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Constructed Wetland Potential in the Sanitation Service Chain in Lake Victoria Basin
Katima J. H.Y.1, Gastory, L.2, Outwater A.3
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1University of Dar es Salaam, College of Engineering and Technology (COET),WSP & CW Research and Development Group, P.O. Box 35131, Dar es Salaam, Tanzania. Email: [email protected]
2WWS Design and Development Co. Ltd. P.O. Box 32312,Dar es Salaam, Tanzania. Email: [email protected]
2Muhimbili University of Health and Allied Studies, P.O. Box 65004, Dar Es Salaam, Tanzania. Email: [email protected]
ABSTRACT
Constructed wetlands (CWs) are now a well established technology for wastewater treatment. The technology is gaining popularity due to its economically and environmentally sound attributes as a wastewater management option. Principally, CWs are designed and constructed to utilize the natural processes involving wetland vegetation, substrates and their associated microbial assemblages to help in wastewater treatment. This is similar to how natural aquatic ecosystems and their associated catchments work.
Research has continuously revealed increasing levels of pollution in Lake Victoria for decades leading to reduction in diversity of fish species, reduced levels of oxygen, increased salt loading, and emergence of water hyacinth, all of which eventually impact on the increased incidence of diseases and general health of the people. Yet, discharges and the complexity of untreated and partially treated wastewater into the Lake are increasing due in large part to population growth as well as expansion of human activities within the Lake Victoria Basin.
This paper disseminates CW technology thereby sharing its potential in addressing sanitation challenges currently experienced in Lake Victoria. It focuses on reduction of organic matter and nutrients which are responsible for the eutrophication as evidenced by explosion of water hyacinths in the Lake. It also evaluates and proposes technological coupling with CW towards achieving complete wastewater treatment cycles and meeting recommended effluent discharge standards. The experience presented experience shared base on case studies from our long term research work on wastewater with 20 pilot and full scale CW units in East Africa. Overall, obtained results revealed satisfactory pH control as well as BOD, NO3 – N and NH3- N, COD, tannins and zinc removal from wastewaters whereby most effluents were found to be below the recommended national standards. The study recommends incorporation of CW technology in wastewater treatment within Lake Victoria Basin since it has potential to protect the Lake ecosystem. The study also, calls for joint efforts to formalize and disseminate CW technology for decentralized wastewater treatment, water reuse and biodiversity enhancement.
1.0 INTRODUCTION
Constructed wetlands (CWs) are now a well established technology for wastewater treatment. Globally, there are several thousand wetland systems treating municipal, agricultural and industrial wastewaters and a rising number of systems treating point source and non-point pollution source. The technology is gaining popularity due to its economicallyand environmentally sound attributes as a wastewatermanagement option.Compared with technology-basedwastewater treatment systems, CWs require nomachinery, chemicals, anthropogenic energy inputsand result in modest operation and maintenance requirements.
Principally, CWs are designed and constructed to utilize the natural processes involving wetland vegetation, substrates and their associated microbial assemblages to help in wastewater treatment. In other words, CWs attempt to replicate the function of natural wetlands and other aquatic systems but in more controlled manner (US EPA, 2004) thereby involving the physical, chemical, biological processes. The most stable microbiota in these systems is found in the biofilm formed inside the system: on root plants or filter bed material surface. The complex microbial community associated with the filter material or roots, created by interactions with the wastewater, is most responsible for the degradation performance of the system (Sleytret al., 2009). This is similar to how natural aquatic ecosystems and their associated catchments work. Lake catchments for example, have been regarded since time immemorial as cleansing media and sink for pollutants, particularly nutrients.
