I. Nhapi*, H.J. Gijzen** and M.A. Siebel** * Department of Civil Engineering, University of Zimbabwe, P.O. Box MP167, Mt. Pleasant, Harare, Zimbabwe (E-mail:
[email protected]) ** IHE Delft, The Netherlands Abstract The aim of this study was to formulate an integrated wastewater management model for Harare, Zimbabwe, based on current thinking. This implies that wastewater is treated/disposed of as close to the source of generation as possible. Resource recovery and reuse in a local thriving urban agriculture are integrated into this model. Intervention strategies were considered for controlling water, nitrogen and phosphorus flows to the lake. In the formulation of strategies, Harare was divided into five major operational areas of high-, medium-, and low-density residential areas, and also commercial and industrial areas. Specific options were then considered to suit landuse, development constraints and socio-economic status for each area, within the overall criteria of limiting nutrient inflows into the downstream Lake Chivero. Flexible and differential solutions were developed in relation to built environment, population density, composition of users, ownership, future environmental demands, and technical, environmental, hygienic, social and organisational factors. Options considered include source control by the users (residents, industries, etc.), using various strategies like implementation of toilets with source separation, and natural methods of wastewater treatment. Other possible strategies are invoking better behaviour through fees and information, incentives for cleaner production, and user responsibility through education, legislative changes and stricter controls over industry. Keywords Decentralised; onsite; reuse; strategies; sustainability; wastewater
Water Science and Technology Vol 47 No 7–8 pp 11–18 © IWA Publishing 2003
A conceptual framework for the sustainable management of wastewater in Harare, Zimbabwe
Introduction
Harare, the capital city of Zimbabwe, has systematically destroyed its major source of potable water supply, Lake Chivero. The lake is located about 35 km downstream of Harare as shown in Figure 1. It receives pollution from sewage effluent, agricultural, solid waste, industrial (un-sewered), and natural sources. Harare has a population of about 1,900,000 with the majority housed in 106,950 houses in 15 high-density suburbs. About 304,000 m3/d of sewage is produced in Harare with 43% of this directly discharged into the rivers after tertiary treatment (Nhapi et al., 2001c). The rest receives secondary treatment and is used for pasture irrigation. Overloading and numerous plant breakdowns often compromise the quality of effluent discharged to rivers. About 17,800 kg/d TN and 1,900 kg/d TP are generated and discharged to the two major sewage treatment works of Firle and Crowborough (Nhapi et al., 2001b). Urban agriculture is now a notable economic activity with about 16,000 hectares or 75% of the open spaces currently under cultivation. It is estimated that 2,800 and 480 metric tonnes of TN and TP respectively are used in Harare as commercial fertilisers mostly for maize production (Nhapi et al., 2001b). Some of the fertiliser applied eventually leaches into water courses. The central problem in Harare is that sewage effluent seems to be contributing significantly to eutrophication of Lake Chivero. With population increase, the Lake will increasingly receive a higher fraction of sewage effluent whilst raw water abstraction will also increase, further exacerbating the problem. Dry season nutrient levels in the Lake have been reported around 2.4 mg/l TN and 0.8 mg/l TP (Nhapi et al., 2001a). These are higher than the allowable limits of < 0.3 mg/l TN and < 0.01 mg/l TP for drinking water in lakes (JICA,
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I. Nhapi et al. Figure 1 Location of Harare in relation to L. Chivero and illustration of water recycling system
1996). High nutrient levels have led to excessive productivity and periodic fish kills due to ammonium toxicity and low dissolved oxygen levels. The primary objective of sustainable wastewater management in Harare should be to urgently limit the amount of nitrogen and phosphorus reaching L. Chivero. This paper focuses on innovative strategies for this. Basis and formulation of strategies
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Sustainable wastewater management is management that meets current needs without compromising the ability of future generations to meet their own needs. This now requires a move beyond conventional approaches towards new forms and concepts of management. The Dublin Statement and Agenda 21 represent a starting point in the search for more innovative and sustainable approaches to managing wastewater and the environment. In a follow-up meeting at Bellagio, Italy, from 1–4 February 2000, an expert group brought together by the Environmental Sanitation Working Group of the Water Supply and Sanitation Collaborative Council agreed that current waste management policies are abusive to human well-being, economically unaffordable, and environmentally unsustainable. Specifically, the Belagio Statement (Water 21, April 2000) raised the following issues that are of interest to our research: • Export of wastes from the source of generation should be minimised to promote efficiency and reduce the spread of pollution. • Wastewater should be recycled and added to the water budget. • Waste should be managed as close as possible to its source. To “operationalise” the above issues, we developed Figure 2 to conceptualise an ideal wastewater management scheme for Harare. This figure consists of four quadrants dealing with housing density, the appropriate treatment level, applicable technologies at each level, and possible effluent destinations. Residential areas were categorised into high density, medium density and low-density areas. Respective residential stand areas are ±300 m2, ±1,000 m2 and >2,000 m2. Three levels of treatment were considered, namely onsite, decentralised and centralised treatment. These are represented by the three circles. The approach starts at the onsite or property level and tries to reduce water consumption and nutrient release from the site. What cannot be handled at this level is allowed to spill over to the next level – decentralised. The decentralised level deals with groups of dispersed communities and new developments that can be treated as an entity for the purpose of wastewater management. For effective disposal and reuse options, it is very important that only a small and manageable fraction of nutrients is allowed to reach central level treatment. This results in offsite/central treatment performance being less challenged in terms of water volumes and nutrient loads handled than at present. This leads to reduced discharge of
