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INTERNATIONAL JOURNAL OF ENVIRONMENTAL SCIENCES Volume 3, No 5, 2013

© Copyright 2010 All rights reserved Integrated Publishing Association

Research article ISSN 0976 – 4402

Received on December 2012 Published on March 2013 1407

Heavy metal contamination in water and sediment downstream of

municipal wastewater treatment plants, Dar es Salaam, Tanzania Charles Kihampa

Department of Environmental Science and Management, Ardhi University, P. O. Box 35176,

Dar es Salaam, Tanzania

[email protected]

doi: 10.6088/ijes.2013030500011

ABSTRACT

The objective of this study was to determine concentrations of heavy metals leaving seven

municipal wastewater treatment plants (MWWTPs), and reaching streams and rivers

environment. Five metals including Cd, Pb, Cu, Zn and Cr, ranging from 0.01 to 0.27 mg/l

and 0.17 to 225.83 mg/kg dw were detected in water and sediments, respectively. Industries

and institutions wastewater were found to be the most important source for heavy metals in

wastewater treatment plants (WWTPs). Concentrations of Pb, Cd and Cr in water were found

to be higher than the maximum permissible limits as per Tanzania Bureau of Standard. High

concentration in sediments beyond maximum limits for sediment quality guidelines (SQG)

were detected for Pb, Zn and Cu. The results indicate that MWWTPs contribute to heavy

metal pollution to Dar es Salaam environment. Acute health consequences in the investigated

areas can be insignificant, but chronic adverse health effects due to prolonged pollution and

long term exposure through vegetables consumption, washings, animal keeping and fishing

can be inevitable.

Keywords: Environment, heavy metals, municipal wastewater, water, sediment.

1. Introduction

Urban environmental pollution by chemicals from wastewater has become of public interest

particularly in developing countries. Industrial development, modern civilization, poor

planning and rapid population growth that utilize large range of chemicals have led to the

introduction of uncontrolled and unknown contaminants into wastewater, many of them are

not removed completely in conventional wastewater treatment plants (Ouyang, 2005;

Battaglin et al., 2007). Human beings and animals can be exposed to a risk of intoxication by

these chemicals through affecting the quality of water, soil, and production and quality of

crops. Section 11 of the Tanzania's National Environmental Policy 1997 (NEP) and National

Management Act, 2004 (EMA) have identified environmental pollution as one of the key

problems that call for urgent attention (VPO, 2008). Proper disposal of wastewater and

protection of the environment is also among the prerequisite for the achievement of the

Millennium Development Goal (MDG 7), which calls for environmental sustainability by

integrating the principles of sustainable development into country policies and programmes,

and reverse of environmental pollution processes as well as loss of environmental resources

(UNS, 2010). However, like many developing countries, Tanzania is still facing

environmental problems of diverse nature and some of which are growing day by day (Mato,

2002). For instance, industrial effluents have been reported to pollute rivers like Msimbazi in

Dar es Salaam, Karanga in Moshi, Mwirongo in Mwanza and Thermi in Arusha (VPO,

2008). In Dar es Salaam, high levels of heavy metals Pb, Cr, Cu, Cd, Zn and As have been

Heavy metal contamination in water and sediment downstream of municipal wastewater treatment plants, Dar

es Salaam, Tanzania

Charles Kihampa

International Journal of Environmental Sciences Volume 3 No.5, 2013 1408

frequently detected in streams/rivers, soils and vegetables grown along streams and river

banks (Mwegoha and Kihampa, 2010). The foremost source of the heavy metals has being

considered to be untreated or treated wastewater from industries that discharge their

wastewater direct into streams and rivers, effluents from municipal wastewater treatment

plants (MWWTPs) have not been well thought-out to be a significant source.

In Tanzania, only 9 cities Dar es Salaam, Tanga, Morogoro, Dodoma, Arusha, Mbeya,

Mwanza, Tabora and Moshi have sewerage systems, whereby the collected sewage is treated

in waste stabilization ponds (WSPs) before being discharged into receiving water bodies

(VPO, 2008). WSPs are regarded as the method of first choice for treatment of municipal

wastewater in tropical and sub-tropical countries. They are considered to be the most

appropriate method for treating domestic and industrial wastewater to produce effluent that

meets recommended microbial and chemical quality guidelines both at low cost and with

minimal operational and maintenance. However, it has also been established that many WSPs

are discharging effluent that do not conform to the required standards for effluent disposal.

The main reasons range from poor operation and maintenance, overloading, poor design and

technologies used are not very effective to treat wastewater originating both from domestic,

hospitals, industries and institutions (Kaseva et al., 2008; Kayombo et al., 1998; Othman et

al., 1995). Heavy metals are therefore likely to be found in water and sediments of rivers,

streams and groundwater receiving effluents from WSPs.

