IMPACT OF ZIMUNYA TOWNSHIP’S SEWAGE WASTE DISPOSAL SYSTEM
ON MUROWA RIVER IN THE LOWER ODZI SUB-CATCHMENT,
ZIMBABWE
M. Mukwashi1 and M. Masocha2
1
2
mufarogm@yahoo.co.uk
Department of Geography & Environmental Sciences, University of Zimbabwe,
P.O. Box MP 167, Mount Pleasant, Harare, Tel. 263 (0)4 303211 ext. 1265, Fax: 263
(0)4 332059, masocham@arts.uz.ac.zw (Corresponding author)
Abstract
Most rural service centres in Zimbabwe have rudimentary sewage disposal systems that
contribute to water pollution problems. This study examines and explains the impact of
Zimunya township’s sewage disposal system on water quality of Murowa river in the
Lower Odzi sub-catchment of eastern Zimbabwe. Sewage disposal at Zimunya is
characterised by frequent overflows and pipe bursts. Grab water samples were collected
at 6 sampling sites namely 500m upstream of Murowa River, pond inflow, pond outflow,
200m, 500m, and 1500m downstream at two-week intervals between November 2001 and
March 2002. The following water quality indicators were analysed: pH, conductivity,
BOD, phosphate and nitrate loads, and bacteriological counts. Mud samples were also
collected and screened for mud communities. The study found that for most water quality
parameters measured, the mean concentrations were considerably higher closer to
sewage ponds than upstream and downstream. It was observed that Murowa River has
coliform counts that are well above recommended national limits of zero counts/100ml
1
for surface water. This is a cause for concern because many downstream users rely on
Murova River for domestic purposes. Also, it was found that the number and composition
of aquatic species in Murowa River are affected by poor sewage discharge. Generally,
more plankton species were found further downstream of sewage ponds than closer. The
larvae of the midge (Chironomis sp.), which is usually highly tolerant to organic
pollution, was found downstream of the ponds while Ephemero pterans, which is
associated with clean water, was observed upstream. This indicates that downstream of
the ponds, river water is more affected by organic pollution. The study recommends
regular maintenance of sewage pipes and upgrading of existing sewage works so that
they can cope with current population growth trends. This will address the problem of
frequent pipe bursts and overflows thereby minimising water contamination.
Key words: sewage disposal, impact, Murowa River, Lower Odzi sub-catchment,
Zimbabwe.
INTRODUCTION
The establishment and subsequent rapid growth of rural service centres in Zimbabwe
requires efficient sewage waste management systems. To the contrary, rudimentary
sewage disposal systems have been and are still in use in most populous rural service
centres such as Mutambara in the Lower Odzi sub-catchment. Oxidation ponds are the
main sewage treatment plants used in Zimbabwe’s rural service centres since they are
relatively cheap to construct. However, most are poorly maintained and effluent is
2
disposed of directly into rivers. The current sewage management systems pose numerous
environmental, ecological and health problems. Several authors have observed that
generally, bacterial contamination of surface waters is high in Zimbabwean rural service
centres (Ward, 1989; Moyo and Mtetwa, 2000).
Also, the direct discharge of poorly treated sewage effluent rich in nitrogen and
phosphates into aquatic systems has been clearly shown to be a major cause of
eutrophication problems particularly in lakes located close to major urban centres such as
Manyame River and Lake Chivero that are close to Harare (Moyo, 1997). This problem is
compounded by the fact that most local authorities tend to possess insufficient knowledge
on impacts of sewage discharge on water resources. For example, in Victoria Falls town,
the local authority has been discharging raw and partially treated sewage effluent into the
Zambezi River on the assumption that since the river is very turbulent it cleans itself and
areas downstream are not inhabited (Feresu and van Sickle, 1990).
The lack of awareness on the part of local authorities highlighted above to a large extent
explain the absence of robust management plans and properly crafted policies that
address waste management challenges posed by explosive population growth of rural
service centres. Rapid population growth increases sewage waste generation and this
tends to over-stretch existing sewage management systems that were designed to cater for
relatively small populations.
3
Related to the above, data that highlights environmental and ecological problems posed
by sewage effluent in rural areas are not readily available since most past research has
focused on waste management challenges confronting the country’s major urban centres
such as Harare (Chenje et al., 1996). Currently, there appears to have been no study that
has been done to assess the impacts of rural sewage disposal systems on the water quality
of surrounding aquatic resources. Against this backdrop, this study examines the impacts
of Zimunya township’s sewage waste disposal systems on water quality of Murowa
River, in the Lower Odzi sub-catchment located in eastern Zimbabwe.