I. Nhapi et al.
nitrogen and phosphorus to water bodies. Figure 2 also introduces technologies that can be feasibly applied at each level based on locally used systems. The selection of technologies in our scheme should aim at recovering nutrients for reuse in urban agriculture. In the developing world context, technologies should be low-cost oriented, less mechanised, and easily adaptable to local conditions. Socio-economic conditions would also eliminate some possible reuse options. However, some behavioural aspects can be changed over time through aggressive educational campaigns. From Figure 2 we derived a hierarchical “4-step strategic approach” to wastewater management. This starts with (1) pollution prevention (cleaner production approaches) at the point of generation, (2) direct reuse of waste components (water, biogas, nutrients), (3) treatment or conversion of sewage to environmentally safe and usable products (e.g. duckweed), and (4) disposal or dispersion into the environment with stimulation of selfpurification. The implementation should follow this order of hierarchy. Under each step there are several intervention options and these will be discussed in detail in this paper. Residential water consumption in Harare is very high resulting in average unit wastewater production rates of 63 lcd for high density, 210 lcd for medium, and 315 lcd for low density stands. The respective occupancy rates are 7, 11, and 13 persons per stand (JICA, 1996). The inevitable result of this is the design of large treatment structures to accommodate the respective high hydraulic loads and high pumping and aeration costs. This wasting of resources should be strictly discouraged at the property level by drastically reducing water consumption; principally the amount that goes into the sewer system. Wherever possible, the sewer system should be avoided as it merely conveys a problem from one point to another. The current treatment works also receive an assortment of pollutants that cannot be removed by normal sewage treatment processes. In our scheme, we propose the enactment and enforcement of stricter legislation banning the use of products that contaminate wastewater. Toxic elements will hinder effective biological treatment and work against our strategy of using low-cost natural treatment combined with effluent reuse in agriculture. The implementation of our strategies requires changes in the institutional aspects of wastewater management. Strategies at onsite level
It is at the onsite level that real control of what goes in and out can be best achieved. Appropriate options should avoid groundwater contamination as well as the indirect polluEffluent Destination Irrigation River
Housing Density
High Density
Irrigation Aquaculture Poultry Medium Density Onsite irrigation, use control, regulation
Onsite: source separation, combined
Low-cost: Wetlands, Waste Stabilisation, Duckweed Ponds Tertiary: BNR
Technologies
Low Density
Property Level
Spill over to next level
Decentralised
Centralised
Treatment Level
Figure 2 Theoretical framework for the development of sustainable strategies
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tion of lakes and rivers from failing systems. Based on space requirements, we propose onsite systems for individual low-density residential properties, industries, and a small grouping of properties like flats, institutional systems, and enclosed housing complexes. We consider them inappropriate for high density areas because of space limitations in view of the considerable quantities of wastewater produced per stand. Harare low-density areas currently have a population of about 137,000 people, producing about 43,200 m3/d sewage, 1,780 kg/d TN and 190 kg/d TP – about 9% of the total domestic nutrient load (Nhapi et al., 2002). Possible onsite options are; cleaner production approaches, grey/blackwater separation, urine separation, and combined wastewater. Suitable options should take into account the final destination of effluent, linking reuse to treatment level, density of development, and maintenance of present hygienic standards. Cleaner production approaches
Cleaner production approaches are the best way to tackle current high sewage production from all areas: residential, institutional, commercial and industrial. Current figures have serious implications on the sizing of wastewater treatment plants and their efficiency and results in wasting of both resources and energy. Intervention should therefore start by controlling consumption. We should ask ourselves whether some of the things we use are really necessary and consider substitutes if there is a danger to the environment. The other question is whether we should really consume at current levels of inefficiency, e.g. use water, nitrogen and phosphorus only once? Cleaner production involves not only technology, but also planning, good housekeeping, and implementation of environmentally sound management practices. The “polluter pays” concept and discharge limitations are some of the instruments used to control use practices. Industries can also be compelled by legislation to strictly treat and reuse wastewater within their properties, wherever possible, and thus limit discharges to public sewers and streams. Greywater separation and treatment
