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PASJ 2022. DOI: 10.47787/pasj.v1i02.12 www.panafricajournal.com
Pan Africa Journal of Sciences
ISSN 2709-1473 | Open Access
Assessment of Heavy Metal Concentrations (Cu, Cd, Pb, and
Zn) in Wastewater from Gusii Treatment Plant in Kisii
County, Kenya
Rayori Douglas1*, Getabu Albert1, Omondi Reuben1, Orina Paul2, Nchore Hellen3, Gisacho Boniface4, Nyabaro
Obed4, Omondi Argwings5, Omweno Job1
1 Department of Environment, Natural Resources and Aquatic Sciences, Kisii University, P.O. Box 408-40200, Kisii, Kenya.
2 Kegati Aquaculture Centre, Kenya Marine and Fisheries Research Institute (KMFRI), P.O. Box 3259-40200, Kisii, Kenya.
3 School of Chemistry and Material Sciences, Technical University of Kenya, P. O. Box 52428-00200, Nairobi, Kenya.
4 Department of Chemistry, School of Pure and Applied Sciences, Kisii University, P.O. Box 408-40200, Kisii, Kenya.
5 Department of Medical and Applied Sciences, Sigalagala National Polytechnic, P.O. Box 2966, Kakamega, Kenya.
* Corresponding author: drayori@kisiiuniversity.ac.ke
Received: 7th January 2022; Accepted: 8th February 2022; Published: 10th February 2022
DOI: 10.47787/pasj.v1i02.12
Volume 01 – Issue 02
RESEARCH ARTICLE: Environmental Ecology, Technology & Engineering
Abstract
The concentrations of heavy metals were determined from wastewater samples collected from the Gusii wastewater
treatment plant, from May to July, 2021. Heavy metal analysis was done using a flame atomic absorption
spectrophotometer, model AA 7000 Shimadzu, Japan. The results showed that the concentrations of Zinc and
Cadmium were below the detection limit for all the sampling sites. The concentrations of Lead and Copper (Mean
± SE) ranged between 0.34 ± 0.06 mg/L and 0.86 ± 0.08 mg/L and 0.25 ± 0.05 and 0.34 ± 0.01 mg/L respectively.
The month of July exhibited a higher mean Cu concentration of 0.35 ± 0.004 mg/L compared to the mean Cu
concentration (0.2 ± 0.02 mg/L) of May. Likewise, the mean lead concentration of May (0.60 ± 0.04 mg/L) was
higher than the mean (0.53 ± 0.05 mg/L.) of July. The independent sample t-test showed that mean Cu concentration
difference was significant between the sampling months (t (34) = 21.58; p < 0.05) while for Pb it was not significant
between the sampling months (t (30) = 1.241; p = 0.274). The percentage removals of Copper and Lead were generally
low at 12.61 % and 6.27 %, respectively. The continued discharge of effluent into River Riana may lead to
accumulation of heavy metals in the environment, which in turn poses health risks to the general public. Therefore,
the study recommends that Gusii Water and Sanitation Company continue monitoring and assessing the levels of
heavy metals in the treatment plant for its sustainability.
Keywords: Wastewater; Heavy metal concentration; Cadmium; Copper; Zinc; Lead; Permissible limits
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1. INTRODUCTION
The increased level of heavy metal discharge into the environment and their associated risks has
necessitated in-depth research into their occurrence in waste waters. Heavy metals are non-biodegradable
and often bio-accumulate in animals through food chains, building up their concentrations to toxic levels.
Human beings can also get exposed to heavy metals through consumption of foods [1, 2, 3, 4, 5]. The
growing concern of heavy metal pollution in aquatic ecosystems has attracted high scientific and policy
interest. Their sources can be traced to the discharge of waste water from domestic, municipal and
industrial sources, which have become widespread globally [1, 4, 6, 7, 8]. Monitoring of heavy metals
concentration in the aquatic environment has been done through determination of their concentrations in
lotic and lentic water systems or wastewater discharge, especially sewage treatment sites, effluent, sludge,
sediments, and the plankton community (phytoplankton and zooplankton) [3, 5, 9, 10, 11, 12].