MATERIALS AND METHODS
Study Area
The Lower Odzi sub-catchment lies in Manicaland Province (eastern Zimbabwe) and has
a drainage area of about 26 730km2. It covers parts of Mutare and Chimanimani districts
which are serviced by about 1 037 boreholes and 23 557 blair latrines (Ministry of Local
Government, Public Works and National Housing, 1999; 2001). The parent rock
comprises sediments of sand, silt and gravel that are covered by acidic soils. Major land
uses in the sub-catchment include subsistence agriculture (86%), small-scale rural
farming (13%), and forestry (1%) (ZINWA, 2001). Many rivers including
Umvumvumvu, Nyambewa, Wengezi, Mpudzi and Murowa (Figure 1) drain the subcatchment.
4
Figure 1: Study area.
Most of the rivers provide water for smallholder irrigation and domestic purposes. Other
sources of domestic and irrigation water are small dams such as Zimunya dam, boreholes,
as well as both protected and unprotected wells. Currently, Zimunya dam supplies water
to Zimunya and Chitakatira townships that have an estimated population of over 8 000.
5
The dam covers an area of 96 706,45m2 and has a gross capacity of 180 x 103m3
(ZINWA, undated).
The Mpudzi sub-catchment which is drained by Murowa River has a relatively low mean
annual rainfall of 720mm, an estimated mean annual evaporation of 2 010mm and a mean
annual runoff of 150mm (ZINWA, undated). The catchment predominantly lies in a
communal area. Communities in Mpudzi sub-catchment like their counterparts in the
lower Odzi sub-catchment have limited access to safe potable water. They do not have
environmentally safe disposal systems for domestic and livestock waste since most
households use of pit latrines, a number of which were destroyed by cyclone Eline
induced floods.
The lower Odzi sub-catchment is home to population centres such Zimunya Township,
which is located about 20km south east of Mutare City. The Township was established in
1952. Currently, it relies on sewage maturation ponds for sewage treatment. Although
some of the sewage effluent is used to irrigate pastures, numerous cases of effluent
contaminating Murowa River are common. These emanate mostly but not exclusively
from frequent overflows and burst pipes. The pipes are located along the river.
Sampling and laboratory analysis
Grab water samples were collected in 1-litre plastic bottles from the water surface at six
sampling sites namely 500m upstream of Murowa River, pond inflow and outflow, 200m,
6
500m and 1500m downstream at two-week intervals between November 2001 and March
2002. Mud samples were also collected at the six sampling points and screened for mud
communities using sieves. Water samples were packed in an ice filled cooler box and
sent to Zimbabwe National Water Authority (ZINWA) laboratory for bacteriological,
physical and chemical analysis within 48 hours. Phytoplankton was collected using a
20µm net while a 64µm mesh net was used for zooplankton. All samples collected for
plankton analysis were preserved in 10% formalin. Analysis was carried out in the
laboratory using a Neubauer haemocytometer. Water quality parameters measured were
compared and assessed according to distances sampling points were from the sewage
ponds, which were assumed to be major sources of contaminants found in Murowa River.
Cluster analysis was used to classify sampling stations and plankton groups based on
their similarity coefficients.
RESULTS
Biophysical and Chemical Water Parameters
Table 1 shows the average results of chemical and biophysical parameters measured. It
was found that pH was low in sewage inflow but increased sharply to 9.58 in the outflow
before decreasing to 7.02 about 1500m downstream. Similarly BOD decreased with
distance along Murowa River from 107.6mg/l in pond inflow to about 2mg/l 500m
downstream. A slight increase was recorded 1500m downstream of the river. A similar
trend was noted for conductivity, which decreased considerably from 913μs/cm (pond out
7
flow) to 189μs/cm 1 500m downstream. Regarding phosphates and nitrates,
concentrations were higher in the sewage outflow than downstream although raw sewage
effluent obtained from the inflow did not record any nitrate. Results of bacteriological
analysis were shocking in that 3 699 coliform counts/100ml of water were recorded
immediately below the pond outflow. This is well above required standards of zero
counts/100ml water for surface waters in Zimbabwe (ZINWA, 2000) and is a cause for
great concern because in many rural areas downstream users rely on river water to meet
their domestic requirements.
Plankton Species
Figure 2 shows that sewage ponds have a direct impact on the numbers of plankton
species along Murowa River. For both zooplankton and phytoplankton, the number of
species was lower closer to sewage ponds than further downstream. As distance from the
sewage ponds along Murowa River increased, the number steadily increased downstream.