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After reducing the amount of water and nutrient produced as wastewater the next step is direct reuse of greywater or the use of it after basic treatment. We define greywater as household wastewater minus blackwater (faeces and urine). The collection of greywater is easier in Zimbabwe as the plumbing system already directs it into a trap gully that filters out large suspended matter. Our conceptualisation of the greywater treatment system is shown in Figure 3. Our calculations show that out of the 167,500 m3/d domestic sewage produced in Harare, about 86,800 m3/d (52%) is actually greywater that can be reused at property level. Blackwater contains most of the nutrients in the wastewater (about 90% TN and 50% TP) and can be treated separately and used as fertiliser (Lindstrom, 1998). It can also contain more than 50% of the BOD5 in household sewage (DLG, 1998). Faecal coliforms are low, while viruses are usually not found unless someone in the home has an infection. The content of heavy metals and organic micro-pollutants should also be low (domestic) and can be easily controlled at this level. If necessary, greywater can be treated in a simple way, close to the home and reused within the property boundary, especially for watering gardens, car washing, and toilet flushing. After greywater separation, the toilet waste also needs to be handled at the property level or discharged into the municipal sewer system. Septic tanks are a usual option but the faecal sludge (FS) produced still have to be dealt with. In some countries FS is disposed of untreated mainly due to lack of treatment options adapted to the socio-economic conditions of developing and newly industrialised countries. For effective treatment of FS, the toilet waste can be reduced by using water-saving cisterns. In Dzivaresekwa Extension, a peri-urban area of Harare, faecal sludge has been composted for three months with ash and
Kitchen Greywater Treatment
Bath
Laundry
+ Other
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Onsite Residential
Gardening Treatment ? WC
Urine
Faeces
Figure 3 Schematic presentation of an onsite greywater treatment and reuse system
soil and later used as a soil conditioner. The composting kills pathogens and also keeps groundwater from being polluted by nitrates. Urine separation
Urine separation has been practised in peri-urban areas of Harare and offers an attractive option for the sustainable management of wastewater. It should be recognised that nutrient control is only necessary due to the presence of urine in wastewater (Larsen and Udert, 1999). It can be adopted for high- to low-density areas provided more decent structures are developed than the simple ones currently in use at Dzivaresekwa Extension, Harare. These are home-made from plastic containers and a short piece of hosepipe. Each person produces about 500 litres of urine and 50 litres of faeces per year (Winbald, 1996). To flush away 550 litres of faeces and urine requires about 15 000 litres of purified water every year. If urine and faeces are kept apart problems of odours and fly-breeding are eliminated, facilitating storage, treatment and transport. Appropriate handling and treatment methods for urine need further development. Faeces can be subjected to primary treatment, basically dehydration, which also effectively destroys most of the pathogenic organisms. In Harare, faeces are currently composted with ash and earth before use as manure. Human urine is estimated to contribute about 80% to the nitrogen and 50–60% to the phosphorus found in ordinary household wastewater although it forms 1% of the volume (Jönsson et al., 1998). This implies that urine separation for medium and low density areas of Harare would potentially recover 3,000 kg/d TN and 223 kg/d TP; representing 15% and 10% respectively of the domestic N and P loads in Harare. Combined handling of wastewater
Septic tanks and leach fields are generally used for the combined handling of wastewater. The basic (design) problem with current conventional septic systems is that they introduce nutrients and microbes too deeply into the ground for any of the natural processes of decomposition and plant uptake to take place (Lindstrom, 1998). Preferably, properly designed septic tanks should form the first part of a treatment train aimed at reusing the nutrients after stabilising the organic part. Plant uptake should be the last and crucial stage in the recycling of these nutrients. Constructed wetlands are also being used for treating wastewater at property level in Zimbabwe. They have been used in conjunction with septic tanks for compound dwellings and polishing brewery effluent in industrial water recycling. 15
Strategies at community or decentralised level
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The middle circle in Figure 2 represents a decentralised system of wastewater management, particularly for new housing developments and medium density areas. The decentralised concept aims at providing a framework for producing alternative wastewater systems which are more financially affordable, more socially responsible, and more environmentally benign than conventional practice. Many considerations would determine how close to the source of generation it is practical to address treatment and disposal. One of these is if and how the wastewater could be reused in a beneficial manner. Other considerations include topography, soil conditions, development density, and type of land use. To minimise the operations and maintenance liabilities of this strategy requires technologies that are appropriate to the volume of flow, the nature of the development served, the nature of the reuse opportunities, limitations on disposal options, etc. A schematic illustration of a conceptual decentralised scheme is given in Figure 4 below. It is possible to use combined or separated wastewater streams in this scheme. This concept can best be applied for new developments, minimising the costly construction of trunk sewers and pumping systems. The local authority can require that any new subdivision include a separate sewage treatment and disposal system. Suitable local technologies that can be used in this manner in Zimbabwe are waste stabilisation ponds, constructed wetlands and duckweed-based pond systems. Strategies at a central level