In previous studies to assess the pollution levels of heavy metals and other industrial

chemicals in Dar es Salaam environment, some results on types and concentrations of heavy

metals in streams and rivers have been documented (Mwegoha and Kihampa, 2010). This

study aimed at evaluation of type and levels of heavy metal leaving the WWTPs and reaching

streams and rivers system. In addition, sediments adjacent to WWTPs have been analysed to

evaluate the discharge’s impact on metal accumulation.

2. Materials and method

2.1 Study area and sampling

The study was conducted along the channels from seven MWWTPs in Dar es Salaam City,

Tanzania (Figure 1). The WWTPs receive wastewaters from domestic, institutions, hospitals

and industries such as manufacturing batteries, textile, steel, paints, food processing, abattoir,

and electrical products. WWTPs discharge their effluents into the Dar es Salaam rivers and

streams that eventually flows into Indian Ocean. Different activities are carried out along the

rivers and streams, including irrigation for green vegetable farms, fishing, washings and

extraction of sand for construction. The treatments in the seven WWTPs are conducted

through anaerobic, facultative and maturation ponds.

A total of 42 samples comprised of 21 water samples and 21 sediment samples were collected

between March and May 2012. Three water samples and three sediment samples were

collected about 10 m downstream of discharge point of each of the WWTP. The name of the

plant, geographical location, wastewater source, significant industrial discharge and the

receiving water body are presented in Table 1. Water samples were collected using 500 mL

plastic bottles. The sampling bottles for heavy metal determination were pre-soaked

overnight with 10% HCl, rinsed with distilled water and later rinsed with effluent/stream

water before sample collection. Sampling bottles for the determination of physicochemical

parameters were cleaned and rinsed using distilled water only. Preservation of water samples

was done by adding 2 drops of concentrated HNO3 to each water sample before storage


es Salaam, Tanzania

Charles Kihampa


below 4°C until analyzed. Sediment samples were collected at depths of 0 to 15 cm. The

samples were wrapped in aluminum foils then stored in plastic bags and transported to the

laboratory for heavy metal extraction and analysis.

Figure 1: Site descriptions and sampling points’ allocation

2.2 Sample preparation

Water samples were analysed directly into AAS, without any treatment. Sediment sample

was oven dried at 105°C for 24 h, followed by grinding and sieving using 0.18 mm sieve. 0.5

g of dry sediment sample was poured into a graduated test tube and mixed with 2 ml of aqua

regia 1:3 (1 conc. HCl: 3 conc. HNO3). The mixture was digested on a hot plate at 95°C for 1

h and allowed to cool to room temperature. The sample was then diluted to 10 ml using

distilled water and left to settle overnight. The supernatant was filtered prior to analysis using

AAS.


es Salaam, Tanzania

Charles Kihampa


2.3 Analytical methods

2.3.1 Reagents

All standard reagents used were analytical grade obtained from Romil limited, England.

2.3.2 Heavy metals

Heavy metals were determined using Perkin Elmer Analyst 100 AAS with Perkin Elmer

HGA 850 Graphite Furnace and Perkin Elmer AS 800 Auto-sampler made in Germany. For

analytical quality assurance, after every five sample readings, standards were run to make

sure that the margin of error is within 5%. In every analytical batch, 10% samples of all were

analyzed repeatedly to ensure the precision and accuracy of analysis. Standard reagents and

blanks were also used in the process of analysis to ensure the precision. A 10 cm long slot-

burner head, a lamp and a standard air-acetylene flame were used. The detection limit was

0.01 ppm (0.01 mg/kg), slit width 0.70 nm and elements wavelength were 228.8, 357.9, 324.8

and 283.3 nm for Cd, Cr, Cu and Pb, respectively.

Table 1: Description of the seven wastewater treatment plants investigated

Sampling site

(Plant)

Geographical

position

Wastewater sources Significant industrial

discharges

River/Strea

m

S1 (UDSM) S 06.77768

E039.21402

Domestic, laboratories,

workshops, health centre

70% Institution

30% Residential

Mlalakuwa

S2 (Mabibo) S06.81151

E039.22733

Industries, institutions,

residential

50% Industrial

50% Residential

Msimbazi

S3

(Vingunguti)

S06.83721

E039.23685

Industries 85% Industrial

15% Residential

Msimbazi

S4 (Buguruni) S06.82560

E039.24585

Residential 75% Residential

15% Industrial

Msimbazi

S5 (Kurasini) S06.85297

E039.29103

Industrial, residential 80% Residential

20% Industrial

Kurasini

S6

(Mikocheni)

