Scielo RSS <![CDATA[Water SA]]> vol. 34 num. 3 lang. en <![CDATA[SciELO Logo]]> <![CDATA[<b>A mathematical technique for the design of near-zero-effluent batch processes</b>]]> Wastewater minimisation in chemical processes has always been the privilege of continuous rather than batch plants. However, this situation is steadily changing, since batch plants have a tendency to generate much more toxic effluent compared to their continuous counterparts which are usually encountered in bulk manufacturing. Past methodologies for wastewater minimisation in batch processes have focused on operations based on mass transfer. They do not take into consideration the reuse of wastewater as part of product formulation. Reusing wastewater in product formulation has the major advantage of negating much of the effluent produced, thereby enabling a process to operate in an almost zero-effluent manner. Presented in this paper is a mathematical technique for the simultaneous design and scheduling of batch operations operating in a near-zero-effluent manner. The technique determines the number and size of the processing vessels, while ensuring maximum water reuse in product. The technique was applied to an illustrative example, and an 80% savings in wastewater was achieved, with a corresponding plant design that achieves the required production. <![CDATA[<b>Scour from the interstitial spaces in cobble-bed rivers</b>]]> The periodic removal of sand from the interstitial spaces between cobbles is extremely important for ecosystem functioning in cobble-bed rivers. In order to flush fine sediments from the interstitial spaces between cobbles in river reaches downstream of dams, specific dam releases known as flushing flows or sediment maintenance flows are utilised. This paper describes the development and calibration of a mathematical model to predict the equilibrium depth of scour of fine sands from between cobbles in terms of applied stream power principles. The model was developed with the aid of physical model experiments and is founded on a stream power model which defines the condition of dynamic equilibrium in a deformed sand-bed river. Calibration was done in the laboratory under clear water conditions and with uniform cobble sizes. The scouring of fine sands in cobble-bed rivers is associated with an increase in absolute bed roughness and an associated decrease in the unit stream power applied along the bed as the cobbles become exposed. When scour ceases, the sand particles on the bed are at the movement threshold and critical conditions exist. In order to establish the relationship between equilibrium scour depth and bed particle characteristics, the power which is required to suspend sand particles under laminar boundary conditions is equated with the turbulent power being applied along the bed. <![CDATA[<b>Water erosion prediction at a national scale for South Africa</b>]]> Erosion is a major soil degradation problem in South Africa, confronting both land and water resource management throughout the country. Given the increasing threat of soil erosion, a need to improve techniques of estimating the soil-erosion risk at a national scale was identified by the National Department of Agriculture and forms the basic premise of this study. Principles and components of the Revised Universal Soil Loss Equation are applied here since the model combines sufficient simplicity for application on a national scale with a comprehensive incorporation of the main soil-erosion factors. Indicators of erosion susceptibility of the physical environment, including climate erosivity, soil erodibility and topography were improved over earlier assessments by feeding current available data into advanced algorithms. Two maps are presented: an actual erosion-risk distribution, and a potential erosion-risk map that excludes the vegetation cover factor. Actual soil-erosion risk, which relates to the current risk of erosion under contemporary vegetation and land-use conditions, was accounted for by regression equations between vegetation cover and MODIS-derived spectral index. The area of land with a moderate to severe potential risk is found to total approximately 61 m. ha (50%). Although more than 91 m. (75%) are classified as having only a very low to low actual risk, approximately 26 m. ha (20%) of land is eroded at a rate greater than a soil-loss tolerance of 10 t/ha-yr, showing the potential to target erosion control to problem areas. The Eastern Cape, Limpopo and KwaZulu-Natal Provinces have the highest erosion potential. Comparison of potential and actual erosion risk indicates that over 26 m. ha (>30% of national land) could be subject to high erosion risk without maintenance or careful management of the current vegetation cover and land use. Although the distribution of the actual erosion risk broadly follows that outlined previously, this study provides an advance on previous assessments of erosion; results are validated more comprehensively than before, and show an overall accuracy of 77%. The paper also describes many of the limitations inherent in regional erosion studies. <![CDATA[<b>Development of a framework for an integrated time-varying agrohydrological forecast system for Southern Africa: Initial results for seasonal forecasts</b>]]> Uncertainty about hydro-climatic conditions in the immediate future (today), as well as the near (up to one week) and more distant futures (up