Wastewater in Harare is generally treated in centralised systems with Crowborough (103,000 m3/d) and Firle (171,000 m3/d) being the main treatment plants. At a central level, natural systems (waste stabilisation ponds, wetlands, aquatic weeds, land treatment systems) and technology systems (artificial input of oxygen) can be used. Natural systems are advantageous for developing countries because they do not need skilled manpower or sophisticated imported equipment. They, however, require large areas of land of about 1–2 m2/PE (Veenstra, 1992). Secondary effluent can be used for commercial irrigation and biogas collected whilst tertiary effluent is discharged into rivers. Additional disinfection will be required for technology systems to meet WHO pathogen standards for safe reuse (Khouri et al., 1994). A conceptual offsite scheme is shown in Figure 5 below. In the proWater m 3/d Residential Areas Combined/separated
Natural Treatment CW, WSP, DPS
Protein biomass
Anaerobic Treatment UASB, Anaerobic Pond, Septic Tanks
Biogas Sludge
Disinfection?
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Disposal Options Irrigation; pastures, golf courses, plantations
Disinfection?
Figure 4 Schematic presentation of an intermediate or decentralised system (CW = constructed wetlands; WSP = waste stabilisation ponds; DPS = duckweed-based pond systems)
posed scheme (Figures 2 and 5), less hydraulic and nutrient loads would be allowed to reach central level treatment. This will be mostly commercial effluents which currently constitutes about 20% flow, 23% TN, and 21% TP from the current wastewater loads. Technology selection and potential benefits I. Nhapi et al.
The strategies proposed in this paper have serious implications on the current institutional setup. It will result in much more cumbersome management requirements than at present, necessitating well-organised and responsive institutions. Individual plot owners can manage onsite systems provided there are economic gains in the cost of treatment and income from reuse. Private developers can participate at a decentralised level with incentives coming from the reduction of conveyance and treatment costs if appropriate technologies are selected. Economic benefits derived from reusing effluent water for commercial irrigation would also attract private investors, especially those involved in peri-urban farming. Technology selection, with emphasis on costs, recovery and profitable reuse of nutrients, is very important in the implementation of our strategies. We recommend technologies that achieve the above criteria and are backed by local expertise as a starting point. The results from implementing different options and strategies are summarised in Table 1. In Table 1 the following treatment efficiencies for TN and TP removal were assumed: WSP (45% TN, 45% TP); BNR systems (90%, 90%); trickling filters (35%, 30%). The results show a potential reduction in water consumption and nutrient discharges. These results would favour systems like urine separation, duckweed-based pond systems (DPS) and BNR systems. However, the use of the latter should only be resorted to where reuse options are not economically attractive because they use energy to destroy valuable nutrients. Possible constraints are land availability, manpower and electricity costs, imported parts, and sludge handling. Commercial and industrial loads are favoured for central systems. Different approaches and technologies would apply to specific areas. The best way to achieve optimal results is by integrating the different approaches. Conclusions and recommendations
The water pollution problems being experienced in Harare and its downstream water supply source of Lake Chivero can be solved by a sustainable integrated approach to wastewater management as demonstrated in this paper. A more revolutionary and innovative approach is required. This approach starts at onsite level, progressively reducing pollution loads at different levels of aggregation. Reuse needs to be optimised at each step. Further work is required to model pollution paths and reductions in order to quantify the actual impact on downstream water quality. W a ter m 3 /d
R esid en tial + In d u strial + C om m ercial A reas (E n forced legislation + d em an d m an agem en t)
A n aerob ic P re-treatm en t
B iogas
L an d fill
A q u acu ltu re H orticu ltu re
S lu d g e
S econ d a ry T reatm en t
T ertiary T reatm en t L ak e C h ivero
S torage or n atu ral d isin fection (p on d s)
W ater P u rification
S ch ools, O p en sp aces, P laygroun d s, G olf cou rses
C om m ercial irrigation
Figure 5 Schematic presentation of a centralised treatment strategy
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Table 1 Potential impact of strategies on nutrient outflows System
Pop.
