The objective of this work was to investigate the occurrence and fate of five heavy metals in water, sludge, and sediments from a conventional municipal wastewater treatment facility in Kisumu City, Kenya. The effluent quality was compared with the effluent quality parameters stipulated by the National Environmental Management Authority (NEMA) to assess the efficiency of the plant and potential effect of the discharged effluent on the recipient river. The levels of the heavy metals recorded in the sludge samples were significantly higher than those in the corresponding water samples. The order of the metal percentage removal efficiency (%R) from the treatment plant was Mg>Cu>Mn>Fe>Zn. It is concluded that the plant is a point source for Zn loading into the recipient waters which poses potential risk to end users downstream. The heavy metal-laden sludge was within permissible limits for utilization in agricultural lands.
Top1. Introduction
Heavy metals have been defined differently by different authors. However, heavy metals are generally considered as elements having atomic masses ranging between 63.5 and 200.6, specific gravity above 5.0, are toxic or carcinogenic and cannot be metabolized by living organisms (Srivastava and Majumder, 2008). Unlike organic pollutants, heavy metals are persistent environmental contaminants due to their non-biodegradable nature and tend to be biomagnified in living organisms. Of specific interest in wastewater treatment processes include Zn, Hg, Cd, Cu, Pb, Cr and Ni.
Accumulation of heavy metals in different environmental compartments, especially water resources, and the associated negative impacts have been directly or indirectly associated with industrial processes, especially in developing countries. Most of the aforementioned heavy metals are characteristic of most effluents from industrial sources and constitute finished products that end in waterwater at the end of their use or through improper disposal (Olujimi et al., 2012).
In urban settings, the major source waters to wastewater treatment plants (WWTPs) include household effluents, hospital effluents, business effluents (e.g. car washes), and traffic-related emissions (e.g. motor exhaust, brake linings, gasoline/oil leakage etc) which find way into the sewage via storm water.
The toxic effects of heavy metals ions on living organisms are well documented and are therefore briefly mentioned here.
Some heavy metals are essential for metabolic processes at low concentrations and become toxic above certain tolerable limits. Other heavy metals are wholly toxic with no metabolic benefits reported. For instance, Zn is essential for human health. It is a regulator of various biochemical processes and other physiological functions of living tissue. However, excess zinc causes apparent health dysfunctions, such as anemia, skin irritations, vomiting and stomach cramps among others (Oyaro et al., 2007). Noteworthy, Zn is the most profuse of heavy metals found in extremely elevated levels in municipal wastewater (Carletti et al., 2008). Cu is also reported to play essential role in animal metabolism. But as zinc, above certain limits copper causes serious toxicological effects, such as vomiting, cramps even death (Paulino et al., 2006).
Lead intake can potentially damage the central nervous system (CNS), kidney, liver, reproductive system, and brain functions. The symptoms associated to lead poisoning include anemia, insomnia, headache, dizziness, hallucination and renal damages among others (Naseem and Tahir, 2001). Mercury on the other hand is also a neurotoxin that can cause damage to the CNS, as Pb. High exposure to Hg causes kidney dysfunction, impairment of pulmonary glands and dyspnoea (Namasivayam and Kadirvelu, 1999).
Heavy metals are increasingly becoming environmental priority contaminants due to the aforementioned problems. Consequently, more stringent regulations and policies on allowable limits of heavy metals in wastewater from discharge point sources and in treated portable and wastewater are being adopted. Such policies demand more efficient technologies for removal of these toxic heavy metals from the wastewater to for environmental protection. Conventional wastewater treatment processes consist of a combination of physical, chemical, and biological processes and operations to eliminate solids, organic matter and nutrients from wastewater. Common terms employed to describe different scales of treatment, in order of increasing treatment level, are preliminary, primary, secondary, and tertiary and/or advanced wastewater treatment. The next section discusses the convectional processes for the removal of heavy metals from wastewater. Their advantages and limitations in application are also evaluated.