Sewage Treatment by Waste Stabilization Pond Systems

Abstract

Sewage generated in Ghana is commonly discharged into the environment without any form of treatment to reduce the degree of contamination and mitigate potential public health and environmental issues. Although some attempts have been made in some parts of Ghana to utilize the waste stabilization pond (WSP) system to treat domestic sewage, the ponds often fail to achieve their purpose due to lack of basic maintenance and supervision. To assess the utility of the WSP system for treating sewage, wastewater samples were collected from the raw sewage, anaerobic, facultative and maturation ponds of WSPs at Obuasi in Ghana, and analyzed for physicochemical and microbiological contaminants. The results show that the final pond effluent meets recommended microbiological and chemical quality guidelines. The waste stabilization pond system demonstrates high removal efficiencies of wastewater contaminants. The biochemical oxygen demand, total suspended solids, nitrate and faecal coliforms reduction efficiencies of 97.3%, 97.6%, 83.3% and 99.94% respectively are highly significant, and compare well with reported removal efficiencies in the literature. Additionally, the ponds have high reduction efficiencies for heavy metals and pathogenic microorganisms. The wastewater treatment system complies with standard wastewater management practices, and provides a useful method for treating and disposing wastewater in Ghana.

Full text

JENRM, Vol. 3, No. 1, 7-14, 2016
Research Article
Sewage Treatment by Waste Stabilization Pond
Systems
Kenneth J. Bansah *, Raymond S. Suglo **
Abstract
Sewage generated in Ghana is commonly discharged into the environment without any form of treatment to reduce the degree of
contamination and mitigate potential public health and environmental issues. Although some attempts have been made in some
parts of Ghana to utilize the waste stabilization pond (WSP) system to treat domestic sewage, the ponds often fail to achieve
their purpose due to lack of basic maintenance and supervision. To assess the utility of the WSP system for treating sewage,
wastewater samples were collected from the raw sewage, anaerobic, facultative and maturation ponds of WSPs at Obuasi in
Ghana, and analyzed for physicochemical and microbiological contaminants. The results show that the final pond effluent meets
recommended microbiological and chemical quality guidelines. The waste stabilization pond system demonstrates high removal
efficiencies of wastewater contaminants. The biochemical oxygen demand, total suspended solids, nitrate and faecal coliforms
reduction efficiencies of 97.3%, 97.6%, 83.3% and 99.94% respectively are highly significant, and compare well with reported
removal efficiencies in the literature. Additionally, the ponds have high reduction efficiencies for heavy metals and pathogenic
microorganisms. The wastewater treatment system complies with standard wastewater management practices, and provides a
useful method for treating and disposing wastewater in Ghana.
Keywords
Wastewater—Pathogens—Waste Stabilization Ponds—Faecal Coliform—Sewage
*Department of Mining and Nuclear Engineering, Missouri University of Science and Technology, Rolla-Missouri, USA, Email:
kjbt3c@mst.edu; kjbansah@yahoo.com
**Department of Mining and Geological Engineering, Botswana International University of Science and Technology, Botswana, Email:
rsuglo@yahoo.ca
Contents
1 Introduction 7
2 Location, Relief and Climate of Obuasi 8
3 Waste Stabilization System 8
4 Methods 105
5 Results and Discussion 10
6 Conclusions and Recommendation 12
References 12
1. Introduction
Waste stabilization ponds (WSPs) utilize shallow basins10
for wastewater treatment through natural disinfection
mechanisms by integrating the activity of phototrophic,
autotrophic and heterotrophic microorganisms [
1
]. Ef-
fectively treating wastewater effluents can efficiently con-
tribute to water conservation, expansion of irrigated agri-
15
culture, environmental and public health protection [2].
Although WSPs are most commonly used for treating
domestic wastewaters, they have been recommended by
the World Health Organization (WHO) for the treatment
of agro-industrial wastes for reuse in agriculture and
20
aquaculture due to their effective removal of excreted
pathogens and helminth eggs [
3
]. Stabilizing sewage by
utilizing WSP is considered an appropriate technique for
the treatment and removal of pathogenic microorganisms
from wastewater in tropical and subtropical regions of
25
the world.
The ponds require little energy (only sunlight) and
no disinfectants [
4
], while playing an important role in
the removal of contaminants such as crude oil and heavy
metals. These low energy consuming ecosystems that use
30
natural processes in contrast with complex high mainte-
nance treatment systems (such as trickling filters, acti-
vated sludge treatment, rotary biological contactors, etc.)
have become well established as methods for wastewater
treatment in tropical climates [
5
]. The treatment tech-
35
nique uses minimal operational and maintenance input
to produce an effluent that meets the recommended mi-
crobiological and physicochemical quality guidelines [6].
Additionally, it requires daily minimum supervision for
its operation, by simply cleaning the outlets and inlet
40
channels.
WSP system generally consists of a series of ponds
involving anaerobic, facultative and maturation ponds
[
5
,
7
,
8
]. Anaerobic and facultative ponds are respec-
tively used for primary treatment and secondary treat-
45
ment. They are both for the removal of biochemical
oxygen demand (BOD), Vibrio cholerae and helminth
eggs. The maturation ponds are used for tertiary treat-