to one season) remains a fundamental problem challenging decision makers in the fields such as water resources, agriculture, and many other water-sensitive sectors in Southern Africa. Currently many institutions, such as the SA Weather Service, provide weather and climate forecasts with lead times ranging from 1 d to one season. However, disconnects exist between the weather/climate forecasts and their links to agrohydrological models, and in the applications of forecast information for targeted agricultural and water-related decision-making. The skills level of the current weather and climate forecasts, and the mismatch in scales between the output from weather/climate models and the spatial scales at which hydrological models are applied, as well as the format of seasonal forecasts in that they cannot be used directly in agrohydrological models, are some of the problems identified in this study and are being addressed. This has necessitated the development of a GIS-based framework in which the 'translation' of weather and climate forecasts into more tangible agrohydrological forecasts such as streamflows, reservoir levels or crop yields is facilitated for all the inter-linked quaternary catchments for enhanced economic, environmental and societal decision making over South Africa in general, and in selected catchments in particular. For monthly and seasonal (i.e. 3-month lead time) forecasts, two methods, viz. the Historical Sequence Method and the Ensemble Re-Ordering Method have been developed to translate the triplet of forecast rainfall probabilities (i.e. above, near and below normal) into daily quantitative values of rainfall for use in hydrological models. The first method was applied together with the daily time step ACRU Model to simulate seasonal flow forecasts in the Mgeni catchment in KwaZulu-Natal, South Africa. In taking account of uncertainty in the seasonal rainfall forecasts through the process of translating these to daily streamflow simulations by the ACRU Model, some skilful initial forecasts of streamflows can be obtained which can assist decision makers to take protective action against the impacts of hydro-climatic variability. <![CDATA[<b>The development and assessment of a regionalised daily rainfall disaggregation model for South Africa</b>]]> The temporal distribution of rainfall, viz. the distribution of rainfall intensity during a storm, is an important factor affecting the timing and magnitude of peak flow from a catchment and hence the flood-generating potential of rainfall events. Rainfall intensity is also one of the primary inputs into hydrological models used for the design of hydraulic structures. In the absence of continuously recorded rainfall data, one method of estimating the temporal distribution of rainfall is to disaggregate coarser-scale data into a finer resolution, e.g. from daily data into hourly rainfall information. In this study, a daily to hourly disaggregation model developed in Australia, and modified for application in South Africa, is used. However, this model requires input obtained from short-duration data at the desired location. Owing to the paucity of short-duration data in South Africa, the methodology is regionalised to enable the application of the model at a national scale, particularly at locations where only daily data are available. The regionalised model was independently tested at 15 locations in differing climatic regions in South Africa. At each location, observed hourly data were aggregated to yield daily values and were then disaggregated using the methodology. Results show that the regionalised model is capable of replicating the results obtained when 'at-site' short duration rainfall data are used as input to the disaggregation model, and is able to retain the daily totals and the statistical characteristics of the hourly rainfall. <![CDATA[<b>Implementation of the TOPKAPI model in South Africa: Initial results from the Liebenbergsvlei catchment</b>]]> Flash floods and droughts are of major concern in Southern Africa. Hydrologists and engineers have to assist decision makers to address the issue of forecasting and monitoring extreme events. For these purposes, hydrological models are useful tools to: • Identify the dominant hydrological processes which influence the water balance and result in conditions of extreme water excess and/or deficit • Assist in generating both short- and long-term hydrological forecasts for use by water resource managers. In this study the physically-based and fully distributed hydrological TOPKAPI model (Liu and Todini, 2002),which has already been successfully applied in several countries in the world (Liu and Todini, 2002; Bartholomes and Todini, 2005; Liu et al., 2005; Martina et al., 2006), is applied in Africa for the first time. This paper contains the main theoretical and numerical components that have been integrated by the authors to model code and presents details of the application of the model in the Liebenbergsvlei catchment (4 625 km²) in South Africa. The physical basis of the equations, the fine-scale representation of the spatial catchment features, the parsimonious parameterisation linked to field/catchment information, the good computation time performance, the modularity of the processes, the ease of use and finally the good results obtained in modelling the river discharges of Liebenbergsvlei catchment, make the TOPKAPI model a