Wastewater management in Zimbabwe

Wastewater management in Zimbabw

Occurrence of cyanobacteria genera in the Vaal Dam: implications for potable water production

The occurrence of cyanobacteria genera in the Vaal Dam was analysed and the factors that influence its dominance in the particular reservoir were also investigated. The study was motivated by the effects of the secondary metabolites of cyanobacteria genera on potable water production. Cyanobacteria genera have been found to be potentially toxic and capable of producing taste and odour secondary metabolites such as geosmin. Historical data from the Department of Water and Sanitation on percentage composition of cyanobacteria genera in the Vaal Dam, were collected for the 2006, 2007, 2011 and 2012 years. The concentrations of NO2-N and NO3-N and total phosphorus as well as water temperature data were collected from the same sampling point for the study period. This data, together with weather data, was statistically analysed for trends and relatedness between variables. It was found that Microcystis and Anabaena were the dominant cyanobacteria genera in Vaal Dam and they were jointly dominant over other phytoplankton genera during February and March. It was also found that the dominance of cyanobacteria genera significantly correlated with air and water temperature and concentration of NO2-N and NO3-N. It was concluded that the dominance of Microcystis and Anabaena genera among the cyanobacteria genera has significant implications for potable water production as the genera are associated with taste and odour metabolites and toxins. It was recommended that depth profiling be employed in order to identify an abstraction depth in the multilevel intake of the reservoir with relatively low levels of cyanobacteria cells. This would assist in minimising taste and odour events in potable water production.Keywords: Anabaena, cyanobacteria dominance, environmental conditions, Microcystis, Vaal Dam, water treatment

Potential for the use of duckweed-based pond systems in Zimbabwe

Duckweed systems are a form of natural wastewater treatment method that is ideal for developing countries. They demand less in terms of financial resources for construction and maintenance, manpower sophistication, electricity requirements, and machinery. This paper looks at the duckweed technology as a new phenomenon in Zimbabwe, reviews its requirements and problems, and finally explores its potential in the Zimbabwean environment. A simple spreadsheet model was developed to assess a water and nutrient balance of an ideal duckweed system. It was concluded that under ideal or optimum operating conditions, duckweed systems could achieve the required Zimbabwean nutrient standards of 10 mg.l-1 total nitrogen and 1 mg.l-1 total phosphorus. Duckweed systems would suit areas of moderate to high water consumption to avoid toxicity problems and also to increase the surface area available for duckweed growth. It was recommended that further experiments be carried out locally to improve and validate the model developed and used in this paper. Water SA Vol.30(1): 115-12

Options for wastewater management in Harare, Zimbabwe; Proefschrift Wageningen Universiteit.

The sustainable management of wastewater should aim at pollution prevention and reduction first, followed by resource recovery and reuse. This thesis shows that substantial water quality improvements could be achieved through a so-called 3-Step Strategic Approach to wastewater management. This thesis focused on Harare, in the Lake Chivero catchment area of Zimbabwe. A water quality monitoring study established that the current situation leads to water quality deterioration in the lake and is wasteful in terms of water use. Using the 3-Step Approach, short-term, medium-term, and long-term solutions were developed. The short-term solutions include pollution prevention and the reduction of wastewater volume. The medium-term solutions involve treating wastewater to high standards, introducing or improving resource recovery and reuse options, and controlling upstream point and non-point sources of pollution. A long-term solution was suggested based on pollution prevention and direct reuse, treatment at onsite and decentralised levels followed by reuse, and disposal of the remaining effluents combined with stimulation of the self-purification capacity of the receiving rivers. The long-term measures would aim at sustaining acceptable lake water quality under increased urbanisation conditions. The frameworks for managing wastewater at onsite, decentralised, and centralised levels developed in this research can be used in re-designing current systems or designing new systems that make optimal use of wastewater components whilst minimising pollution to the environment