Gusii wastewater treatment plant is a conventional plant for wastewater treatment and is the only sewer
system in Kisii County, South-Western Kenya. The lagoon design capacity is 15,000 m3/day an upgrade
from a previous design capacity of 8,000 m3/day. This was purposely to cater for the increased amount of
domestic and industrial wastewater resulting from increased population coupled with heavy rains [13].
Gusii wastewater treatment plant is surrounded by several households who practice mixed subsistence
farming of crops and animals and could either benefiting or affected by the treatment plant. The notable
crops grown include maize, bananas, sugarcane, kales, and Napier grass among others. The processed
sludge from sewage ponds is at times used as fertilizer for these plants [14]. However, limited information
is available on its conformity to standards recommended for application as fertilizer. On the other hand,
the common source of clean water for domestic and agricultural uses for the surrounding households
included either from water springs, boreholes or roof catchments and River Riana as well (Figure 1) [13].
Few studies have been conducted on the treatment plant thus limited information is available on its
effectiveness in wastewater treatment. The selected physical-chemical parameters measurements data
available from GWASCO (2015) indicates that the previous design of the treatment plant effluent
discharged did not meet the stipulated standards by the National Environment Management Authority
(NEMA).
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Figure 1: Some of the activities that occur around the Gusii wastewater treatment plant. Clockwise:
Napier grass planted at the effluent discharge point to river Riana; Banana and sugarcane farming along
River Riana; Water point source for domestic use; Animals (goats) grazing along River Riana water bank
(Source: Author)
The most recent study focused on phytoplankton diversity [15]. Previously, a study conducted by [16]
was on the effectiveness of constructed wetland in polishing wastewater effluent from the treatment plant.
Both of these studies were carried out on the previous treatment plant design. To this end, limited
information is available concerning heavy metals both for the previous and the current treatment plant.
Therefore, the present study was undertaken to determine the concentrations of selected heavy metals (Cu,
Cd, Pb, and Zn) in wastewater from Gusii wastewater treatment plant, to inform policies concerning
human and environmental health.
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2. MATERIALS AND METHODS
2.1 Study area
Gusii wastewater treatment plant is a conventional plant for wastewater treatment and is the only sewer
system in Kisii County, South-Western Kenya. The geographical location of the wastewater treatment
plant is latitudes 00 39’ 30’’ S and Longitude 340 42’ 30’’ E. The treatment plant consists of receiving
units in the screens, grit chambers, and channels to the lagoon. Wastewater moves through the grit
chambers and channels to the lagoon for the removal of debris and is channelled into two anaerobic ponds,
then facultative pond, and two maturation ponds (that’s first and second maturation pond) for treatment
before the effluent is discharged into River Riana (Figure 2).
Figure 2: A sketch showing the wastewater treatment stages and sampling points (marked with star) in
Gusii wastewater treatment plant (Author).
2.2 Wastewater sampling
Wastewater samples were collected in the months of May and July, 2021. There were nine sampling points,
including the inlet, two anaerobic (two composite samples) ponds, one facultative, and two maturation
ponds, the effluent before discharge into the river besides to three points in the river, that is, at the
confluent and 100 meters up and downstream at the effluent discharge point into River Riana (Figure 2).
Wastewater samples were collected in duplicate using 500 ml different transparent plastic bottles totalling
18 samples per sampling session. The sampling bottles were washed thoroughly using a detergent and
rinsed twice with distilled water followed by 10% Nitric (HNO3) acid before embarking on fieldwork.
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The collected wastewater samples were preserved by adding 5 ml of concentrated Nitric acid (HNO3) to
prevent the precipitation of the metals, algae growth and microbial activity. All the collected samples were
clearly labelled, packed and then transported to the laboratory in a cooler box containing ice for heavy
metals analysis.
In the laboratory, 100 ml of each wastewater sample was measured using a measuring cylinder and then
transferred into 250 ml separate beakers. Each sample was digested on a hot plate using 10 ml of Aquaregia
(a mixture of HNO3 and HCl in the ratio 3:1) inside a fume hood. The digested solution of each sample
was then filtered using Whatman No. 42 filter paper and transferred into a separate 100 ml volumetric
flask. The solution was then diluted to the mark with distilled water. The samples were transferred into
clearly labelled separate plastic bottles. In the place of a sample, for the blank, distilled water was used
and the same procedure for digestion was followed. Heavy metal analysis was done using the flame atomic
absorption spectrophotometer, AA 7000 Shimadzu, Japan model.
