ABSTRACT
An assessment of the efficiency of heavy metals removal by a constructed wetland system at Egerton University, Kenya, was conducted between August 2013 and January 2014. The aim of the study was to evaluate the physico-chemical characteristics of wastewater and investigate heavy metals retention in the wetland. Water samples were collected monthly in plastic bottles at the inlet, along the wetland, and at the outlet; sediment samples were collected from the gravel bed and the three wetland cells using a core sampler. Whole plants were randomly collected and pooled together to form composite samples for each species in every site. In every sampling occasion; temperature, pH, electrical conductivity (EC) and Dissolved Oxygen (DO) of water samples were measured in situ. In the laboratory the samples were processed and the concentrations of metals; lead (Pb), cadmium (Cd), copper (Cu) and zinc (Zn) determined using Atomic Absorption Spectrophotometry (AAS). Minitab software was used to determine spatial variations of heavy metals concentrations and physico-chemical characteristics of water using Analysis of Variance (ANOVA). Further, correlation analyses were performed to establish relationships between the physico-chemical parameters and the retention of metals in the wetland. The study results showed significant variations in temperature and conductivity across the wetland (p < 0.05). On the contrary, there was no significant difference in DO across the wetland. Influent levels for lead, copper and zinc were 1.25 ± 0.75 mg/L, 1.09 ± 0.49 mg/L and 0.15 ± 0.11 mg/L respectively while the level of these metals in the effluent were 0.07 ± 0.07 mg/L and 0.32 ± 0.11 mg/L for lead and copper, respectively with zinc being below detection limit. Removal efficiencies of 94%, 70% and 100% for lead, copper and zinc respectively were observed. There was a significant negative correlation between zinc in sediment and pH in water (r = - 0.55), and a moderate positive correlation between copper in plants and pH in water (r = 0.47). These findings indicate that the constructed wetland is efficient in removing heavy metals from the wastewater. The study recommends that the wetland should be rehabilitated to enhance and sustain its function of removing heavy metals from wastewater in order to safeguard human and environmental health.
CHAPTER ONE
INTRODUCTION
Background to the study
The world’s freshwater resource is scarce and vulnerable to pollution by waste streams generated by human activities. The decline of water quality across the world is constraining global freshwater supplies and millions of people lack access to adequate supply of drinking water (WHO/UNICEF, 2012). Hence most of the problems that humanity face in the twenty-first century are linked to water quantity and quality issues (UNESCO, 2009).
The major activities responsible for increase in water pollution are industrialization, urbanization and rapid growth in population. Pollutants, whether organic or inorganic, severely impact human health and the stability of natural ecosystems. It is projected that by the year 2050, untreated wastewater could reduce the world’s freshwater resources by as much as 18,000 km3 annually (Bao and Kuyama, 2013).
The practice of discharging wastewater into watercourses and natural wetlands was a common practice as a means of disposal in the past (McEldowney et al., 1998). This method is effective where discharges do not exceed the capacity of the receiving system to assimilate, absorb or detoxify contaminants present (Bayes et al., 1989). The major chemical pollutants in wastewater include ionic forms of nitrogen and phosphorus, heavy metals, detergents, pesticides, and hydrocarbons (Larsdotter, 2006). Unlike pollution associated with major plant nutrients in aquatic environment such as nitrogen and phosphorus, which can be assimilated to usable biomass at different trophic levels, heavy metals tend to accumulate and magnify at higher levels of the food chain. This is attributed to their non-biodegradability and persistence in the environment.
Heavy metal contamination can have devastating effects on human health and the ecological integrity of the receiving environment (Farombi et al., 2007). Although living organisms require varying amounts of heavy metals such as Iron, cobalt, copper, manganese, molybdenum and zinc, excessive levels can have deleterious effect on organisms (Rainbow, 2007). Zinc, for example, when present in high levels inhibit many plant metabolic functions resulting to retarded growth and senescence (Ebbs and Kochian, 1997), while high concentration of copper may severely damage gills, adversely affect the liver and kidneys of fish or cause some neurological damage (Flemming and Trevors, 1989).
Removal of heavy metals from wastewater can be accomplished through various treatment options, including chemical precipitation, coagulation, complexation, activated carbon adsorption, ion exchange and reverse osmosis. Although chemical precipitation has been extensively practiced in the removal of heavy metals from wastewaters (McEldowney et al., 1998), this technique is costly, requires continued supervision and sometimes present operational challenges (Upadhyay, 2004). Thus, effluents released from wastewater treatment plants may contain various heavy metals (Mapanda et al., 2005).