8
Table 1: Chemical and physical parameters of water samples from Murowa River.
Sampling site
Parameter
pH
Upstream
Inflow
Outflow
Downstream
Downstream
Downstream
500m
200m
500m
1500m
(UP500m)
(DS200m)
(DS500m)
(DS1500m)
7.30
6.88
9.58
7.20
7.22
7.02
BOD (mg/l)
<2
107.6
71
<2
<2
3.9
Conductivity
130
800
913
240
215
189
Nitrates (mg/l)
0.3
0.0
3.0
0.7
0.7
0.7
Phosphates (mg/l)
0.02
1.4
5.0
0.02
0.02
0.03
-
-
-
3 699
-
-
-
-
-
252
-
-
-
-
-
4 354
-
-
(μS/cm)
E. coli
(Counts/100ml)
Faecal strept.
(Counts/100ml)
Total coliforms
(Counts/100ml)
Note: – indicates that bacteriological samples were not taken
9
14
Zooplankton
Phytoplankton
12
number of species
10
8
6
4
2
0
UP500m
DS200m
DS500m
DS1500m
Distance from ponds (metres)
Figure 2: Number of plankton species in Murowa River upstream and down stream of
sewage ponds.
Results of correlation analysis of vectors of values for plankton species between sampling
stations show weak correlation coefficients (Table 2). This indicates that that the
sampling stations had different number of plankton species and this is attributed to the
influence of sewage treatment ponds. Although correlation coefficients showed that
sampling stations were different in terms of planktonic species diversity, results of single
linkage cluster analysis grouped the upstream and down stream stations within the same
cluster (Figure 3).
10
Table 2: Correlation between sampling stations.
Distance from sewage ponds (m)
Correlation between Vectors of Values
UP500m
DS200m
DS500m
DS1500m
Up Stream 500m (UP500m)
1
0.429
0.211
0.213
Down Stream 200m (DS200m)
0.429
1
0.439
0.086
Down Stream 500m (DS500m)
0.211
0.439
1
0.162
Down Stream 1500m (DS1500m)
0.213
0.086
0.162
1
Results from cluster observations (Figure 4) of plankton species found at sampling
stations showed that Phragmites sp. and Cymbella sp. were in the first cluster. This
reflects their preference to heavy nutrient enrichment.
Figure 3: Hierarchical cluster analysis of sampling stations.
11
Figure 4: Hierarchical cluster analysis of plankton species.
Benthic fauna found was dominated by Ephemero pteraus (mayflies). Cordalegaster
boltena was observed at all sampled sites. The mayflies were placed in the second cluster
with phytoplanktonic diatoms, Synedria. Pinnuliria was also encountered at all sites. In
the Chlorophyta group Spirogyra dominated all sites and was also placed in the second
12
cluster. Volvox sp., Selenastrum sp. and Ankidostodeum sp. were found at 1500m
downstream the sewage ponds. Sampling sites after the sewage ponds were dominated by
mud burrowing worms Chironomid sp. a clear indication that closer to the sewage ponds
the river water was nutrient rich.
DISCUSSION
The outflow water quality of the sewage ponds was observed to record higher
concentrations of water quality parameters than inflow measurements. The inflow and
outflow sewage recorded high conductivity concentrations, which decreased considerably
downstream. This notable decrease could be a result of ion leaching and nutrient uptake
by aquatic plants. High pH levels were also recorded at the pond outflow (9.58). This
subsequently dropped downstream to levels around neutral. The pH levels recorded
downstream were similar to those observed upstream of the sewage ponds. It is likely that
along both before and after the sewage ponds, pH levels were lowered by the dense
growth of Phragmites australis, which has been shown to absorb dissolved ions
(Mitchell, 1973).
Although nitrate concentrations were slightly high in the sewage effluent, their
concentrations in Murowa River were generally low. Once again, this could be attributed
to up-take by aquatic plants. From the high BOD recorded it was inferred that the sewage
effluent was high in organic matter hence more oxygen was required to break down the
organic compounds it contained. In contrast less oxygen was required to break down the
13
organic compounds in the river as reflected by low BOD concentrations recorded. The
low BOD observed in Murowa River could be explained largely by the fast flowing and
turbulence nature of the river that result in increased aeration and oxygenation (Nduku,
1976).
