated effluents polluted with heavy metals poses a great challenge to the environment as well as human health.

In order to ensure adequate and proper protection of human health and environmental sustainability, numerous treatment technologies have been studied and applied. Application of these methods depends on the pollution load, cost implications, efficiency and simplicity in the design, effectiveness, and sustainability. Adsorption have been singled out as a better alternative to the physicochemical methods.

In particular, the use of agricultural by-products and wastes has majorly been investigated for use as low-cost adsorbents. This has therefore created a new spectrum in the remediation of heavy metals polluted wastewater and sludge. However, the use of a blend of various agricultural wastes and anaerobically digested agricultural wastes have not been zealously explored in this endeavor. Therefore, there is need for more studies in this area in order to fill this gap.