Sewage Treatment by Waste Stabilization Pond Systems — 8/13
ment of wastewater effluents and responsible for the re-
moval of faecal viruses (e.g. rotavirus, astrovirus and
50
norovirus), faecal bacteria (e.g. Salmonella spp., Shigella
spp., pathogenic strains of Escherichia coli) and, nitrogen
and phosphorus nutrients [9].
The relative advantages of WSPs compared to other
conventional wastewater treatment systems (such as ro-
55
tary biological contactors, activated sludge treatment,
trickling filters, aerated lagoons, biological filters and
reed-bed systems) in relation to effluent standards with
respect to algae removal, input of external energy, nutri-
ent removal and relative costs include simplicity, low cost,
60
low maintenance, low energy consumption, robustness,
and environmental sustainability [10].
WSPs are particularly well suited for tropical and
subtropical countries such as Ghana. Stabilization ponds
are widely used for treatment of municipal wastewater in
65
many countries with ample sunshine such as Colombia, El
Salvador, Guatemala, Honduras, Israel, Jordan, Morocco,
Nicaragua, Tunisia and Uganda [
11
]. They are typically
used in smaller towns where land availability and cost is
less of a constraint. In some cities (e.g. Amman in Jordan
70
and Adelaide in Australia), larger stabilization ponds
have been replaced in the early 2000s by activated sludge
wastewater treatment plants. Due to the temperature and
lengthy duration of sunlight which are key factors that
enhance the performance process and effectiveness of the
75
treatment system, tropical and subtropical countries offer
excellent opportunity for high efficiency and satisfactory
performance [12].
According to [
13
], the commonest secondary treat-
ment technologies adopted for domestic sewage treatment
80
in Ghana are trickling filters, activated sludge and waste
stabilization ponds. WSPs installed in some towns and
communities such as Akuse, Akosombo and Kumasi are
reported to perform remarkably well [
13
,
14
]. However,
the management of domestic wastewater in Ghana still
85
remains a major problem to the local and national gov-
ernments.
To complement the efforts by government in solving
the challenges posed by sewage, six WSPs have been built
by AngloGold Ashanti (Obuasi) mine for the collection
90
and treatment of sewage to reduce the effect of certain
water contaminants on the environment and protect pub-
lic health. The wastewater in these ponds is treated to
ensure that the levels of certain contaminants are brought
below acceptable limits before discharging the effluents
95
into the natural environment. To avoid polluting soils,
receiving waters and endangering human health, flora,
fauna and aquatic lives downstream from the points of
discharge, it is necessary to monitor some water quality
parameters in the treated domestic wastewater before it is
100
discharged. This also provides a means for evaluating the
performance of the treatment mechanism and makes data
available for trend analysis and regulation evaluation.
2. Location, Relief and Climate of Obuasi
Obuasi is located in the Obuasi Municipal Assembly of
105
the Ashanti Region of Ghana. The town is located 64 km
south-east of Kumasi, the regional capital and 175 km
north-west of Accra, the capital of Ghana. It has easy
access to most parts of the country both by road and by
railway. By railway, it is 72 km south of Kumasi, 192
110
km north of Takoradi habour and 330 km north-west of
Accra.
Obuasi is situated in the tropical rainforest zone of
Ghana. It has a long rainy season, which spans from
March to December, followed by a brief dry season with
115
sporadic rainfall from December to February. The highest
rainfall occurs between May and July averaging 77.3 mm
while the least occurs in December with an annual average
rainfall of 39.1 mm. The annual average temperature is
28
o
C. The area has thick vegetation and, where the relief
120
is very high, there are usually shrubs [
15
]. Fig. 1 shows
the location of the study area.
3. Waste Stabilization System
Waste stabilization systems comprise a single series of
anaerobic, facultative and maturation ponds, or several
125
such series in parallel. Anaerobic and facultative ponds
are designed for removal of biochemical oxygen demand
(BOD) and maturation ponds for pathogen removal. How-
ever, some BOD removal also occurs in maturation ponds
and some pathogen removal in anaerobic and facultative
130
ponds [
7
]. Anaerobic and facultative ponds only are re-
quired when the treated wastewater is to be used for
restricted crop irrigation and fish pond fertilization, and
a relatively weak wastewater (up to 150 mg BOD/l) is to
be treated prior to surface water discharge. Maturation
135
ponds are required only when the treated wastewater is
to be used for unrestricted irrigation and when stronger
wastewaters (BOD > 150 mg/l) are to be treated prior
to surface water discharge [4, 12].
Usage of waste stabilization ponds for treatment of
140
sewage wastewaters can be found in over 50 countries
with very different climatic conditions. A great number
of these ponds can be found in Asia, Latin America and
Africa [
16
]. The common treatment technologies adopted
for domestic sewage treatments are trickling filters, acti-
145
vated sludge and waste stabilization ponds. [
17
] describes
the advantages of the WSP system over other treatment
systems including package plant, activated sludge, ex-
tended aeration activated sludge, biological ditch and
aerated lagoon. The WSP system outperforms the other
150
systems by its ability to remove BOD, faecal coliform,
helminth, and virus, the mode and cost of construction,
mode of operation, cost of maintenance, energy demand
and cost of sludge removal. The main limitations of WSPs
are that they require large areas of operation and are very
155
inefficient in the removal of suspended solids.