Research has continuously revealed increasing levels of pollution in Lake Victoria for decades. The causes of rising pollution levels are many and diverse and each of the three East African nations and the other two EA countries which are part of the lake catchment area is culpable. Important pollution components of the Lake include excess nutrient levels, microbiological and chemical pollutants, suspended solids, which result from direct activities on the lake, untreated municipal sewage, agricultural waste brought in by inflowing rivers, maritime transport, and runoff and storm water inflow. These components have led to reduction in diversity of fish species, reduced levels of oxygen, increased salt loading, and emergence of water hyacinth, all of which eventually impact on the increased incidence of diseases and general health of the people (Karanja, 2006). Yet, discharges and the complexity of untreated and partially treated wastewater into the Lake are increasing due in large part to population growth as well as expansion of human activities within the Lake Victoria Basin (Odada, 2006). This pollution acceleration within the Lake will have a tremendous impact on human welfare in the region. Technological solutions, specifically incorporation of CW technology in the sanitation service chain within Lake Victoria Basin, will have long term
positive impacts towards improving the lake sanitary status and biodiversity. Yet, most actors and stakeholders in the provision of sanitation services are unaware of the efficacy of this technology, hence minimal uptake.
This paper disseminates CW technology thereby sharing its potential in addressing sanitation challenges currently experienced in Lake Victoria. Overall, it describes the performance and efficiency of the technology in treating the type of wastewater being generated and discharged into the Lake. It focuses on reduction of organic matter and nutrients which are responsible for the eutrophication as evidenced by explosion of water hyacinths in the Lake. It also evaluates and proposes technological coupling with CW towards achieving complete wastewater treatment cycles and meeting recommended effluent discharge standards.
The experience presented in this paper is based on case studies from our long term research work on wastewater with 20 pilot and full scale CW units in Tanzania, Uganda, Kenya, Ethiopia and Seychelles.
For planners, policy makers, researchers, engineers, health workers and other stakeholders and beneficiaries who wish to see Lake Victoria free ofpollutants from surrounding municipalities, industries and factories, experience presented in this paper promises significant benefits in terms of public health, economic gains and environmental sustainability.
2.0 MATERIALS AND METHOD
2.1 Experimental Designs
Over a period of 14 years (1998 – 2012), 20 Constructed Wetland units have been designed and established by the Waste Stabilization Ponds and Constructed Wetland Research Group of the University of Dar es Salaam in collaboration with several partners. Fifteen have been established in Tanzania, two in Uganda and one each in Kenya, Ethiopia and Seychelles (Table 2.1). Some of the units were developed for research purposes and others to address typical sanitation challenges on the ground. These systems were designed to carry out secondary and tertiary treatment thereby considering the removal of Biochemical Oxygen Demand (BOD), Total Suspended Solids (TSS), Nitrate Nitrogen (NO3-N), Ammonia Nitrogen (NH3-N), Phosphorus (P) and Faecal Coliforms (FC) to required effluent discharge standards. Local standards as provided by respective standard bodies in each country were considered. In the absence of local standards, World Health Organization (WHO) standards were adopted.
Most of the systems were designed to treat domestic wastewater. Others were designed for treatment of municipality sewage, discharges from a paper recycling plant, abattoir and a tannery wastewater and as well as Acid Mine Drainage. All established systems were Horizontal Subsurface Flow Constructed Wetlands (HSSFCW) of varying flow rates and surface area. The filling medium consisted of clean and graded limestone gravel with size ranging from 12 – 20 mm uniformly packed to cover a depth of 0.6m from the bottom of the bed to accommodate the root system of the type of selected plants. Phragmites mauritianus (reeds) were used as macrophytes wetland plants. Other tested plants were Phragmites karka (reeds), Typha spp. (cattails), mangroves and papyrus. On average, the initial planting density for the macrophytes was three plants per square meter. Inlet and outlet pipes were established at 0.6m and 0.5m respectively from the bottom of the bed to cater for hydraulic gradient within the system. As such, water level was maintained at 0.5m from the bottom of the bed. The inlet and outlet zones were packed with boulder stones (50mm – 100mm diameter size) to ensure uniform distribution of wastewater within the systems. The systems were designed for an effluent retention time of 5 – 15 days. Some of the systems were coupled with downstream water reuse activities including aquaculture and agriculture activities.
