THE OPTIMISATION OF A REEDBED FILTER FOR EFFLUENT
TREATMENT AT KASESE COBALT COMPANY LIMITED –
UGANDA
(Byekwaso, Evaristo1 ; Kansiime, Frank2; Logstrup, Jorgen3. and Andersen, Soren3)
1
Kasese Cobalt Company Limited, P. O. Box 524, Kasese, Uganda; 2 Makerere
University Institute of Environment & Natural Resources, P.O. Box 7062 Kampala,
Uganda; 3Transform ApS, Danish Rootzone, Borgergrade 6 DK-1300K Copenhagen,
Denmark. E – Mail = byekwaso.evaristo@kccl.co.ug
ABSTRACT
Wetland systems are cheaper alternatives used to treat wastewater compared to conventional
means. In the last decades, wetland systems are increasingly being applied in the treatment of
various metal mine drainage waters. Kasese Cobalt Company Limited (KCCL) is currently
testing the potential of using a constructed wetland to treat an effluent from its cobalt processing
plant. Eighty per cent of the effluent (1498 m3/day) from the plant is discharged into River
Rukoki. The remaining effluent is discharged into a constructed wetland for treatment. The
constructed wetland is planted with Phragmites mauritianus and consists of two reedbeds Reed
Bed Filter A (RBA) and Reedbed Filter B (RBB) with RBA feeding its effluent to RBB. The
reedbed made up of two units of area 1000m2 and 800m2 designed to treat 20% of the nominal
effluent flow of the plant. This pilot constructed wetland is being optimised for eventual
treatment of all the effluent from the plant facilities.
The study going on involves physical and chemical plant characterisation (growth, plant biomass
density, and stem thickness, health, nutrients and heavy metal uptake) both in the constructed
wetland and River Nyamwamba. The results so far obtained indicate that the reedbeds are
efficient at the uptake and retention of heavy metals Cu, Ni, Co, Cd, Pd and Fe. The reedbeds are
not retaining Zn and are releasing Mn. The reedbeds are also efficient in retention of nitrogen.
There is phosphorous release from the reedbeds. Trend analysis of the performance of the
reedbed in terms of percentage removal across the system indicate that there is no effect on the
performance of the reedbed for nitrate, phosphate and manganese when the flow rate of the feed
is varied. TDS, SO4, Na, NH4-N, Mg, Ca, Pb, Co are inversely dependent on the flow rate of the
effluent feed to the reedbeds. (To me it seems that the if you look at the loads kg / week, the
reduction rate seems quite constant, because it is partly dependent on the evapotranspiration rate).
Increase in pH to 10 of the effluent feed reduces the pollution load that is fed to the reeds. ??????
The results also indicate that Phragmites plants from RBA, which receives effluent directly from
the KCCL processing plant, were taller and had a higher plant biomass density than those in RBB
that receive effluent from RBA. Plants from RBB also had higher biomass density than plants
taken along River Nyamwamba. Generally, the constructed wetland is efficient in removing
heavy metals except for Mn and Zn and the nutrients in the effluent influence the plant biomass
density of Phragmites plants.( I am quite certain that the difference in biomass density mainly is a
result of the very organic soil in RBA, which contains all sorts of micronutrients which is we do
not have in the wastewater, and only in smaller amounts in RBB).
Key Words: Reed Bed, Effluent Treatment, Pyrite Stockpile, Cobalt extraction, Trend
analysis, Phragmites mauritianus.
INTRODUCTION
The Kasese Cobalt Company Limited (KCCL) was established to retreat the pyrite stockpile to
extract cobalt. The pyrite stockpile was constructed during the operation of the Kilembe Mines
(copper mining), which closed in 1976 and subsequently following the closure of the mine the
stockpile began to erode. The erosion resulted in a large acid trail being formed from the
stockpile all the way to Lake George (a Ramsar site) some 10 Km from the site. The acid trail
has contaminated a large area of the Queen Elizabeth National Park resulting in death of most of
the vegetation. The project to extract cobalt from the stockpile has prevented further erosion of
the stockpile and the discharge of acid drainage. The process involves bioleaching, solvent
extraction and Electrowinning to recover cobalt. The plant however generates wastewater from its
production processes.