2.3 Instrument’s operating conditions
The Atomic Absorption Spectroscopy (AAS) operating conditions for heavy metals analysis are
summarized in Table 1. Moreover, the operating conditions were as per the manufacturer's recommended
conditions.
Table 1: The AAS operating conditions
Metal
Lead (Pb)
Zinc (Zn)
Copper (Cu)
Cadmium (Cd)
Wavelength (nm)
283.22
213.73
324.8
228.87
Slit width (nm)
0.7
0.7
0.7
0.7
Lamp current (mA)
10.0
8.0
8.0
8.0
Oxidant flow rate (L/min)
15.0
15.0
15.0
15.0
Fuel flow rate (L/min)
2.0
1.80
1.80
1.80
Burner height (mm)
7.0
7.0
7.0
7.0
Instrument detection limit
0.0038
0.0798
0.0508
0.0092
Flame used
Air/Acetylene
Air/Acetylene
Air/Acetylene
Air/Acetylene
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2.4 Stock solutions, working standards, and calibration curves
All the standards used during this study were freshly prepared whenever the analysis was carried out.
Stock solutions of 1000 mg/L for each metal were prepared using analytical grade salts which were dried
and cooled before weighing. From the prepared stock standard solutions, intermediate standard solutions
were prepared for each element using serial dilution in 50 ml volumetric flasks. The prepared standards
for each element, one after the other, were aspirated into the AAS and their absorbance recorded.
Calibration curves were plotted for each element using the absorbance against concentrations. To obtain
the Y-intercept for each element curve, extrapolation was done while the gradient for each curve was
provided by Microsoft Excel. The respective heavy metal calibration curve prepared was then used to
determine the concentration of heavy metals in the digested wastewater samples.
To validate the method used for heavy metal analysis and ascertain the accuracy of the AAS analytical
procedure, samples with unknown heavy metal concentrations were spiked with standards of known
concentrations and their percentage recovery determined for the respective heavy metals using the
equation 1:
Equation 1
If the calculated values were within 80 – 120% then they indicated good accuracy for the analysis
procedure (Agoro et al., 2020).
2.5 Heavy metals removal efficiency
Any wastewater treatment plant is deemed effective in wastewater polishing when pollutants are removed
so that the effluent discharged meets the required national standards (Water quality Regulations, 2006;
Legal notice No. 121). To determine the effectiveness of the Gusii wastewater treatment plant design in
wastewater polishing, the heavy metal removal efficiency was calculated according to [6] (equation 2).
Equation 2
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2.6 Compliance index
In Kenya, the Kenya Bureau of Standards (KEBS) and the National Environmental Management
Authority (NEMA) provide guidelines on quality requirements for effluent discharge to the environment
with the intent of controlling pollution. The compliance index value was calculated to show the
effectiveness of the treatment plant design in wastewater polishing. The index value of less than 1 indicates
compliance to the set standards for effluent discharge into the environment or surface waters while the
index of greater than 1 indicates non-compliance. In this study, the compliance index for the heavy metals
was calculated using the equation 3 [6]:
Equation 3
2.7 Statistical analysis
Data recorded from heavy metals analysis was analyzed using SPSS 22.0 for windows. One-way ANOVA
(Analysis of Variance) was performed for determination of variation in the mean value of the heavy metal
concentration between the sampling stations while independent sample t-test was performed for
determination of variation in the mean value of the heavy metal concentration between the sampling
months. The significance differences were determined at p < 0.05.
3. RESULTS
3.1 Calibration curves
Figure 3: shows the individual heavy metal calibration curves used for the determination of the respective
heavy metals concentrations in wastewater samples.
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Figure 3: Calibration curves for the determination of the respective heavy metals concentrations in
wastewater samples
3.2 Spatial variation of the heavy metals concentrations
The results of the concentrations of four heavy metals (Cu, Cd, Pb and Zn) determined from wastewater
samples from Gusii treatment plant are shown in table 2.The concentrations of Zn and Cadmium (Cd)
were below the detection limit in all the sampling sites. However, the concentration of lead (Pb) was only
below the detection limit at the confluent sampling point.
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Figure 4: Mean (± SE) spatial variation of heavy metal concentrations (in mg/L) in wastewater from Gusii
treatment plant.