Table 2.1: List of CW units designed and worked by the University of Dar Es Salaam
Year CW Unit Location Wastewater Type
Scale Flow Rate (m3/d)
Surface Area (m2)
Macrophytes
2012 TPCC DSM, Tanzania Domestic Experimental 3.5 9 Cattails
2011 North Mara Musoma, Tanzania AMD Experimental 5 15 Cattails
2010 Seeta High School Mukono, Uganda Domestic Full scale 55 450 Papyrus
2010 Chakechake Pemba, Zanzibar Municipal Full scale 350 1625 NYP
2009 Shimo la Tewa Mombasa, Kenya Domestic Full scale NR NR ND
2009 Mahe Mahe, Seychelles Domestic Full scale NR NR ND
2009 Modjo Addis Ababa, Ethiopia Tannery Full scale NR NR Reeds
2009 Kampala Abattoir Kampala, Uganda Abattoir Experimental NR NR Papyrus
2005 Dr. A. Outwater DSM, Tanzania Domestic Full scale ND 3 Reeds
2004 Ruaha SS Iringa, Tanzania Domestic Full scale 20 108 Reeds
2004 MUWSA Moshi, Tanzania Municipal Full scale 400 972 Reeds
2001 Mallya Prison Shinyanga, Tanzania Domestic Full scale 50 NR Reeds
2001 Bariadi Prison Shinyanga, Tanzania Domestic Full scale 50 NR Reeds
2001 Shinyanga Prison Shinyanga, Tanzania Domestic Full scale 50 NR Reeds
1998 UDSM DSM, Tanzania Domestic Experimental 2 40.70 Reeds
1998 UDSM DSM, Tanzania Domestic Experimental 2 13.80 Cattails
2004 Kleruu College Iringa, Tanzania Domestic Full scale 44 400 Reeds
NR Kibo Paper Moshi Tanzania Industrial Full scale NR NR Reeds
NR Prof. K. Njau DSM, Tanzania Domestic Full scale NR NR Papyrus
NR WAAL´S USR SS Kibaha, Tanzania Domestic Full scale ND 560 NYP
AMD= Acid Mine Drainage, ND = Not Determined, NYP = Not Yet Planted, NR = Not Recorded
Plate 2.1: CW at Ruaha, Tanzania Plate 2.2: CW at Kleruu, Tanzania
Plate 2.3: CW at UDSM, Tanzania Plate 2.4: CW in Shinyanga, Tanzania
Plates 2.5: CW serving Moshi Municipality (MUWSA), Kilimanjaro, Tanzania
Plate 2.6: CW at Seeta School, Uganda Plate 2.7: CW in Mahe, Seychelles
2.2 Monitoring and Operational programmes
Tentative monitoring programmes varied from one system to another depending on the establishment of flow and which macrophytes and microorganisms were incorporated into the systems. The monitoring programmes for the types of selected plants varied from 3 – 6 months. During this time period wastewater was allowed to flow past the CW unitswith careful observance of plants development, bio-films development for microbes and retention times for the systems.
For experimental setups, the operational phases were devised to include continuous sampling of wastewater as well as field and laboratory analysis of wastewater samples to suit the desired objectives. The operational phases for the full scale units i.e. CW units at Ruaha, Kleruu, MUWSA etc., comprised of continuous sampling and laboratory analysis at initial stages only followed by periodical monitoring of the systems as per users arrangements. In most cases, wastewater samples were collected at the inlets and outlets of the CW systems to better examine, among other things, the performance and efficiency of the CW units.
2.3 Statistical and Laboratory Analysis
In most cases, onsite analyses were conducted out to record wastewater flow rates, ambient temperatures and wastewater temperatures. Collected wastewater samples were careful stored and analysed at authorized water quality laboratories in the respective areas. In Tanzania for example, laboratory work was carried out at the College of Engineering and Technology of the University of Dar es Salaam, Ardhi University and at municipal water quality laboratories operated by Urban Water Supply and Sewerage Authorities. Depending on project objectives, wastewater samples were analyzed for multiple parameters covering the spectrum of physical, chemical and biological water quality parameters. In most cases, domestic wastewaters were analysed for pH, Electrical Conductivity (EC) and Total Dissolved Solids (TDS)Biochemical Oxygen Demand (BOD5), Chemical Oxygen Demand (COD), Total Suspended Solids (TSS), Total Dissolved Solids (TDS), Nitrate Nitrogen (NO3-N), Ammonia Nitrogen (NH3-N), Phosphorus (P) and Faecal Coliforms (FC). In addition to the list, other types of wastewater were analysed for tannin and heavy metal concentrations including chromium, nickel, aluminium and zinc. The samples were analyzed in accordance with American Public Health Association (APHA), Standard Methods for the Examination of Water and Wastewater, (1998).