Flows handled per
Nutrient load
Loads reaching
housing category, m3/d
handled, kg/d
rivers, kg/d TN
High
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Onsite systems Greywater separation 301,000 Blackwater separation 301,000 Urine separation 1,786,000 Combined onsite 137,000 Waste minimisation * Industry, 49,324 m3/d * Commercial, 60,284 m3/d Decentralised Duckweed ponds 164,000 Constructed wetlands 164,000 Centralised systems WSP 1,786,000 BNR Trickling filters
1,786,000 1,786,000
Med.
Low
TN
TP
17,240 13,550
18,130 25,030
380 3,380 16,270 1,780
200 1,830 550 190
1,690
530
6,940
760
1,970 1,970
210 210
43,160
30,780 30,780 43,160
30,780
93,560
28,720
3,470
43,160 43,160
30,780 30,780
93,560 93,560
28,720 28,720
3,470 3,470
TP
Reused on site Reused on site Reused on site Reused on site Centrally treated Centrally treated Local irrigation Local irrigation Commercial irrigation 2,870 350 Commercial irrigation
Acknowledgements
The authors would like to thank the Netherlands Government for funding this research through the WREM Project at the University of Zimbabwe. We also acknowledge the University of Zimbabwe, IHE Delft, and City of Harare staff who assisted in various ways in the preparation of this report. References
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JICA Report (1996). The Study of Water Pollution Control in Upper Manyame River Basin in the Republic of Zimbabwe, MLGRUD, Nippon Jogeduido Sekkei Co. Ltd., Nippon Koei Co. Ltd, September. Jönsson, H., Dalemo, M., Sonesson, V. and Vinnerås, B. (1998). Modelling the Sewage System – Evaluating Urine Separation as a Complimentary Function to the Conventional Sewage System, Paper presented at “Systems Engineering Models for Waste Management” International Workshop in Göteburg, Sweden, 25–26 February. Khouri, N., Kalbermatten, J.M. and Bartone, C.R. (1994). Reuse of Wastewater in Agriculture: A Guide for Planners, UNB/WB Water and Sanitation Program, Washington, USA. Larsen, T.A. and Udert, K.M. (1999). Urine Separation – A way of Closing the Nutrient Cycles, Internet Documnt: http://www.blackwell.de/journale/wabo/9911inhalt.html Lindstrom, C.R. (1998). Greywater, Cambridge, Internet site: http://greywater.com Nhapi, I., Siebel, M.A. and Gijzen, H.G. (2001a). Dry Season Inflow and Export of Nutrients from Lake Chivero in Year 2000; in Proceedings of the Zimbabwe Institution of Engineers, Vol. 2(1), pp. 33–41. Nhapi, I., Siebel, M.A. and Gijzen, H.G. (2001b). Sewage Contribution to the Pollution of Lake Chivero: Implications for Nutrient Management; paper presented at the Annual European Geophysical Society Conference in Nice, France, March. Nhapi, I., Hoko, Z., Siebel, M.A. and Gijzen, H.G. (2001c). Assessment of major water and nutrient flows in the Chivero catchment area, Zimbabwe, paper presented at the 2nd Waternet/WARFSA Symposium in Cape Town, South Africa, October 29–31. Nhapi, I., Siebel, M.A. and Gijzen, H.G. (2002). An Inventory of Wastewater Management Systems in Harare, Zimbabwe, paper in preparation. Otterpohl, R., Grottker, M. and Lange, J. (1997). Sustainable Water and Wastewater Management in Urban Areas, in IAWQ Wat. Sci. Tech., Vol. 35(9). Veenstra, S. (1992). Wastewater Treatment Technologies and their Potentials for Reuse, in Al-Layla, M.A. and Veenstra (Eds.) Proceedings of the National Seminar on Wastewater Reuse, May 9 –11, Sana’a University, Yemen. Winbald, U. (1996). Towards an Ecological Approach to Sanitation, Speech delivered at the International Toilet Symposium, Tohama, Japan, October.