S06.76829

E039.22780

Industrial, residential 85% Industrial

15% Residential

Mikocheni

S7 (Lugalo) S06.74358

E039.22780

Ammunition, hospitals,

schools and laboratories

75% Institution

25% Residential

Mlalakuwa

3 Results and discussion

Table 2 summarises the mean values of measured physico-chemical variables in the effluent

water samples from seven sampling sites. The temperature for all samples was relatively

uniform throughout the study area. The temperature range of 26.05 to 26.90°C did not exceed

recommended permissible Tanzanian limits for municipal and industrial wastewater effluents

destined for release in the environment (TBS, 2005). The pH values of the samples were

generally neutral, except samples from S4 and S7 were slightly alkaline with pH values of

9.45 and 9.52, respectively. These pH levels decreases the availability of the metals ions in

water as the metal would remain attached in substrate and complexes (Adhikari et al., 2006).

With exception to sampling points S1 and S7, TDS values for the other sampling sites were

above the TBS maximum limits of 500 mg/l. Salinity levels were generally high compared to

the standard limits by TBS of 0.1. The dissolved solids and salinity play significant roles in


es Salaam, Tanzania

Charles Kihampa


the water quality index and availability of metals in water. The high levels of TDS and

salinity presumed that the availability of heavy metals detected in the water samples were

influenced by elevated salt concentrations, due to the increase of competition between cations

and metal binding sites in which metal have been driven off into overlaying water (Connell

and Miller 1984).

Table 2: Mean concentration of physico-chemical parameters in water effluent samples

collected from MWWTPs

Sampling site

Water sample (mg/l), n=3 CL = 95%

Temp (oC) pH TDS (mg/l) Salinity Conductivity

(S/cm)

S1 (UDSM) 26.05±3.04 7.59±0.01 243.5±2.12 0.2±0.0 503.0±4.24

S2 (Mabibo) 26.6±3.25 8.27±0.23 903.0±24.04 0.9±0.0 1805±46.67

S3

(Vingunguti) 26.9±4.526 8.01±0.22 972.0±111.72 0.95±0.07 1937.5±215.67

S4 (Buguruni) 26.65±3.61 9.52±0.04 1062.0±83.44 1.05±0.07 2108.0±158.39

S5 (Kurasini) 26.9±4.10 8.44±0.65 574.0±0.0 0.55±0.07 1165.5±0.71

S6

(Mikocheni) 26.45±3.47 7.82±0.09 602.5±122.33 0.6±0.14 1221±241.83

S7 (Lugalo) 26.55±3.18 9.45±0.06 150.65±16.62 0.1±0.0 313.5±34.65

Tables 3 shows the range and mean concentration of heavy metals in effluent water and

sediments along the channels of seven WWTPs. The table also indicates recommended

Tanzania permissible levels of heavy metals for the quality of wastewater destined for release

in the environment TZS 860:2005. Since Tanzania has not established the sediment quality

guidelines (SQG) yet, the consensus based threshold effect concentration (CB-TEC) (Table

3) as reported by MacDonald et al., 2000 was used for comparison. The amounts of metal

concentrations in sediment samples were higher than in water samples. Metals in water

normally are adsorbed onto suspended particles and eventually settle to the sediments. They

are slow to degrade in the environment, and sediments in particular are ‘sinks’ where

chemicals tend to concentrate.

The concentrations in water for all the metals except Pb at S3 and S5 were lower than the

maximum permissible limits for municipal and industrial wastewater quality standards

propagated by Tanzania Bureau of Standard (TBS, 2005) TZS 860:2005. This investigation

has also noticed that water and soils in receiving streams and rivers are highly used for

various domestic activities related to human consumption such as fishing, washing, and

vegetable cultivation and irrigation. In order to qualify suitability of this water for human

consumption, concentrations of heavy metals in water (Table 3) were also compared with

Tanzania standards for drinking water TZS 789:2003 (TBS, 2003). Consequently

concentrations of heavy metals Pb, Cd and Cr in S2-S7 were generally high compared to

maximum limits for drinking water quality. Recent data indicate that adverse health effects of

some heavy metals like cadmium may occur at lower exposure levels than previously

anticipated, primarily in the form of kidney damage but possibly also bone effects and


es Salaam, Tanzania

Charles Kihampa


fractures (D’Mello, 2003; Mushtakova et al., 2005). Therefore, the high concentrations in this

water infer the possible hazard to human health.

High concentrations of heavy metals in sediments were detected downstream to Kurasini (S5)

and Mikocheni (S6) WWTPs (Table 3). Figure 2 displays the site variations of the selected

metal concentrations (Pb, Zn, Cd, Cu and Cr) in sediments collected from 7 sampling sites

downstream to WWTPs. The mean concentrations of Pb at S5 and S6 as well as

concentrations of Zn and Cu at S5 were higher than CB-TEC guidelines. Even though most

of the sediment samples had concentration within the acceptable limits, the fact that these

metals have (bio-) cumulative, persistent and synergistic characteristics imply that prolonged

pollution can affect ecosystem health and function, human health and wellbeing, and

undermining human security. These results clearly explain that the sediments along receiving

streams are contaminated by heavy metals and not safe for domestic activities associated to

human consumption.