2.4 Performance and Efficiency Analysis
For the purpose of sharing experiences and the potentials of CW in the sanitation service chain in Lake Victoria, a comprehensive analysis of the performance and efficiency of mentioned CW units was carried out. The analysis focused on the performance efficiency of CW units on reduction of organic matter and nutrients which are the critical parameters responsible for the eutrophication and degradation of Lake Victoria. The analysis also evaluated and contrasted technological coupling with CW towards achieving recommended effluent discharge standards.
3.0 RESULTS AND DISCUSSION
3.1 Performance Efficiency of CWs on Treating Domestic/Municipal Wastewater
Table 3.1 presents a summary of laboratory results for the various CW units that were established to treat domestic/municipal wastewater.
pH Control: The results entail that pH values in the CWinfluent ranged from 7.20 – 8.30 with an average of 7.66±0.57. On the other hand effluent pH values for the CW units ranged from 7.00 – 7.60 with mean value of 7.55±0.31. The results obtained reveals that pH values in the influents varied from time to time and from one source to another possibly due to variations of alkalinity in the raw sewage. Results also entails that pH in the influent is higher than pH in the effluents possibly due to decrease in alkalinity in the CW cells. Performance wise, the results agree with effluent discharge standards as recommended by local authorities which requirepH to be of a range of 6.5 – 8.5. However, the results revealed that for domestic wastewater pH is not a critical parameter as both the influent and effluent met the recommended effluent discharge standards.
BOD Removal: For the assessed CW units, influent BOD concentrationsranged from 51 - 200mg/l with average concentration of 127.75±61.02mg/l. The effluent BOD concentration ranged from 12 – 41 mg/l. The inter-average BOD concentrationis 22.00±12.94 mg/l. This is equivalent to the system efficiency of 82.78% for BOD removal. Generally, the results entails better performed of the CW units as the BOD in the effluents met recommended effluent discharge standards by local authorities.
Nitrate Removal: The results showed that characteristic Nitrate Nitrogen in raw domestic/municipal wastewater range from 26.30 – 35.30 mg/l whereas Nitrate concentration in CW effluents ranged from 11.30 – 11.44 mg/l. The influent and effluent averages were 30.80±4.50 mg/l and 11.37±0.07 mg/l respectively. This is equivalent to the CW efficiency of 63.08% on nitrate removal. The results entails better performance of the CW units as the effluents met recommended effluent discharge standards by local authorities.
Ammonia Removal: Influent ammonia concentrations for the assessed CW units ranged from 24.87– 77.30 mg/l with average concentration of 51.09±26.22 mg/l. The effluent Ammonia concentration ranged from 10.35 – 33.00mg/l. The inter-average Ammonia concentrationin the CW effluent was 21.68±11.33 mg/l. This is equivalent to the system efficiency of 57.57% for Ammonia removal. Though the overall performance efficiency did not meet recommended effluents discharge standards by local authorities, some individual CW met (Table 3.1).
Phosphorus Removal: Laboratory analysis of wastewater samples showed that Phosphorus concentration in raw domestic/municipal wastewater ranged from 18.10 – 56.50 mg/l whereas Phosphorus concentration in CW effluents ranged from 6.72±1.20 – 39.90mg/l. The influent and effluent averages were 50.50±6.00 mg/l and 29.00±6.00 mg/l respectively. This is equivalent to the CW efficiency of 42.57% on nitrate removal. These results entail that CW is fairly poor on the removal of Phosphorus to meet recommended effluent discharge standards as recommended by local authorities in East Africa. However, this might be contributed by the types of substrates used and operation hydrodynamics in the individual CW units.