A 3-step strategic approach to sustainable wastewater managementage/gi

Many cities in developing countries are facing surface water and groundwater pollution problems. This deterioration of water resources needs to be controlled through effective and feasible concepts of urban water management. The Dublin Principles, Agenda21, Vision21, and the Millennium Development Goals provide the basis for the development of innovative, holistic, and sustainable approaches. Whilst highly efficient technologies are available, the infusion of these into a well-thought out and systematic approach is critical for the sustainable management of nutrient flows and other pollutants into and out of cities. Based on cleaner production principles, three intervention steps are proposed in this paper. The first step is to minimise wastewater generation by drastically reducing water consumption and waste generation. The second step is the treatment and optimal reuse of nutrients and water at the smallest possible level, like at the on-plot and community levels. Treatment technologies recommended make the best use of side products via reuse. Once the first two intervention steps have been employed to the maximum, the remaining waste flows could be safely discharged into the environment. The third step involves enhancing the self-purification capacity of receiving water-bodies (lakes, rivers, etc.), through intervention. The success of this so-called 3-step strategic approach requires systematic implementation, providing specific solutions to specific situations. This, in turn, requires appropriate planning, legal and institutional responses. In fact, the 3-step approach could be applied as an overall approach for waste management, although here the focus is on sewage. This paper offers examples under each step, showing that the systematic application of this approach could lead to cost savings and sustainability. Key words: cleaner production, nutrients reuse, 3-step strategic approach, sustainable approaches, urban water cycle, wastewater management Water SA Vol.31(1) 2005: 133-14

The potential and constraints of replacing conventional chemical coagulants with natural plant extracts in water and wastewater treatment

Coagulation is a vital stage in treating contaminated water using coagulants that can be either synthetic or natural. Currently, water is being treated by water treatment plants that use aluminum- or iron-based salts and synthetic polymers for coagulation. However, these synthetic coagulants have flaws, including the production of large amounts of chemical sludge, considerable effects on the pH of the purified water, relatively expensive to use, and potentially toxic effects on the environment. The potential and constraints of replacing chemical coagulants with natural plant extracts for water treatment are reviewed in this paper. Various natural coagulants previously investigated for the removal of heavy metals, turbidity, pathogens, and other contaminants from surface and synthetic water were analyzed based on dosage rates, active ingredients responsible for coagulation, and extraction methods among other criteria. A comparative analysis was performed in studies in which both conventional chemical coagulants and natural plant extracts were used for water treatment. Various studies have shown that it is possible to replace chemical coagulants with plant-based extracts. It has been demonstrated that it is possible to replace chemical coagulants with natural plant-based extracts because seed extracts such as those of fenugreek can achieve turbidity removal efficiencies as high as 98% as compared to 85% for alum. The use of natural coagulants may be beneficial for water treatment plants because they may produce less sludge than chemical coagulants; this increases environmental sustainability while decreasing the cost of handling sludge. They are less toxic than chemical coagulants and do not pose any adverse threats to the environment; therefore, they are a safer alternative. However, studies are needed on how these plant-based coagulants can be commercialized in the same way that chemical coagulants are readily available

Sustainable decentralized sanitation using duckweed-based ponds

This paper focuses on the performance and operational problems at two full-scale duckweed-based systems in the Masvingo province of Zimbabwe

Hydrological Impacts of Urbanization of Two Catchments in Harare, Zimbabwe

By increased rural-urban migration in many African countries, the assessment of changes in catchment hydrologic responses due to urbanization is critical for water resource planning and management. This paper assesses hydrological impacts of urbanization on two medium-sized Zimbabwean catchments (Mukuvisi and Marimba) for which changes in land cover by urbanization were determined through Landsat Thematic Mapper (TM) images for the years 1986, 1994 and 2008. Impact assessments were done through hydrological modeling by a topographically driven rainfall-runoff model (TOPMODEL). A satellite remote sensing based ASTER 30 metre Digital Elevation Model (DEM) was used to compute the Topographic Index distribution, which is a key input to the model. Results of land cover classification indicated that urban areas increased by more than 600 % in the Mukuvisi catchment and by more than 200 % in the Marimba catchment between 1986 and 2008. Woodlands decreased by more than 40% with a greater decrease in Marimba than Mukuvisi catchment. Simulations using TOPMODEL in Marimba and Mukuvisi catchments indicated streamflow increases of 84.8 % and 73.6 %, respectively, from 1980 to 2010. These increases coincided with decreases in woodlands and increases in urban areas for the same period. The use of satellite remote sensing data to observe urbanization trends in semi-arid catchments and to represent catchment land surface characteristics proved to be effective for rainfall-runoff modeling. Findings of this study are of relevance for many African cities, which are experiencing rapid urbanization but often lack planning and design