The Pb mean concentration of the sampling points ranged from 0.34 ± 0.06 mg/L to 0.86 ± 0.08 mg/L.
The facultative pond had the highest Pb mean concentration with 0.86 ± 0.08 mg/L. One-way ANOVA
test showed that mean Pb concentration was not significantly different among the sampling stations (F (7,
24) = 1.827; p = 0.128). The mean concentration of Cu of the sampling stations also ranged from 0.25 ±
0.05 mg/L to 0.34 ± 0.01 mg/L. The confluent sampling station had the highest concentration while the
maturation pond 2 sampling station had the least Cu concentration with 0.34 ± 0.01 mg/L and 0.25 ± 0.05
mg/L, respectively (Figure 4). One-way ANOVA test showed that the mean Cu concentration was not
significant among the sampling stations (F (8, 27) = 0.354; p = 0.935).
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
Concentration (mg/L)
Cu
Pb
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3.3 Temporal variation of the heavy metals concentrations
Table 2 represents the mean (±SE) heavy metal concentrations for the two sampling months. The mean
Pb concentration recorded for the month of May (0.60 ± 0.04 mg/L) was higher than the mean
concentration (0.53 ± 0.05 mg/L) recorded in July. The independent sample t-test showed that mean Pb
concentration was not significant between the sampling months (t (30) = 1.241; p = 0.274). The month of
July measured a mean Cu concentration of 0.35 ± 0.004 mg/L which was higher compared to the month
of May which had a mean Cu concentration of 0.2 ± 0.02 mg/L. The independent sample t-test showed
that mean Cu concentration was significant between the sampling months (t (34) = 21.58; p < 0.05) (Table
2). The mean concentrations for Zn and Cd were below the detection limit in both sampling months during
the study period.
Table 2: Mean (±SE) temporal variation of heavy metal concentrations (in mg/L) in wastewater from
Gusii treatment plant.
Sampling months
Heavy metal concentrations (mg/L)
Pb
Cu
May
0.60 ± 0.04
0.20 ± 0.02
July
0.53 ± 0.05
0.35 ± 0.004
t- value
t (30) = 1.241; p = 0.274
t (34) = 21.58; p < 0.05
3.4 Heavy metals removal efficiency
The performance of the Gusii wastewater treatment plant was assessed in terms of removal efficiency and
the calculated percentage reduction of the respective heavy metals. The percentage removals of Copper
and Lead were generally low at 12.61 % and 6.27 %, respectively. The low percentages of removal of
heavy metals are generally an indication that the treatment plant does not have a good capacity to remove
these elements from the influent wastewater. Nevertheless, the percentage reductions of Cadmium and
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Zinc were not calculated for the treatment plant because their concentrations were generally below
detection limits.
3.5 Compliance index
The compliance indexes for Cadmium and Zinc were not calculated and referenced for the treatment plant
because their concentrations were generally below detection limits. The index value for Copper, 0.25 was
below 1 indicating compliance. However, the compliance index value for Lead was 53, the value which
is greater than 1, indicating non-compliance to the specified NEMA standards for effluent discharge to
the environment (Table 3).
Table 3: NEMA Standards for effluent discharge into the environment
Parameter
NEMA Maximum Allowable
effluent discharge (Limits)
Effluent
discharge
Compliance
index value
Remark
Lead (Mg/L)
0.01
0.53 ± 0.14
53
Non-compliant
Cadmium (Mg/L)
0.01
BDL
Zinc (Mg/L)
0.5
BDL
Copper (Mg/L)
1.0
0.26 ± 0.15
0.26
Compliant
4. DISCUSSION
Unlike in most developing countries, wastewater is treated before discharge to the environment in the
developed countries. Wastewater is of poor quality as it is rich in pathogenic micro-organisms, heavy
metals, organic and inorganic chemicals, and toxic substances hence not suitable not only for domestic
use but also unfit for and agricultural uses and introduction to the aquatic environment [17]. The effects
associated with the release of untreated or partially treated wastewater include degradation of aquatic
ecosystems, outbreak of food and water-borne diseases, and environmental pollution [4, 6, 17, 18]. Despite
the success in using treated wastewater in aquaculture and other agricultural purposes in India with ready
market for the products, the use of the wastewater in the African continent more so Kenya, is uncertain
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mainly on product safety for human consumption [19, 20]. Therefore, knowledge of the nature and
composition of wastewater is critical in wastewater treatment, disposal and re-use. In Kenya, National
Environment Management Authority (NEMA) and Kenya Bureau of Standards (KEBS) provide
guidelines on quality requirements for effluent discharge to the environment.