Table 3.1: A summary of laboratory results for the various CW units treating domestic and municipal wastewaterCW Unit Location pH BOD5 (mg/L) NO3-N (mg/L) NH3-N (mg/L) Phosphorus
(mg/L)Reference
UDSM (Reeds) Inlet 7.2 50.70 NR NR NR Bilha, 2006Outlet 7.0 18.00 NR NR NR
Ruaha SS Inlet 8.30 200.00 35.30 77.30 44.50 Njau et al., 2010Outlet 7.60 41.00 11.30 33.00 18.10
Kleruu College Inlet NR 1245 NR NR NR Katima, 2005Outlet NR 12.00 NR NR NR
MUWSA, Moshi Inlet 7.47 135 26.30 24.87 56.5 Njau et al., 2010MUWSAOutlet 7.45 17 11.44 10.35 39.9
Average Inlet 7.66±0.57 127.68±61.15 30.80±4.50 51.09±26.22 50.50±6.00Outlet 7.35±0.31 22±12.94 11.37±0.07 21.68±11.33 29.00±10.90
Local Requirement 6.5 – 8.5 30 50 10 6
Efficiency (%) 82.77 63.08 57.57 42.57
3.2 Performance and Efficiency of CWs on Treating Industrial wastewater and AMD
Assessment of CWs on treating high strength wastewater focused on treatment of tannin wastewater and acid mine drainage (AMD) which are also taking place in the various areas within the Lake Victoria Basin. Two studies are briefly described below.
The first study was conducted to investigate the application of CW in the treatment of tannin wastewater based at TANWAT, Njombe, Tanzania in 2006. The study focused on the removal of tannins and COD. The results of the study showed satisfactory removal of tannin and COD to recommended effluent discharge standards (Figure 3.1a and 3.2b)
0100200300400500600700800
0 3 6 9 12 15 18 21
Time (days)
Tan
nins
con
cent
ratio
n (m
g/l)
Inlet Raw TWWInlet pre-treated TWWOutlet Raw TWW unit AOutlet pre-treated unit AOutlet Raw TWW unit B
Figure 3.1a: Tannin removal as a function time in CW at TANWAT, Njombe
0
200
400
600
800
1000
1200
0 3 6 9 12 15 18 21
Time (days)
CO
D (m
gO2/
L)
Inlet Raw TWWInlet pre-treated TWWOutlet Raw TWW unit AOutlet pre-treated unit AOutlet Raw TWW unit B
Figure 3.2a: COD removal as a function time in CW at TANWAT, Njombe
The second study was carried out at the University of Dar es Salaam, Tanzania by Rwegoshora (2003). The study was conducted to assess the potential of Constructed Wetland (CW) in the treatment of acid mine drainage (AMD). The study focused on heavy metal removal (specifically zinc removal) from the AMD and the contributing factors, which enhance the removal.Artificial AMD (AMD feed solution) was made in high concentrations, stored in AMD tanks and used for experimentation. Four units of Horizontal Subsurface Flow CW sized 11.12m x 2.3mx 0.6m, planted with Phragmites mauritianus (in two cells), Typha domingensis (in the third cell) and the fourth cell unplanted (control cell) filled with gravel 6-25 mm in size and 0.7m thick were used. The flow rate through the cells was 2010 litres per day. In another set of experiments four small cells of dimension 89.5 cm x 30 cm x 60 cm were planted with Typha, Phragmites, Vetiver and Papyrus and subjected to varying acid concentration (pH 3.5, 3.0, 2.9 and 2.7) to study their resilience to such environments.
The results of the study revealed that zinc concentration as a function of distance within the CW decreasedfrom inlet to outlet for the vegetated CW likely through sulphide precipitation. The CW planted with Typha demonstrated zinc removal from artificial AMD by about 80% -84% while the control cell did not show any significant removal. Also, Phragmites,Typha and Papyrus exhibited acid tolerance in acidic media, hence ability to clean up acid mine drainage.
