WASTEWATER TREATMENT BYA NATURA LWETLAND : THENAKTVUB OSWAMP ,UGAND A PROCESSESAND IMPLICATIONS 0000 0820 2547 (jMofyzo \szi\ s„ WastewaterTreatmen tb y aNatura lWetland : TheNakivub o Swamp,Ugand a Processes andImplications DISSERTATION Submitted in fulfillment of therequirement s of the Board of Deans of Wageningen Agricultural University and the Academic Board of theInternationa l Institute for Infrastructural, Hydraulic and Environmental Engineering for the Degree of DOCTOR tob edefende d in public on Thursday, 20Ma y 1999a t 10:00h i n Delft, the Netherlands by FRANKKANSIIM E bornin Bushenyi District, Uganda and MAIMUNANALUBEG A bornin Kampala, Uganda r> Thisdissertatio nha sbee napprove db yth epromoters : DrG .J .Alaert s Professor inSanitar yEngineerin g(IHE ) DrR Denny Professor inWetlan dan dAquati cEcolog y(IHE ) DrL . Lijklema Professor inAquati cEcolog yan dWate rQualit y Management(LUW ) Co-promoter: DrJ .J .A .va nBrugge n Seniorlecture ra tth eDepartmen to fEnvironmenta l Sciencean dWate rResource sManagemen t(IHE ) Authorization tophotocop y items for internalo r personal use,o r theinterna l orpersona l useo f specificclients ,i sgrante db yA.A.Balkema ,Rotterdam ,provide dtha tth ebas efe eo fUS$1.5 0pe r copy,plu sUS$0.1 0pe rpag ei spai ddirectl yt oCopyrigh tClearanc eCenter ,22 2Rosewoo dDrive , Danvers,M A01923 ,USA .Fo rthos eorganization stha thav ebee ngrante da photocop ylicens eb y CCC,a separat esyste mo fpaymen tha sbee narranged .Th efe ecod efo ruser so fth eTransactiona l ReportingServic eis :9 0541 042 0 1/99US$1.5 0+ US$0.10 . Publishedb y A.A.Balkema,RO.Bo x 1675,3000B RRotterdam ,Netherland s Fax: +31.10.4135947;E-mail :[email protected] ;Interne tsite :http://www.balkema.n l A.A.Balkem aPublishers ,Ol dPos tRoad ,Brookfield , VT05036-9704 ,US A Fax:802.276.3837 ;E-mail : [email protected] ISBN9 0541 0420 1 © 1999A .A .Balkema ,Rotterda m Printedi nth eNetherland s BIBUOTHEEK LANDBOUWUNIVERSTTHTT WAGENTNGEN Contents Acknowledgments Abstract Chapter 1 Introduction 1 Chapter 2 Description of the Nakivubo Swamp 15 Chapter 3 Water Balance and Hydrodynamics of the Nakivubo Swamp 39 Chapter 4 Water Quality of the Nakivubo Swamp 83 Chapter 5 Nutrient Transformations by Dominant Vegetation Types in the Nakivubo Swamp 105 Chapter 6 Transport and Fate of Faecal Coliforms in the Nakivubo Swamp 139 Chapter 7 Nitrification in the Nakivubo Swamp 169 Chapter 8 Phosphorus Retention in the Sediments of the Nakivubo Swamp 191 Chapter 9 Mass Balance of the Nakivubo Swamp 239 Chapter 10 Socio-economics of the Nakivubo Swamp 265 Chapter 11 Summary and Conclusions 283 Appendices A1 Curriculum Vitae Acknowledgment First we are very grateful to the Dutch Government whothroug h the International Institute for Infrastructural Hydraulic and Environmental Engineering (IHE) and Wageningen Agricultural University (WAU), in The Netherlands, financed this study, under the Ecotechnology Project. This project also involved Makerere University and Fisheries Research Institute (FIRI), both in Uganda. We are particularly indebted to our promoters: Professors Alaerts, G. J., Denny, P., and Lijklema, L. and Co-promoters: Dr. Bugenyi, F.W.B. and Dr. van Bruggen, J. J. A., for their constructive discussions and advices both during the Research work and write up of the thesis. We are very grateful to Aalderink, R.H. for his kind assistance and contribution throughout this research. Additional thanks got o Azza, N. G. T.; Berhane, M.; Kaarsemaker, S.; Omoit, S.; Oosthoek, J.; Otoi, S. P.; Sloot, J. and Van der Veen, M., who participated in the data collection. Our sincere thanks also go to Dr. de Ruyter, E and Dr. Tukahirwa, E. for coordinating the project activities in the Netherlands and Uganda respectively. We would also like to thank Schakenraad, R., Melis, F. and Personnel in their section for administrative arrangements. Thanks also go to Dr. Kiggundu, Department of Civil Engineering Makerere University for allowing us to use the Public Health Engineering Laboratory in his Department. At this juncture, we also thank members of the Laboratory who inon ewa y or another made our work a success. We extend further thanks to Vermaat, J. Hootsmans, M., Jacobs, A.A.J., (for statistical advice), Lubberding, H., Kelderman, P. and deLaat , P.J.M., who read some chapters of this thesis and made constructive suggestions and Dr. Hooijmans, T., for her efforts during the first part of the study. Wear e also grateful toth e Computer Groupan d Laboratory staff at IHE for their technical and moral support. We also acknowledge and thank our field assistants, who made sure that both we and our supervisors never did our research while 'swimming' under the swamp mat. Toth e residents of Bukasa Village and the wardens of Luzira Prison who never denied us access to the Nakivubo Swamp during data collection, we say thank you. Lastly, our heartfelt thanks go to our parents who have always prayed for our success. Abstract