Sewage Treatment by Waste Stabilization Pond Systems — 9/13
Figure 1. Study Area
Under the tropical conditions in Ghana, the Waste
Stabilization Ponds (WSPs) have been found to be more
suitable and appropriate compared to conventional treat-
ment systems because of the ease of operation and mainte-
160
nance and the high level of treatment efficiencies they are
able to achieve [
14
]. Fig. 2 illustrates a typical scheme of
WSP.
Figure 2. Typical Scheme of a Waste Stabilization
System: Anaerobic, Facultative and Maturation Ponds
in Series (source: [18])
Fig. 3 is a schematic diagram of the WSP at Obuasi
[
19
]. These ponds consist of two anaerobic, four secondary
165
facultative, and two maturation ponds. The raw sewage
with high organic loading (compared to the other ponds)
is discharged into a small receptacle lined with sieves to
prevent large solids from entering the anaerobic ponds.
The anaerobic ponds are about 4 to 5 m deep. Their
170
primary function is BOD removal. In anaerobic ponds
sludge is deposited on the bottom and anaerobic bacteria
break down the organic matter by anaerobic digestion,
releasing methane and carbon dioxide. Viruses, bacteria,
helminth, Ascaris eggs and other pathogens can also be
175
inactivated by sedimentation when associated with solids.
Figure 3.
Schematic Diagram of the Waste Stabilization
Ponds at Obuasi
Nitrogen, phosphorus and potassium can also be reduced
by sludge formation and the release of ammonia into
the air [
20
]. Anaerobic ponds help to settle undigested
material and non-degradable solids as bottom sludge,
180
dissolve organic material and break down biodegradable
organic material.
The facultative ponds, which are 1 to 2 m deep, receive
settled wastewater from the anaerobic ponds. They are
also designed for BOD removal on the basis of a relatively
185
low surface loading (100 to 400 kg BOD/ha day) to per-
mit the development of a healthy algal population as the
oxygen for BOD removal by the pond bacteria is mostly
generated by algal photosynthesis [
5
]. The algae grow
using sunlight and produce oxygen in excess to their own
190
requirements. The excess oxygen is used by bacteria to
further break down organic matter via aerobic digestion
transforming the organic pollutants into carbon dioxide.
In addition to the aerobic and anaerobic digestion of BOD
in the facultative ponds "sewage BOD" is converted into
195
"algal BOD" [
5
]. As a complete process, the facultative
pond serves to further treat wastewater through sedimen-
tation and aerobic oxidation of organic material, reduce
odor, reduce some disease-causing microorganisms if pH
rises and store residues as bottom sludge [20]. 200
The maturation ponds (which are 1 to 1.5 m deep)
receive the effluent from the facultative ponds. The size
and the number of maturation ponds depend mainly on
the required bacteriological quality of the final effluent.
The primary function of maturation ponds is the removal
205
of excreted pathogens. Pathogens die off due to the high
temperature, high pH or radiation of the sun leading