The use of natural and artificial (constructed) wetlands for wastewater treatment has been proposed as an intermediate technological solution for handling wastewater (McEldowney et al., 1998). These systems are economically attractive and relatively energy-efficient for wastewater treatment, especially for isolated populations, yet with capacity to improve aesthetic value of an area. The basic concept of these systems is deceptively simple and many constructed wetlands have not performed as expected.
Improvement in water quality in a wetland system may be assessed by monitoring and comparing a range of water-quality parameters at the inflow and outflow. Some of the parameters commonly used for monitoring wetland performance include dissolved oxygen content, biochemical oxygen demand, total phosphorus, total nitrogen, faecal coliform, pH, suspended solids, electrical conductivity, and heavy metal content. Corrective measures are then applied when results show that the system is not working according to the set objectives (Kayombo et al., 2001).
The wastewater stabilization ponds at Egerton University were constructed several decades ago when the institution was an agricultural college with a few hundreds of students and staff. Upon upgrading to a fully-fledged University in 1987, the population on campus increased steadily and so did the volume, as well as the complexity of wastewater. In 2007, a wetland was constructed to polish the effluent from the waste stabilization ponds before discharging it into River Njoro.
Statement of the problem
The discharge of treated wastewater into River Njoro has been an issue of concern to the general public and other stakeholders. The river not only serves as a source of water for downstream communities but also provides the bulk of the total freshwater inflow into Lake Nakuru – a wetland of international importance. Previous studies have reported the working status of the WSPs at Egerton University as only moderate, with respect to nutrients, pathogens and traces of heavy metals in the effluent. Heavy metals have been detected in water, plants, fish and sediments in Lake Nakuru. Heavy metals persist in the environment, bio-accumulate in organisms and bio-magnify along the food chain. There have been episodes of flamingo deaths in Lake Nakuru and heavy metals were found accumulated in the organs of their carcases. This led to the listing of heavy metal toxicity as one of the possible causes for their death. Therefore, water contaminated with heavy metals may adversely affect human health for the River Njoro riparian populations and the Lake Nakuru biodiversity.
Objectives
General Objective
To assess the efficiency of Egerton University constructed wetland in removing heavy metals from wastewater.
Specific objectives
1. To evaluate the physico-chemical characteristics (pH, temperature, electrical conductivity and dissolved oxygen) of water across the wetland profile.
2. To determine the concentrations of lead, copper, cadmium and zinc in water, plants and sediments in the wetland.
3. To assess the effect of selected physico-chemical parameters on retention of heavy metals in the wetland.
Hypotheses
H01: The physico-chemical parameters of water have no significant variation along the wetland profile.
H02: The concentrations of lead, copper, cadmium and zinc in water, plants and sediments have no significant difference across the wetland profile.
H03: There is no significant relationship between selected physico-chemical parameters of water and the retention of heavy metals in the wetland.
Justification of the study
Egerton University is obligated to protect human health and environmental integrity as well as comply with regulations and related standards for effluent discharge into the environment. There is also the need to implement sustainable environmental management programmes that meet international standards (ISO 14001:2015) and actively participate towards the achievement of Vision 2030 of Kenya and the SDGs (Goal 6), (UN, 2015). It is also a moral/ethical obligation for the University to ensure that the water quality of River Njoro does not change to the detriment of downstream users. There is also need to protect the flora and fauna in Lake Nakuru, a Ramsar site, from the effects of heavy metals pollution. Effectively treated wastewater has great potential for re-use by humans and livestock as well as supplement existing freshwater supplies. Data obtained in this study will provide valuable information on the wetland and the entire wastewater treatment system at Egerton University.
Scope of the study
The study covered Egerton University constructed wetland only and investigated selected physico-chemical parameters of wastewater (pH, temperature, Electrical conductivity and dissolved oxygen) and the content of selected heavy metals (Pb, Cu, Cd and Zn) in water, dominant macrophytes (P. stratiotes, C. alopecuroides, S. lacustris, and E. crassipes) and sediments along the entire wetland profile. The study was conducted over a period of six months (August 2013 to January 2014).
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Item Type: Kenyan Topic | Size: 55 pages | Chapters: 1-5
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