During the approval of the KCCL Environmental Impact Statement (EIS) the National
Environment Management Authority (NEMA) regarding the predicted effluent concentrations
raised concern. It was agreed between KCCL and NEMA to trial a passive treatment system
using reeds, which had been found on some projects, could improve effluent water quality.
For polishing effluent at Kasese Cobalt Recovery Plant, a two step process flow will ensure that
the water goes through the required processes for optimum treatment. The first one is the wetland
sedimentation plant with limestone with a second part containing a rootzone with active soil.
Sedimentation wastewater, which has not completed flocculation, or which is still chemically
reactive will pass thorough a shallow planted wetland lagoon. The wastewater in this wetland
sedimentation lagoon will be aerated with atmospheric oxygen. Due to surface tension forces
between the liquid and the plant stems, a more marked precipitation will occur in the proximity of
the stems, thus, the surface tension forces of the plants, which will catalyze molecular formation
and flocculation. Sulphate contaminant will react with organic compounds and precipitated as
elemental and organic sulphur. The lagoon is constructed over a limestone filter, from where
water is drained, thereby activating precipitation.
The rootzone method was developed based on the fact that certain soil types have a high
adsorption capacity. The rootzone/filter plant is a biological filter, where biological treatment of
wastewater takes place in a soil volume, which is penetrated by roots. The rootnetwork is
composed of suitable plant species Phragmites mauritianus. The root structure ensures that the
wastewater flows horizontally thorough the soil mass. During wastewater flow, watercontaminating compounds are bound to the soil colloids or other constituents of the root zone or
are released to the atmosphere, through respiration, nitrification and denitrification or other
processes. The turnover of organic substances occurs as a result of a diverse complex of aerobic
and anaerobic microbial activity in the soil. The aerobic activity is supplied with oxygen from the
plant roots, as well as via the surface of the system. Thus aerobic activity is concentrated near the
plant roots and at some distance from the roots, anaerobic activity prevails. The mosaic of aerobic
and anaerobic pockets provides optimum conditions for a wide range of active microbial
organisms. An aerobic as well as anaerobic group of organisms is necessary for breakdown of
wastewater constituents.
The effluent from KCCL plant mainly consists of non-precipitated sulphurous salts. Precipitation
of sulphurous salts is well known in the rootzone filters from a number of industries (textile,
dying, chemical industry). Mainly Ca-sulphates (gypsum) and Mg-sulphates will precipitate in
the wetland sedimentation lagoons. Other sulphates will mainly adsorb to the soil colloids in the
rootzone. Residual traces of heavy metals will also precipitate and adsorb. Long term studies and
field scale data collections indicate that metal salts crystallize in the aerated zone close to root
hairs, i.e. the metal salts are immobilized. At low pH will the salts again be released in the soil
liquid.
KCCL engaged a Danish company Transform Aps (Denmark) to assist in the design and
construction of a trial biological treatment system (reedbed filter).
A survey “The Viability of Establishing Constructed Wetlands as a Process Effluent Polishing
Method for Kasese Cobalt Company Ltd.” investigating the technological aspects of the project
was completed in 1998. The conclusion of the study was that rootzone technology might be
appropriate for the treatment of the wastewater. Based on the study it was concluded that a pilot
project should be initiated in order to clarify whether the technique in practice could be used for
the treatment of wastewater from the KCCL processing plant.
MATERIALS AND METHODS
Study Area
The system is divided into two ponds (RBA and RBB) known as the wetland and the rootzone
respectively. The wetlands pond was planted with reeds (Phragmites Mauritianus), with the floor
of the pond covered by hydrophilic grasses. The rootzone pond was planted solely with
Phragmites Mauritianus, with any other species of plant being removed.