Sewage Treatment by Waste Stabilization Pond Systems — 10/13
to solar disinfection. Virus removal occurs by adsorp-
tion onto settleable solids and consequent sedimentation
[
20
]. Maturation ponds achieve only a small removal of
210
BOD, but their contribution to nutrient (nitrogen and
phosphorus) removal can also be significant [5].
4. Methods
Samples were taken from the raw sewage and from the
effluents of Ponds 1 to 6. The American Public Health As-
215
sociation standard methods for the examination of water
and wastewater [
21
] were followed. The physicochemical
and microbiological parameters of WSP samples were
analyzed. Wastewater quality parameters analyzed in-
cluded total suspended solids (TSS), total dissolved solids
220
(TDS), pH, biological oxygen demand (BOD), nitrate,
total coliforms, faecal coliforms, Escherichia coli (E. coli),
oil and grease, dissolved oxygen, conductivity, colour,
lead (Pb), iron (Fe), copper (Cu), arsenic (As), zinc (Zn)
and chloride (Cl
−
). The quality of the final pond ef-
225
fluent was compared with recommended microbiological
and physicochemical quality guidelines (typically adapted
from the World Health Organization, U. S. Environmen-
tal Protection Agency, and the World Bank Group) by
the Environmental Protection Agency of Ghana (EPA)
230
to determine conformance. Additionally, the reduction
efficiencies of the various parameters were calculated by
using equation 1.
η=Cr−Cf
Cr
×100 (1)
Where:
η
is removal or reduction efficiency in %,
Cr
is the concentration in the raw sewage,
Cf
is the
235
concentration in the final pond effluent.
5. Results and Discussion
The discharge of effluents with high TSS concentrations
can cause sludge depositions and anaerobic conditions in
the receiving water body. The TSS concentration of the
240
raw sewage is 1260 mg/l and that of the final effluent is 30
mg/l, giving a reduction efficiency of 97.6%. Fig. 4 shows
the TSS content of the effluents from the pond system.
The final effluent of the WSP system compares well with
recommended guidelines. The high TSS concentrations of
245
the effluents from the anaerobic and facultative ponds can
be attributed to the high TSS levels of the raw sewage.
The TSS removals in Ponds 1 to 5 support the findings of
Arthur [
17
], that the ability of WSP systems to remove
TSS is poor. However, it must be noted that the TSS
250
concentrations of the pond effluents decrease along the
treatment series as expected, and that the overall high
reduction efficiency achieved is in contrast with Arthur
[
17
]. The high TSS reduction efficiency demonstrates that
the WSP systems at Obuasi are efficient in reducing TSS.
255
Effluents with high concentrations of nitrates can
cause unwanted phytoplankton growth in the receiving
water bodies. According to Metcalf and Eddy [
22
], ni-
trate is typically absent in domestic sewage. The nitrate
concentration of the raw sewage is 2.39 mg/l while that
260
of the final effluent is 0.4 mg/l. The nitrate concentration
of the final effluent is acceptable as it is far less than
the guideline value of 50 mg/l. The nitrate removal ef-
ficiency of 83.3% achieved by the treatment ponds can
be described as appreciable. Fig. 5 shows the nitrate
265
concentrations of the effluents. The results show that
the nitrate levels in all the ponds are far lower than the
recommended guideline value.
When effluents with high concentrations of BOD are
discharged into the natural drains, they can cause deple-
270
tion of natural oxygen resources which may lead to septic
conditions. The BOD of the raw sewage is 280 mg/l and
that of the final pond effluent is 7.6 mg/l. The average
BOD removal efficiency of the ponds is calculated to be
97.3%. Fig. 6 shows the BOD levels in the effluents. A
275
study by Gloyna [
23
] indicates that it is uncommon to
obtain better than 90% BOD removal in waste stabiliza-
tion. The findings in this research suggest that the WSP
systems at Obuasi are very effective in the removal of
BOD. 280
The faecal coliforms level of the raw sewage is 65000
counts/100 ml while that of the final pond effluent is
40 counts/100 ml. The average faecal coliforms removal
efficiency is determined to be 99.94% which is signifi-
cant. Waste stabilization ponds usually give such high
285
micro-organism removal efficiency. Effluents with high
concentrations of faecal coliforms have high potential of
endangering public health. The concentration of the fae-
cal coliforms of the final effluent is very low compared to
the recommended guideline value of 1000 counts/100 ml
290
(see Fig. 7). This indicates that the final pond effluent
can be discharged into the natural environment without
posing threat to humans and the ecosystem in general.
Other wastewater parameters (pH, conductivity, TDS,
oil and grease, As, Fe, Cu, Pb, Zn, Cl
−
, total coliforms,
295
E. coli, colour and DO) of the final pond effluent compare
well with their respective recommended levels. The raw
sewage contains small concentrations of metals which can
be attributed to the relatively “normal” pH level. While
the metal concentrations of the final pond effluent remain
300
low and meet regulatory requirements, the pH of the
maturation ponds are higher than that of the anaerobic
and facultative ponds. It is common to find variations
in pH in WSP systems. Maturation ponds usually have
high pH which aid in the pathogen die-off mechanism.
305
Kayombo et al [
24
] also report diurnal pH changes in WSP,
and indicate that increase in pH up to 11 is common in
WSPs, with highest pH levels usually reached during the
late afternoon.
It is also demonstrated that the ponds can be effective
310