An investigation to assess the capacity of the Nakivubo swamp, Kampala-Uganda (which has been receiving partially treated sewage from the city for more than 30year s now), to remove nutrients and pathogens was carried out. The aim of the study wast o evaluate the potential of this swamp to remove nutrients and pathogens from wastewater in a sustainable way, with emphasis on describing and quantifying their pathways, transformations and budgets. From field studies, water balance terms of channel discharges, rainfall, subsurface flows, evapotranspiration and seiches were measured or calculated from existing hydrometeorological data to form a water balance. Nutrients (N and P) and faecal coliforms (FC) transformations in the swamp were studied from four transects cut acrossth e swamp. Vertical and longitudinal profiles of nutrients and pathogens were also constructed. Laboratory simulations were carried out to estimate nutrient fluxes into the plant and sediment compartments and to estimate the removal mechanisms of FC from the water column. In this study differences in the morphological, hydraulic, physico-chemical, floristic and overall wastewater treatment performance between areas covered by the two major vegetation types Cyperuspapyrus L . and Miscanthidium violaceum Robyns (about 80% and 20% of the study area, respectively) were elucidated. Papyrus is emergent at the swamp edges where the water level is more affected by the seasons (rainfall). It floats towards the centre and closer to the lake. The loose rhizomatous raft over which papyrus floats allows for fairly free fall- through of plant debris and decomposing matter onto the sediment via the water column resulting in high suspended solids content in the underlying water. This possibly slows, and sometimes restricts water flow in some areas. Due to the lower flows closer to the edges, a thick (up to 60 cm) layer of peaty material is also formed. The loose mat facilitates vertical mixing between the interstitial mat water and the water beneath the mat during the rise and fall of water/mat levels. This lead to a less steep gradient of nutrients over the vertical profile and facilitates nutrient uptake from the water column by papyrus vegetation. In comparison, Miscanthidiumvegetatio n is restricted to the middle of the swamp and is characterised by a thick (0.9 to 1.6 m) mat with highly interlaced roots, but low bulk density (60 - 300 kg/m3, surface to bottom). The thick mat helps the retention of falling plant debris on to its surface, where low rate decomposition and further mat accretion take place. The combination of material retention onto the mat surface and high water flows beneath results into a clearer water column and a very thin peat layer (maximum 10 cm) of poorly decomposed plant material. Further, thema t structure prevents free vertical and lateral mixing of the mat water with the water column beneath. This leads to reduced interactions of the plants with wastewater in these zones, and therefore less nutrient abstraction by plants from the wastewater in these zones. The average waste water discharge in the swamp was estimated at 103,575 m3/d. Water flow is highly channelised and hydraulic retention times in the swampdurin g the rainy periods may be as low as 18 hours. Seepage is negligible. Water quality variations within the swamp showed that wastewater is notevenl y transported to allpart s of the swamp as it flows through. The nutrient load into the swamp was 770 gN/m2/yr and 66 gP/m2/yr. Different nutrient uptake rates and plant tissue contents (N=1.3%, P=0.21% for papyrus and N=0.64% and P=0.15% for the Miscanthidium vegetated zones) plus the above structural differences in flows and retention times are partly responsible for the disparate purification efficiencies between the vegetation zones. In the papyrus vegetated zones, the average purification efficiencies were 67% N, and TP and 99.3% FC while in theMiscanthidium vegetated zones, it was lower at 55% N, 33% TP and 89.3% FC. The lower flows (about 20%) that went through thepapyru s vegetated zones enabled higher retention times for these zones. The major mechanisms of nutrient removal in papyrus vegetated zones were identified to beplan t uptake for the nutrients and attachment onto particulates followed by sedimentation, for FC and P. Predation and natural die-off of FC may be high especially in the root zones where micro- aerobic zones exist (mostly in papyrus zones). The thick mat of Miscanthidiumlimit s the number of live roots that can reach the water column to get nutrients from there. Since the bulk (80% near the lake) of the wastewater goes through this zone, then itmean s that theoveral l (swamp-wide) nutrient and pathogen removal efficiency from the wastewater is low (56% N, 40% TP and 91% FC). Very low levels of oxygen were observed in the Nakivubo swamp (and very infrequently) due to the high oxygen demand exerted by decomposing organic matter in the swamp.