The effluent from the plant first enters the system into the wetland pond A where it is distributed
along a trench and collected at the bottom of pond A. The wetland pond acts as a sedimentation
pond, settling out the majority of the suspended solids from the effluent. The atmosphere, causing
some further precipitation of solids such as gypsum, particularly around the reed stems where
surface tensions differ also aerates the effluent.
The effluent is then directed to pond B and drains (via a manhole) into the rootzone pond where
water is collected via 3 subsurface pipes spaced evenly through the lower half of pond B. In this
pond, the water is passed under ground horizontally through a permeable layer of limestone,(the
filtermaterial in pond B is build up by mixing limestone with volcanic tuff, red soil, straw, sand
and nutritious organic material, so the limestone is less than 10% in the media). which is
inhabited by the complex of roots from the reeds above. This limestone and root layer act as a
biological filter, as many micro-organisms are present in the soil and roots and as the effluent
passes through, the microbes breakdown the harmful substances. The anaerobic conditions in the
pond cause the plants (it is the microorganisms) to obtain their oxygen by breaking down the
sulphate in the effluent, releasing the oxygen. Metal sulphides then form using heavy metals in
the effluent. These sulphides are then stored in the plant itself (and mainly as salts in the soil),
transpired into the atmosphere via the leaves or precipitated to the surface.
From the rootzone pond the effluent is passed through another manhole (facilitating flow rate
measurements) and then released into an existing stream or used for irrigation of an existing
garden with eucalyptus trees.
The study is extended to River Nyamwamba where the plants growing in the reed beds were
obtained.
Sampling
Water
Sampling is being carried out weekly with assistance from KCCL at all the predetermined sites,
RBFA, RBFB, RBEB, at the constructed reed bed and once a month along River Nyamwamba at
the predetermined sites upstream location, 36N0166990, UTM 0023279 (Nyp1), around the
middle 36N 0169956, UTM 0021351(Nyp2), and downstream 36N0179135, UTM 0020379
(Nyp3).
Sampling is usually done between 1000 – 1200 hrs. A fixed sampling procedure that is followed
involves. Measurement of physico-chemical parameters viz; electric conductivity (EC), pH, and
temperature are carried out in situ using of WTW portable meters. Taking water samples to be
analyzed for heavy metal (Cu, Mn, Ni, Co, Pb, Fe, and Zn) in 500-mls plastic bottles and acidify
to pH 2 with concentrated HNO3 in the field.
Plants
Phragmites plant samples include leaves, stems, and roots. For data presented in this report,
samples were obtained from the constructed wetland and along River Nyamwamba. The plant
samples were taken from three sampling plots located diagonally within each reed bed (two sites
were located 5 m from the edge of each of the reed bed while the third was located in the middle)
and along River Nyamwamba at the predetermined sites upstream location, 36N0166990, UTM
0023279 (Nyp1), around the middle 36N 0169956, UTM 0021351(Nyp2), and downstream
36N0179135, UTM 0020379 (Nyp3).
After harvesting the heights of the above ground biomass of all the plants from the each plot were
measured. Thereafter, the wet weights of the above and below ground biomass from each of the
plots were also measured. Finally, sub samples (I – 2 kg wet weight) of plant parts (leaves, stems
and roots) samples were sun dried to reduce their moisture content before oven drying at 70 oC to
constant weight. The wet weights and dry weights of the plant samples were recorded and used to
estimate the biomass density.
Sediments
Sediment samples were also obtained from the edge of the sample plots located diagonally in reed
beds as well as the edges of the sampling location along River Nyamwamba.
Analytical techniques
Nitrogen
Ammonium-Nitrogen
Direct Nesslerization method was used and the concentration (mg/l) was determined using HACH
DR/2010 Spectrophotometer at a wavelength of 425nm.