0 200 400 600 800 1000 12000
0.10.20.30.40.50.60.70.80.9
Zinc concentration against distance within the wetland -Typha
Zn ppm (Typha)
Exponential (Zn ppm (Typha))
Distance[cm]
Zinc
Con
cent
ratio
n [p
pm]
Figure 3.2: Variation of zinc levels with distance within the CW
3.3 Technological coupling of CW with other conventional approaches Constructed wetland is not a stand-alone technology. Technically, CW systems are designed to accomplish either secondary treatment thereby removing organic matter (BOD) or tertiary treatment in reducing microorganisms and excessive nutrients from wastewater; or both. In other words, CW systems are not appropriate for carrying out primary wastewater treatment for solids removal since solid matter are detrimental when they clog the systems. However, suspended organic solids concentrations of less than 200mg/l are allowable as they take part in the formation of biofilms for treatment microbes but not to the extent to affecting wastewater pathways since sediment build up rate is normally less than 1 cm per year (Pries and Brassard, 2009). Note that, sedimentation and filtration are also pollutant removal mechanisms in CW systems and CW can be designed to recover nutrients (such as nitrogen and phosphorus) preferably for downstream irrigation agriculture. This is due to the fact that the design of CW systems is based on characteristic wastewater parameters. The land requirement for ammonia removal for instance, is much larger than the land requirement for BOD removal since the ammonia removal rate constant is much smaller than other critical pollutant parameters especially in domestic/municipal wastewater (Kayombo et al., 2003). The paragraphs and illustrations below provide the typical technological configuration with CW for the various treatment purposes which are appropriate in the East African Region.
3.3.1 CW for Secondary and Tertiary Wastewater Treatment For successful accomplishment of secondary and tertiary treatment of wastewater, CW system needs to be incorporated with preliminary and primary wastewater treatment facilities. Appropriate preliminary facilities are bar screen and grit chamber and are required for removal of solids, unwanted solid particles and/or fibrous materials. Primary treatment is required to separate organic particles in suspension and floating materials (oil, grease and lighter solids) from liquid waste for discharge for secondary treatment. Appropriate primary facilities are septic tanks and anaerobic ponds for small and large scale levels respectively. The effluent from well designed and operated preliminary and primary treatment facilities can be adequately treated by CW systems to meet recommended effluent discharge standards.
3.3.2 CW for Tertiary Wastewater Treatment Tertiary treatment of wastewater, also called “final polishing” is required to provide treatment to raise the effluent quality to nationally required standardsbefore it is discharged to the receiving environment (sea, river, lake, ground, etc.). The critical wastewater characteristic parameters are nutrients, nitrogen and phosphorus as well as the microorganisms. CW can be designed and operated to successfullyremove nitrogen
(nitrate and ammonia), phosphorus and microorganisms from wastewater through biological removal mechanisms. To accomplish this, the CW system needs to be coupled with appropriate preliminary, primary and secondary treatment facilities. At small scale level, a bar screen, grit removal and septic can suffice. However, large scale configurations require a bar screen, grit removal, anaerobic and facultative ponds. In other words, for a typical wastewater treatment system, CWs replace the soakaway pits and maturation ponds at small and large scale levels respectively. Yet, CWs provide the potential for reuse of treated water especially in agriculture and aquaculture.
3.4 Technological Challenges
3.4.1 Poor operation and maintenance Constructed Wetlands (CW) must be managed properly if they are to perform well (Beharell, 2004). Thus, CW require regular monitoring and maintenance to ensure it remains functional and in a 'healthy' condition. The operational and maintenance needs includes the requirements for safety, water management, cleanout of sediment, maintenance of structures, embankments, and vegetation, control measures for vectors and pests, and containment of potential pollutants during maintenance operations (Kuginis, 1998; Beharell et al., 1998; Beharell, 2004).
The survey results on the operation of existing CWs carried out by the WSP & CW Research and Development Group of the University of Dar Es Salaam in 2010 indicated that 86% of the surveyed CWs experienced various forms of operational problems.The major problem experienced was a combination of blockage and over flooding (57.1%) whereas blockage itself constituted 14.3%. Other operational problems included seepage through the walls or leakage, storm water runoff especially during and after rainfall events and, cracks which altogether constituted 28.6%.