Sewage Treatment by Waste Stabilization Pond Systems — 11/13
in reducing oil and grease, which otherwise can affect
aquatic activities, and contaminate soils and plants. The
high oil and grease reduction efficiency is corroborated by
the findings of Shpiner et al [
25
]. They concluded that
waste stabilization ponds can treat synthetic produced
315
water (PW) and achieve high oil and grease removal levels
(
>
90%). Although the chloride concentration of the final
pond effluent meets regulatory requirements, there are
observed variations in the chloride concentration along
the pond series. These changes in the chloride concen-
320
trations as the sewage passes through the ponds are also
reported by [
26
]. The ponds also achieved high total
coliform and E. coli reduction efficiencies. The progres-
sive removal of pathogens along the series of ponds is
consistent with the findings of Mara et al [
12
]. Stabiliza-
325
tion ponds are generally reported to demonstrate high
pathogen removal efficiencies [
27
,
28
,
29
]. Table1 shows
results of the wastewater parameters analyzed for the
pond series.
Figure 4. TSS Concentration of Pond Effluents
Table 1. Physicochemical and Microbiological Parameters of WSP Samples
Parameter Raw Sewage Pond Pond Pond Pond Pond Pond EPA
1 2 3 4 5 6
pH 7.6 7.6 7.4 7.8 8 9.5 8.9 9-Jun
Conductivity 11860 11480 10800 6360 4620 3119 1410 1500
TDS 6103 5800 5500 3200 2400 1560 854 1000
TSS 1260 1002 720 380 250 150 30 50
Oil & Grease 12.98 12.33 7.24 3.18 1.02 0.11 0.01 100
As 0.89 0.81 0.79 1.48 1.21 1.04 1.3 1.5
Fe 0.71 0.69 2.05 0.99 0.24 0.05 0.14 2
Cu 0.02 0.01 0.05 0.01 <0.01 <0.01 <0.01 2.5
Pb 0.09 0.06 0.02 0.02 0.01 <0.01 <0.01 0.1
Zn 0.08 0.08 0.35 0.012 0.05 0.01 0.02 5
Cl- 261 248 274 212 323 263 177 250
NO3- 2.39 2.08 2.01 0.02 5.19 1.66 0.4 50
Total Coliforms 183744 161600 144000 55200 33200 1256 380 1000
Faecal Coliforms 65000 61000 5200 4000 3100 280 40 1000
E. Coli 298 255 94 53 30 13 3 10
Colour 1198 1120 560 270 140 40 20 200
BOD5 280 103 73 51.4 45.7 15.1 7.6 50
DO 6 6.3 6.4 6 6 6.7 6.8 -
(All concentrations in mg/lexcept pH, Conductivity (µS/cm), Colour (PCU) and the Coliforms(counts/100ml)

Sewage Treatment by Waste Stabilization Pond Systems — 12/13
Figure 5. Nitrate Concentration of Pond Effluents
6. Conclusions and Recommendation
330
The waste stabilization pond system at Obuasi is assessed
to achieve high removal efficiencies of wastewater contam-
inants. The ponds demonstrate high reduction efficiencies
in the physicochemical, microbiological and heavy metal
contaminants levels of the wastewater. Given that the
335
temperature in Obuasi ranges from 22
oC
to 34
oC
, with
rainfall throughout the year, the contaminant removal
efficiencies are not expected to vary significantly during
the year. The waste stabilization pond system is appro-
priate for treating wastewater and produces effluents that
340
meet the recommended microbiological and chemical qual-
ity guidelines at low cost and minimal operational and
maintenance requirements. The wastewater treatment
system is effective and complies with standard wastewa-
ter management practices. The quality of the final pond
345
effluent is not anticipated to have adverse effects on the
environment when discharged into nearby surface water
sources. The waste stabilization pond system provides a
useful method of wastewater treatment and disposal for
growing communities, and therefore should be regarded
350
as a method of choice for treating wastewater in Ghana.
Figure 6. BOD Levels of Pond Effluents
Figure 7. Faecal Coliform Levels of Pond Effluents
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