Nitrate- Nitrogen
Cadmium reduction method was used. The concentration mg/l was read directly using Hach
DR/2010 Spectrophotometer at 507 nm.
Total nitrogen
Cadmium reduction method was used. Nitrogen concentration in mg/l was then read at 543 nm
using CECIL Spectrophotometer model 1000 series. The standard for reference (0.5 mg/l) was
treated in the same manner as the samples.
Phosphorus
Soluble Reactive Phosphorus (SRP)
Ascorbic acid method was used. After 15 to 20 minutes of colour development, absorbance was
read at 880 nm using CECIL Spectrophotometer model 1000 series.
Total phosphorus (TP)
TP in the water sample was determined using the persulphate digestion method. The samples
were autoclaved at 120oC for 30 minutes. After cooling, digested samples were analysed for
phosphate by ascorbic acid method.
Total suspended solids
Total suspended solids (TSS) in the wastewater samples were determined directly using Hach
DR/2010 spectrophotometer.
RESULTS
Biomass Characterization
Plant heights
Phragmites plants that are growing in RBA are generally taller than those in RBB. The growth
rate before July 2001 were highest after which the plants reached maturity (Figure 1). The plants
growing in reedbed B (RBB) are more or less the same as those growing along River
Nyamwamba (Table 1).
6
height /m
5
4
3
2
1
RBA
Figure 1: Heights of Phragmites in the reedbeds
RBB
Mar-02
Jan-02
Nov-01
Sep-01
Jul-01
May-01
Mar-01
Jan-01
Nov-00
0
The Phragmites plants growing along the Nyamwamba, before the river receives the effluent from
Kilembe mines (Nyp1) were slightly taller than those that were growing after the discharge point
of the effluent from Kilembe mines (table 1)
Table1: Mean heights, (HSD) of Phragmites plants growing in the reed beds and along River
Nyamwamba.
Reed bed A
2
3
Plots
1
Height
(m)
4.84 0.03
4.3 0.02
Reed bed B
5
6
4
3.96 0.03
3.57 0.0.3
3.85 0.03
3.06 0.03
River Nyamwamba
Nyp1
Nyp2
Nyp3
4.15 0.03
3.45 0.05
Variation of biomass density
The biomass density (weights of plants per unit area) was higher for Phragmites growing in RBA
(plots 1, 2 and 3), followed by RBB (plots 4, 5 and 6), while Phragmites plants growing along
River Nyamwamba had the lowest biomass density for all the plant parts (Figure 2).
Dry weight( Kg m-2)
8
6
4
2
0
Roots
RBA
Stems
RBB
Leaves
RNY
Figure 2: Biomass density of Phragmites plants in the sampled plots at the reed bed and along
River Nyamwamba.
Plant density
The number of plants in each square meter of area in the reedbeds indicates higher numbers in
RBA. The number of plants in each square meter shows an increase before July 2001 after which
it remains relatively constant (Figure 3). This could be attributed to the maturity of the plants
after this time
3.41 0.07
RBA
Mar-02
Jan-02
Nov-01
Sep-01
Jul-01
May-01
Mar-01
Jan-01
Nov-00
stems/m^2
120
100
80
60
40
20
0
RBB
Figure 3: Number of stems of Phragmites per square meter in RBA and RBB.
Dry weight( Kg m-2)
Biomass density of plant parts
Stems had the highest biomass density for Phragmites plants harvested from all the study sites,
followed by roots and leaves (Figure 4).
8
7
6
5
4
3
2
1
0
Roots
RBA
Stems
RBB
Leaves
RNY
Figure 4: Biomass density in Phragmites plant parts from the study sites
Stem area
Stems have the highest stem area for Phragmites plants in RBA than in RBB (Figure 5).