Table 3: Mean concentration of heavy metals in water effluent and sediment

samples collected from MWWTPs

Sampling site Water sample (mg/l), n=3 CL = 95%

Pb Zn Cd Cu Cr

S1 0.09+0 0.01+0 0.01+0 0.08+0.01 0.02+0.01

S2 0.11+0.02 0.04+0.03 0.05+0.02 0.02+0 0.01+0

S3 0.17+0.15 0.04+0.01 0.04+0.04 0.01+0 0.10+0.07

S4 0.12+0.05 0.06+0.02 0.03+0 0.02+0.01 0.11+0.1

S5 0.27+0.13 0.04+0.16 0.04+0.03 0.01+0 0.13+0.1

S6 0.11+0.02 0.02+0.01 0.06+0.03 0.01+0 0.03+0.02

S7 0.16+0.03 0.03+0.01 0.05+0 0.03+0.01 0.06+0.02

TZS 860:2005 0.1 5 0.1 2.0 1.0

TZS 789:2003 0.1 0.05 0.05

Sediment sample (mg/kg-dw)

S1 10.08+0.02 42.68+0.01 0.19+0.02 5.55+0.02 9.53+0.03

S2 13.2+2.21 74.29+1.07 0.45+0.03 17.25+0.25 9.75+0.15

S3 10.13+1.03 14.63+1.04 0.17+0.06 1.38+2.31 5.77+3.16

S4 13.13+2.03 50.55+2.02 0.29+0.11 6.75+0.25 13.88+3.05

S5 81.9+1.9 225.83+0.99 0.22+0.12 175.05+0.0

1 6.6+0.7

S6 153.9+2.2 96+0.1 0.6+0.15 15.88+0.41 20.54+0.03

S7 10.95+0.04 25.58+0.96 0.3+0.11 3.08+0.01 8.4+0.1

SQGa 35.8 121 0.99 31.6 43.4

a - Consensus based threshold effects concentration (CB-TEC)


es Salaam, Tanzania

Charles Kihampa


Pb

0

20

40

60

80

100

120

140

160

S1 S2 S3 S4 S5 S6 S7

Sampling sites

Co

nce

ntr

atio

n (

mg

/kg

dw

) March

April

May

Zn

0

20

40

60

80

100

120

140

160

180

200

220

240

S1 S2 S3 S4 S5 S6 S7

Sampling sites

Con

centr

atio

n (

mg/k

g d

w)

March

April

May

Cd

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

S1 S2 S3 S4 S5 S6 S7

Sampling sites

Co

nce

ntr

atio

n (

mg

/kg

dw

)

March

April

May

Cu

0

20

40

60

80

100

120

140

160

180

200

S1 S2 S3 S4 S5 S6 S7

Sampling sites

Co

nce

ntr

atio

n (

mg

/kg

dw

)

March

April

May

Cr

0

5

10

15

20

25

S1 S2 S3 S4 S5 S6 S7

Sampling sites

Conce

ntr

atio

n (

mg/k

g d

w)

March

April

May

Figure 2: The distribution of selected heavy metals (Pb, Zn, Cd, Cu and Cr) in sediments

from 7 sampling sites in Dar es Salaam MWWTPs

4. Conclusion

The results of this study revealed compositions and distributions of heavy metal contaminants

in water and sediments along the channels downstream of MWWTPs that enters Dar es

Salaam rivers environment. The findings indicate that despite of the conventional treatment

of municipal wastewater by WSPs, the disposed treated wastewater effluents are still


es Salaam, Tanzania

Charles Kihampa


contaminated with heavy metals. The levels of the heavy metals detected are not remarkable

to cause acute health consequences in receiving water, but chronic adverse health effects due

to prolonged pollution and long term exposure through vegetables consumption, washings,

animal keeping and fishing can be inevitable. These problems signify prerequisite to initiate

technologies that can remove heavy metals before effluents are discharged. They also call for

constant monitoring in order to ensure the environmental sustainability of streams and rivers

environments.

Acknowledgements

The author is very grateful to the work well done by Ms Glory Marandu during sample and

data collection, organisation and analyses. Mr Stalin Mkumbo and Mr. Addo Ndimbo from

the School of Environmental Science and Technology, Ardhi University are appreciated for

their invaluable technical assistance during the field sampling, laboratory extraction and

heavy metals analysis in the laboratory.

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