Phosphate concentrations were characterised by high outflow concentrations and low
inflow loads. This could be attributed to the fact that the element is relatively stable hence
it tends to concentrate in the settling ponds. It is only released in water as phosphate ions
under anoxic conditions in sewage maturation ponds. The phosphate concentration
decreased along the river to values as low as 0.03mg/l (1500m downstream). The
decrease could be attributed to self-purification processes which occur with increased
aeration of water due to turbulence induced by the rough riverbed. Aquatic organisms
also play a key role in the self-purification process, as the riverbed was observed clogged
with dense growths of P. australis. These obtain their nutrients from the river hence they
act as good nutrient traps (Mitchell, 1973). Apart from leaching and uptake by plants,
phosphates could have been lost through adsorption with ferric and aluminium
hydroxides to produce insoluble phosphates complexes.
Bio-indicators of organic pollution were also investigated. The composition of different
species of aquatic organisms assessed included the Chironomis sp. (Figure 2), larvae of
the midge which is usually highly tolerant to organic pollution due to their ability to
respire at low oxygen tension (Moyo, 1997). The larvae are also detritus feeders, which
14
burrow into muddy and sandy bottoms were found in abundance at sampling sites
associated with organic enrichment especially those close to and downstream of the
sewage ponds. The presence of these organisms provides ample evidence that Murowa
River is prone to organic pollution. Although the sampling site upstream of the sewage
ponds showed signs of nutrient enrichment, the presence of Ephemero pterans, which is
usually associated with clean water, suggests that the water upstream is less exposed to
organic pollution. This is supported by low BOD measurements recorded upstream of the
sewage ponds.
Results of single linkage cluster and correlation analysis of sampling stations showed that
they were different in planktonic species diversity. Only notable similarities were found
between the UP500m and DS200m sampling stations (Figure 3). Dense growths of P.
australis dominated riparian vegetation both upstream and downstream of the sewage
ponds. This shows that the upstream sampling station was not totally free from nutrient
enrichment. The UP500m station could have been affected by the outflow discharge
emanating from Zimunya population centre. However, this needs to be further
investigated. The results from cluster observations of plankton species observed at
sampling stations placed Phragmites sp. and Cymbella sp. in the first cluster (Figure 4)
reflecting their preference to heavy nutrient enrichment.
The study found that although the direct discharge of sewage effluent into Murowa River
greatly impacts on the distribution and composition of aquatic organisms, the river still
15
possesses the natural abilities to purify itself of organic pollution. However, this selfpurification process has limits beyond which the river becomes ecological stressed
(Machena, 1995). A notable example is the current eutrophication problems being
experienced in some portions of Mukuvisi River that drains the Greater Harare area
(Moyo, 1997).
While the local authority in charge of Zimunya Township has explored the possibility of
using effluent water to irrigate pastures, sewage management at the centre is still dogged
by poor sewage pipe maintenance, which constantly burst and were poorly sited along the
River. Therefore, unless corrective measures are taken to improve sewage management
pollution of water resources is likely to increase. This disrupts the functioning of river
ecosystems and jeopardises the health of downstream communities especially those who
use the water for domestic purposes.
CONCLUSION
This paper has shown that for most water quality parameters analysed (e.g. phosphate
loads, BOD and conductivity), the mean concentrations were lower upstream and further
downstream of sewage ponds than closer. Although further investigations are necessary,
the findings of this study provide ample evidence that Zimunya Township’s sewage
disposal system causes contamination of Murowa River. Bacteriological analysis
revealed that downstream of the sewage ponds, coliform counts were well above
recommended national limits of zero counts/100ml water for surface waters in
16
Zimbabwe. This is a cause for concern because many downstream users rely on Murova
River for domestic purposes. Also, it was found that the number and composition of
aquatic organisms in Murowa River is affected by discharge of sewage effluent. More
plankton species were found downstream of sewage ponds than upstream. The larvae of
the midge (Chironomis sp.), which is usually highly tolerant to organic pollution, was
recorded downstream of the ponds while E. pterans, which is usually associated with
clean water, was recorded upstream of the ponds. The presence of these organisms
indicates that Murowa River is prone to organic pollution. The study found out that
although the direct discharge of sewage effluent into Murowa River greatly impacts on
the distribution and composition of aquatic organisms, the river still possesses the natural
ability to purify itself of organic pollution. On the basis of the results discussed above,
the study recommends regular maintenance of sewer pipes as well as upgrading of
existing sewage works so as to cope with current population growth trends. This will
address the problem of frequent pipe bursts and overflows thereby minimising water
contamination.
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