30000
20000
10000
stem area RBA
Mar-02
Jan-02
Nov-01
Sep-01
Jul-01
May-01
Mar-01
Jan-01
0
Nov-00
stem area mm^2
40000
stem area RBB
Fig 5: Total stem area of Phragmites in RBA and RBB.
Above ground (AG) and below ground (BG) biomass density of Phragmites
The AG biomass density of Phragmites plant was higher than the BG for all the study sites (Figure 6). The
ratio of AG: BG biomass density for the study sites RBA, RBB and RNY is 3.6: 3.4: 3.2.
The above ground and below ground relation is interesting. Here in Denmark we would see
almost the opposite picture, meaning that BG is a bit higher than AG. It is most likely that the
ratio is a lot different in Uganda, but still I would like to know how you have measured the BG ?.
I know from own investigations how difficult it is to get the hole root.
Dry weight (Kg m-2)
10
8
6
4
2
0
RBA
RBB
RNY
Sites
AG
BG
Figure 6: AG and BG biomass density of Phragmites from the study sites
Overall biomass density
When the biomass from all the plots in each of the reed beds and along River Nyamwamba were
pooled together, the biomass density of Phragmites was highest in RBA followed by RBB.
Biomass density was lowest for Phragmites harvested along River Nyamwamba (Figure 7). This
is probably because, plants from RBA directly receives nutrients (N & P) from the wastewater
that is getting out of KCCL plant. The nutrients are used in the processing plant to stimulate the
growth of bacteria, which are used in the bio-leaching process. The nutrients are absorbed by the
plants and converted to biomass.
Dry weight (Kg m-2)
15
10
5
0
RBA
RBB
RNY
Sit es
Figure 7: Overall biomass density of Phragmites plants in the study sites
Estimation of overall biomass density of Phragmites in the reed beds
The total biomass density in the reed beds is presented in table 2. RBA has higher total biomass
density than RBB although RBA has a smaller area than RBB (Table 2). Taking a hypothetical
area of 1000 m2 covered with Phragmites along River Nyamwamba as is shown in table 2, the
ratio of biomass density of Phragmites in the sampled sites RBA, RBB and RNY is 3.5: 2.3: 1.
Table 2: Total biomass density of plant parts in the reed beds. Values indicate mean ±
Standard deviation, n=3
Sites
Area (m2)
RBA
800
RBB
1000
RNY
1000
Plant parts
Biomass (Kg m-2)
Total Biomass (Kg)
Leaves
Stems
Roots
Leaves
Stems
Roots
Leaves
Stems
Roots
0.87 0.11
7.31 0.09
2.27 0.01
0.39 0.01
3.88 0.31
1.11 0.05
0.34 0.03
1.46 0.09
0.56 0.06
693 106.84
5848 883.71
1819  29.23
393 20.93
3877 383.03
1110 60.83
340 38.44
1457 112.37
563 68.50
Overall biomass (Kg)
8360 1019.78
5380464.79
2360219.31
Figure 7 indicates that your mean +- should be larger in table 2 ?
Water Quality
Experiments of water quality (effluent) began far back to 2000 and the parameters analyzed were
suspended solids, pH , TDS, SO4, No3, Na, PO4, NH4-N, Mg, Ca, Mn, Pb, Co and conductivity. Effective
November 2001, additional components were analysed. These were Cu, Ni, Zn, Fe, and Cd.
Electrical Conductivity
The electrical conductivity of the effluents at the actual sampling points of the reedbeds are
shown in Figure 8. The electrical conductivities were high with a variation from 6000 to 10,000
S/cm. They was a rise in the levels in June 2001 because of increase in the concentration of
sodium that was dosed to increase the pH to 7.
12000
10000
EC
8000
6000
4000
2000
RBFA
RBFB
May-02
Apr-02
Mar-02
Feb-02
Jan-02
Dec-01
Nov-01
Oct-01
Sep-01
0
RBEB
Figure 8: Trend in electrical conductivity at sampling points
pH
The variation in the pH in the reedbeds is as shown in Figure 5. Before July 01, the pH of the
effluents reduces to around 7 as it goes through RBFA. In June 2001, the pH was adjusted to 10
by dosing the effluent with caustic soda solution.