3.4.2 Inconsistent Design Procedures A constructed wetland is not just a hole on the ground. Although they are often relatively simple in engineering terms, they are extremely complex ecological systems which require the involvement of expertise in a variety of fields including chemistry, hydrology, soil science, plant biology, natural resources, environmental management, ecology, environmental/civil engineering, surveying and project management. Yet, information regarding most aspects ofthe design and implementation of CWs is lacking. This is in line with the lack of an integrated approach in the planning and design, lack of appreciation by many designers of the complex, physical, biological and chemical processes within CWs; lack of consistency in design, construction and operation aimed at optimal performance; lack of appropriate design tools and methodologies suitable for
local conditions; and the changing nature of rapidly-developed technology. This has led to the development of many CWs that are inappropriate, under-performing, or poorly designed or maintained.
3.4.3 Minimal Uptake of the Technology Despite all the advantages of CW there has been minimal uptake of the technology partly due to in-formalization of the technology by local authorities within East Africa.
4.0 CONCLUSION AND RECOMMENDATIONS
4.1 Conclusion This study was carried out to assess the performance efficiency of CW technology in East Africa and share its potential in addressing sanitation challenges currently experienced in Lake Victoria. The CW units currently operating in East Africa were considered for performance evaluation. The overall performance of the technology on treating domestic/municipal wastewater was satisfactory for pH control, BOD, NO3 – N and NH3- N removal. The mean removal efficiency of BOD, NO3-N, NH3-N and P were found to be 82.78%, 63.08%, 57.57% and 42.57% respectively. Most effluents were found to be below the recommended national standards except for phosphorus removal.
On the other hand the study assessed the performance of CWs on treating high strength wastewater, specifically tannin wastewater and acid mine drainage (AMD) which as well form the types of wastewater generated within the Lake Victoria Basin. Obtained results revealed satisfactory removal of tannin, COD and Zinc to recommended national effluent discharge standards. The performance efficiency on zinc removal for example was observed to range from 80% - 84%.
The study also evaluated technological coupling of CW with other conventional approaches to successful accomplish the overall cycle in the process of wastewater treatment. It was outlined that compulsory stages prior to CW unit are preliminary and primary wastewater treatment stages. Based on the Tanzanian experience, the study proposed the typical bar screen and grit removals as appropriate preliminary facilities. Well designed and operated septic tanks and anaerobic ponds were also suggested as appropriate options for primary treatment prior to discharge into the CW system depending on the scope of the project.
4.2 Recommendations
From the results discussed above, it is hereby recommended that: CW systems should be incorporated in wastewater treatment systems in the Lake
Victoria Basin since has potential to protect the Lake Victoria ecosystem That more data should be generated through research works to further investigate
the effectiveness of CW system in treating wastewater while meeting other purposes in integrated manner. Specifically it is proposed to investigate the removal of such parameters of heavy metals and toxins using CW.
That joint efforts are required to formalize and disseminate CW technology for decentralized wastewater treatment, water reuse and biodiversity enhancement.
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APHA (American Public Health Association) (1998), Standard Methods for Examination of Water and Wastewater, 20th edition, Water Environment Federation, Washington, D.C
Baharell, M. (2004).Operation and Maintenance of Constructed Wetland. In M.H. Wong (Ed.) Wetland Ecosystems in Asia: Function and Management. Elsevier B.V. pp 347-359
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Kayombo, S., Mbwette, T.S.A., Katima, J.H.Y., Ladegaard, N. and Jørgensen, S. E (2003).Waste Stabilization Ponds and Constructed Wetlands Design Manual, WSP & CW Research Project, College of Engineering and Technology, University of Dar es Salaam.
Odada, E.O., Olago, D.O. and Ochola, W., Eds., (2006). Environment for Development: An Ecosystems Assessment of Lake Victoria Basin, UNEP/PASS.
Pries J., and Brassard C., (2009). Is a constructed wetland a potential treatment solution for my community or industry? In brochure on Natural Treatment Systems
Sleytr K, Tietz A, Langergraber G, Haberl R, Sessitsch A. (2008) Diversity of abundant bacteria in subsurface vertical flow. Ecological Engineering. V.5:1021-1025.
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