In the current study, heavy metals were analyzed in wastewater from the Gusii wastewater treatment plant
and three points along river Riana at the effluent discharge point. Cd and Zn concentrations in the
wastewater samples were below the detection limit of the instrument that was used for heavy metal
analysis and this can be attributed to the fact that they have a lower solubility characteristics compared to
Pb. This corroborates with a study by [21] in a study that investigated heavy metals on fish, water, and
sediments sampled from Southern Caspian Sea, Iran. Therefore, the high levels of Pb in wastewater should
be of public health concern in terms of wastewater reuse and discharge into water bodies. The high levels
of Pb in wastewater can be attributed to greater solubility characteristics and effluents from institutions
(that’s higher learning and research institutions), hospitals and industries within the town. Some studies
have also shown that lead dominates most of the commonly used metal products, paints, pesticides,
pipelines and cables and this can be the reason for the higher levels in the wastewater sample [22]. The
confluent sampling station had the highest concentration of Cu and this can be attributed to anthropogenic
activities and institutional effluents that’s learning and research institutions), hospitals and industries
within the town. On the other hand, the downstream sampling station had the least Cu concentration and
this can be attributed to bioaccumulation by aquatic organisms [23].
Simultaneously, the low concentrations of heavy metals (Zn and Cd) in the wastewater treatment plant
can be attributed to several factors. These include dilution by heavy rainfall in the region during the
sampling period and uptake by phytoplankton and zooplankton which might have bio-accumulated the
heavy metals [11]. In most instances, the sediments might have also act as sinks removing heavy metals
from the water column. Consequently, [6] reported heavy metal variations in wastewater and sewage
sludge from municipal treatment plants in Eastern Cape Province, South Africa. Furthermore, [11] also
studied heavy metal concentration in the bottom sediments Lake Koka in Ethiopia, which they found to
be higher, compared to the wastewater. The retention period of wastewater in treatment plant respective
ponds might be short, therefore, some of the metals might have been transported out of the plant resulting
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in their low concentrations in the treatment plant [24, 25]. Low connectivity of the sewer lines in the
region has led to improper heavy metals contaminated wastewater being disposed to open channels by the
residents. A previous study by [7] showed that wastewater and soil samples were drawn from open waste
channels in industrial areas in Nairobi contained heavy metals.
5. Conclusions
Gusii wastewater treatment plant plays an important role in the treatment of urban and municipal
wastewater from Kisii town and its environs. The observed results showed that Cadmium and Zinc were
below the detection level in all sampling stations. The concentration of Copper was within the permissible
limits by National Environment Management Authority for effluent discharge into the environment while
those of Pb were above the set limits. Therefore, the continued discharge of the effluent to River Riana
may lead to the accumulation of heavy metals in the environment in turn causing a variety of health and
environmental impacts. As a result, Gusii Water and Sewerage Company should consistently monitor and
assess the levels of heavy metals in the treatment plant for its sustainability. Use of permanent heavy
metals sensors linked to a central computer processing point will not only provide data but also inform
need for management improvement over time. There is great need for further research as the County’s fast
growing population exerts pressure on the wastewater treatment plant service into the future.
Acknowledgements: The authors are grateful to Kisii University and the African Development Bank
(AfDB) for the financial and logistical support of this research. We also thank National Commission for
Science Technology and Innovation (NACOSTI) for permission to carry out this research. We are grateful
to Gusii Water and Sanitation Company, Kisii County for the free access to the wastewater treatment plant
during sampling. Also, we are grateful to the Technical University of Kenya (TUK) for laboratory
assistance.
Author Contributions: Conceptualization, R.D., O.A; methodology, R.D., and O.A; validation, G.A.,
O.R., O.P., O.R., and O.A; formal analysis, R.D., O.J., and O.A; data curation, R.D.; writing—original
draft preparation, R.D., O.J, O.A., O.R., N.H., G.B., and N.O
Conflict of Interest: The authors declare no conflict of interest. The funding sponsors had no role in the
design of the study and in the decision to publish the results.
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