12
Conc (mg/l)
10
8
6
4
2
Pond A Feed
Pond B Feed
22/03/2002
22/01/2002
22/11/2001
22/09/2001
22/07/2001
22/05/2001
22/03/2001
22/01/2001
0
Pond B Effluent
Figure 9: pH of the effluents at the sampling points of the reedbeds
The increase in pH of the effluent feed to the reedbeds lowered the concentration of heavy metals
that goes through the constructed wetlands. The increase to pH 10 precipitates the heavy metals
out of solution and the heavy metal load decreases (Fig 10).
10
6
pH
8
4
2
18-Mar-02
11-Feb-02
07-Jan-02
03-Dec-01
29-Oct-01
24-Sep-01
20-Aug-01
16-Jul-01
11-Jun-01
07-May-01
02-Apr-01
26-Feb-01
0
22-Jan-01
Conc/mg/l
12
1
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
Pond A Feed
Pond B Feed
Pond B Effluent
pH
Fig 10: Effect of pH on cobalt load in the wetland.
Rate of Flow of Effluent
The performance of the wetland system depends on the flow rate of the effluent that is fed to the
constructed wetland. At lower flow rates, the constructed wetland was removing heavy metals up
to above 90%. At higher flow rate approaching the design capacity, the performance of the
constructed wetland was 20% (Figure 11). The high performance at low flow rates may be
attributed to the high residence time the effluent takes through the constructed wetland.
120
% removal
100
80
60
40
20
0
2.41
2.6
5.08
6.46
10.8
15.81
Rate m3/hr
cobalt
Lead
Fig 11: Dependence of constructed wetland on rate of feed.
As far as I can see the reduction in kg is quite constant.
Total Dissolved Solids (TDS)
The load of TDS at the sampling points RBFA, RBFB, and RBEB in the effluents is shown in
Figure 12. The retention of TDS by RBA is represented by the difference in the reading between
RBFA and RBFB, meanwhile the retention by RBB is represented by the difference in the load of
TDS between RBFB and RBEB. Generally, both RBA and RBB are reducing the load of TDS in
the effluent. This is exemplified by the appreciable reductions in the loads of the TDS from the
inlet of RBA to the outlet of RBB.
TDS (Kgs)
20000
15000
10000
5000
0
Jan Feb Mar April May June July Aug Sept Oct Nov Dec
Months 2001
RBFA
RBFB
RBEB
Figure 12: Retention of total dissolved solids by RBA and RBB
Heavy Metals
Heavy metals
The characteristics of the effluents with reference to heavy metals are presented in table 3. The
results show that the load in Kilograms per day of Manganese, Lead, Cobalt, Nickel, Copper,
Cadmium, Iron, and Zinc at the sampling points at the reed bed varied from 0.05 – 0.31, 0.016 –
0.01, 0.039 – 0.02, 0.02 – 0.007, 0.009 – 0.001, 0.002– 0.0011, 0.019 – 0.014, 0.004 – 0.004 at
the inlet of RBA and at the outlet of RBB.
Table 3: Variation of load of given heavy metals at the sampling points along the
constructed wetlands
Heavy metals
RBFA
Manganese Kg/day
Lead
Kg/ day
Cobalt
Kg/ day
Nickel
Kg/ day
Copper
Kg/ day
Cadmium Kg/ day
Iron
Kg/ day
Zinc
Kg/ day
0.050
0.016
0.039
0.020
0.009
0.002
0.019
0.004
RBFB
RBEB
0.140
0.013
0.031
0.010
0.006
0.0016
0.020
0.006
0.310
0.010
0.020
0.007
0.001
0.0011
0.014
0.004
A comparison of the load of the effluent with respect to heavy metals at sampling points RBFA,
RBFB, and RBEB is shown in Figure 9. The results show that the effluent becomes richer in Mn
as it gets out of the reed beds. This suggests that there is release of Mn from within the reed beds
probably due the weathering of the rocks, which have been used in the construction of the reed
beds.
0.35
Load (Kg/day)
0.3
0.25
0.2
0.15
0.1
0.05
0
Zn
Fe
Cd
Cu
Ni
Mn
Co
Pb
Heavy metals
RBFA
RBFB
RBEB
Figure 13: Load of Heavy metals at the sampling points RBFA, RBFB and RBEB of the
constructed wetlands.
Removal efficiency of heavy metals by constructed wetland system.
The result of the variation in the removal of heavy metals by the constructed wetland
system is presented in table 4.
Table 4: Removal efficiency of heavy metals by constructed reed beds
RBA
Heavy metals
Manganese
Lead
Cobalt
Nickel
Copper
Cadmium
Iron
Zinc
Inlet
(Kg/day)
0.05
0.016
0.039
0.02
0.009
0.002
0.019
0.004
Outlet
(Kg/day)
0.14
0.013
0.031
0.01
0.006
0.0016
0.02
0.006
RBB
%
Removal
-180
18.8
20.5
50
33.3
20
-5.3
-50
Inlet
(Kg/day)
0.14
0.013
0.031
0.01
0.006
0.0016
0.02
0.006
Outlet
(Kg/day)
0.31
0.01
0.02
0.007
0.001
0.0011
0.014
0.004
%
Removal
-121.4
23
33.3
30
83.3
31.3
30
33.3
The result shows that the efficiency of RBA at retention of heavy metals is in the decreasing
order of Ni, Cu, Co, Cd, and Pb respectively. RBA however show negative efficiency at the
retention of Zn, Fe, and Mn. RBA in this respect is releasing these heavy metals to RBB.
Meanwhile, the efficiency of RBB at heavy metals retention is in the decreasing order of Cu, Co
and Zn , Ni and Fe, Cd, and Pb respectively. RBB also show negative efficiency at retaining Mn
in the wastewater. The effluent from the reed beds is discharged to the irrigation bed and
eventually to the pyrite trails in the Queen Elizabeth National Park. Although RBA show negative
efficiency at retaining Fe, RBB show an equal efficiency at retention of Fe, which is discharged
to it from RBA (Table 4).
Comparison of efficiency of RBA and RBB at retaining heavy metals
A comparison of efficiency of RBA and RBB at retaining heavy metals from the effluent is
shown in Figure 10. RBA is more efficient at retaining Ni from the effluent that goes to it than
RBB. RBB is however more efficient at retaining most of the heavy metals that goes to it than
RBA and is retaining Fe, and Zn which is being released by RBA instead. It is however also
inefficient at retaining Mn in the wastewater like RBA.
100
% efficiency
50
0
Mn
Pb
Co
Ni
Cu
Cd
Fe
Zn
-50
-100
-150
-200
Heavy metals
RBA
RBB
Fig: 15: Comparison of heavy metal retention efficiency of RBA and RBB
Overall efficiency of the reed beds in retaining heavy metals
The overall efficiency of RBA and RBB at the retention of heavy metals from the effluent is
shown in the table 5.
Table 5: Overall efficiency of RBA and RBB at retaining heavy metals.
Heavy metals
Mn
Pb
Co
Ni
Cu
Cd
Fe
Zn
Inlet of RBA
Kg/day
0.05
0.016
0.039
0.02
0.009
0.002
0.019
0.004
Outlet of RBB
Kg/day
0.31
0.01
0.02
0.007
0.001
0.0011
0.014
0.004
% Efficiency
-520
37.5
48.7
65
88.9
50
26.3
0.0
The results in table 5 shows that the efficiency of the reed beds decreases in the order of
Cu, Ni, Co, Cd, Pb and Fe. It also shows that the reed beds are not retaining Zn. The reed
beds are instead releasing more Mn to the effluent being discharged from the plant.
Nutrients
The characteristic of the effluent with respect to nutrients at the sampling points RBFA,
RBFB and RBEB is shown in table 6. The loads are expressed in Kilograms per day.
Table 6: Load of nutrient at the sampling points RBFA, RBFB and RBEB of constructed
reed beds at KCCL.
Nutrients
NH4-N
NO3-N
O-PO4
TP
Kg/day
Kg/day
Kg/day
Kg/day
RBFA
RBFB
RBEB
5.90
2.64
0.04
0.20
3.78
2.07
0.10
0.46
1.88
1.23
0.21
0.83
Retention of nutrients by the constructed wetlands
The efficiency of RBA and RBB at retention of nutrients that is loaded to them is presented in
Table 7. RBB however receives effluents that come from RBA.
Table 7: Efficiency of RBA and RBB at retention of nutrients in the effluents
RBA
Nutrients
Ammonia
Nitrates
Phosphates
Total Phosphorus
Inlet
(Kg/day)
5.90
2.64
0.04
0.20
Outlet
(Kg/day)
3.78
2.07
0.10
0.46
RBB
%
Removal
35.9
21.6
-150
-130
Inlet
(Kg/day)
3.78
2.07
0.10
0.46
Outlet
(Kg/day)
%
Removal
1.88
1.23
0.21
0.83
50.3
40.6
-110
-80.4
Overall %
Removal
68.1
53.1
-425
-315
The reed beds are efficient at retention of all forms of nitrogen but it is releasing
phosphates to the effluent from the plant.
Summary of results
1. Phragmites from RBA were taller and had higher biomass density than those in RBB.
2. Although RBA (800 m2) is smaller than RBB (1000 m2), RBA had a higher total biomass
density than RBB.
3. Phragmites growing along River Nyamwamba has low biomass density compared to those in
the reed beds.
4. Biomass density of Phragmites is higher in the above ground (AG) parts than below ground
(BG) parts.
5. The reed bed systems are efficient at the uptake and retention of heavy metals in the
decreasing order of Cu, Ni, Co, Cd, Pb and Fe. The reed bed systems are however not
retaining Zn, while it is releasing Mn to the effluents from the plant that is being fed to the
reed beds.
6. The reed beds are efficient at the uptake and retention of all forms of nitrogen from the
effluents while it is releasing phosphates to the effluent.
7. The performance of the constructed wetland is highest for low flow rates. Increase in pH
lowers the pollution load of the effluent to the constructed wetland.
References
APHA (American Public Health Association).1992. Standard Methods for the Examination of Water and
Wastewater, 18th edition. Washington, DC.
Brodie, G.A., C. R. Britt, T.M. Tomaszewski, and H. N. Taylor. 1993. Anoxic limestone drains to enhance
performance of aerobic acid drainage treatment wetlands: experiences of the Tennessee Valley Authority.
pp 129-138 in Constructed Wetlands for Water Quality Improvement, G. A. Moshiri (ed.). CRC Press,
Boca Raton, FL.
Faulkner, B. B., and J. G. Skousen. 1994. Treatment of acid mine drainage by passive treatment systems.
pp 250-257 in Volume 2 of Proceedings of the International Land Reclamation and Mine Drainage
Conference and the Third International Conference on the Abatement of Acidic Drainage, Pittsburgh, PA,
April 24 – 29, 1994.
Hedin. R. S., and R. W. Nairn. 1991. Constructing wetlands to treat coal mine drainage. Course notes for
National RAMP Workshop, Pittsburgh, PA, May 8, 1991.
Kansiime F., and van Bruggen J.J.A., 2001. Distribution and retention of faecal coliforms
in the Nakivubo wetland in Kampala, Uganda.