promising tool for hydrological modelling of catchments in South Africa. <![CDATA[<b>Hydrolytic enzymes in sewage sludge treatment: A mini-review</b>]]> Biological wastewater treatment processes can be classified as either aerobic or anaerobic. These two biological treatment processes are each characterised by groups of micro-organisms and their associated enzymes. Hydrolytic enzymes secreted by these micro-organisms are vital for the rate-limiting step of hydrolysis in the treatment of highly polymeric substrates present in sewage sludge. In this mini-review, the effects of mass transfer limitation, metabolic intermediates, extracellular polymeric substances (EPS), electron acceptor conditions and pH and temperature on the activity of these enzymes are summarised. The most salient and current perspectives of the significance and the role that hydrolytic enzymes play in sewage sludge treatment are highlighted. <![CDATA[<b>Long-term emissions from mechanically biologically treated waste: Influence on leachate quality - Part II</b>]]> Mechanical biological pretreatment of waste prior to disposal is proven to effectively reduce the long-term polluting potentials of landfilled waste. The combined effect of waste pretreatment and flushing, as is possible in landfills operated in tropical or sub-tropical countries, has the potential to further reduce the landfills' environmental impact. In this study, long-term emissions from pretreated waste were monitored in anaerobic leaching columns operated at increasing liquid-to-solid ratios. The efficiency of the pretreatment, conducted in full-scale passively aerated windrows, was assessed by comparing different treatment periods (8 and 16 weeks). In order to understand the influence of sorting (separated collection) on the pretreatment, the treated waste was sieved in a 50mm diameter sieve and the coarse and fine fractions separately analysed in the leaching columns. The results showed that treating the waste markedly reduces the COD and NH3-N loadings while the coarse fractions show a greater long-term pollutant risk. <![CDATA[<b>Oestrogenicity and chemical target analysis of water from small-sized industries in Pretoria, South Africa</b>]]> Increasing concern about endocrine disrupting chemicals (EDCs) and their effects on humans, animals and the environment resulted in this study being conducted. Water from 7 sites in the Pretoria West area (South Africa), with significant numbers of small-sized industries, was screened for oestrogenicity, using the Recombinant Yeast Cell Bioassay (RCBA). Target chemical analyses were carried out to establish the presence of EDCs, including p-nonylphenol (p-NP), bisphenol A (BPA), phthalate esters, polychlorinated biphenyls (PCBs) and various organochlorine pesticides, including dichlorodiphenyltrichloroethane (DDT). p-NP, PCBs and organochlorine pesticides were detected using LECO Pegasus II MSTOF and BPA and phthalates were detected using the GC-MS method. Oestrogenic activity was detected in all the samples collected from these sites. Lindane, an organochlorine pesticide, was detected at one site. p-NP, PCBs and phthalate esters were detected at some of the other sites. Small-size industries were found to contribute to EDC pollution of water in the Pretoria West area. <![CDATA[<b>Prevalence of enterohaemorrhagic <i>Escherichia coli</i> O157</b>: <b>H7 in drinking water and its predicted impact on diarrhoeic HIV/AIDS patients in the Amathole District, Eastern Cape Province, South Africa</b>]]> Immunosuppressed persons such as HIV/AIDS patients are at risk of acquiring diarrhoeal infections from water-borne E. coli O157:H7. In the present study, we investigated the prevalence of E. coli O157:H7 in drinking water collected from selected distribution systems within the Amathole District of the Eastern Cape and its predicted impact on diarrhoeic conditions of HIV/AIDS persons living in this area. One hundred and eighty water samples and 360 stool swabs from confirmed and non-confirmed HIV/AIDS diarrhoeic patients were analysed. Escherichia coli O157:H7 were isolated using enrichment culture and confirmed using molecular techniques. Of the 180 drinking water samples, 46 (25.56%) were positive for E. coli O157. The prevalence of E. coli O157 in the stools was at 36.39% (131/360) of which 56.5% (74/131) and 43.5% (57/131) were from stools of confirmed and non-confirmed HIV/AIDS patients, respectively. Molecular analysis of 27, 25 and 29 representative presumptive E. coli O157 from water and stools of confirmed and non-confirmed HIV/AIDS patients, respectively, revealed that 14.81%, 36% and 17.24% of the isolates were E. coli O157:H7. The findings predicted a possible link between E. coli O157:H7 isolated from drinking water and diarrhoeic conditions of both confirmed and non-confirmed HIV/AIDS patients visiting Frere Hospital for treatment. <![CDATA[<b>Effect of chloramine concentration on biofilm maintenance on pipe surfaces exposed to nutrient-limited drinking water</b>]]> This study addresses the effect of specific monochloramine concentration on biofilm density and bacterial functional potential in nutrient-limited water. The efficacy of monochloramine residual maintenance on biofilm density was studied at a range of 0.5 to 2.0 mg/ℓ, using a 3:1 (w/w) dosing ratio of chlorine to ammonia, with the provision of low-nutrient water (0.18 mg/ℓ as total organic carbon, 0.055 mg/l as biodegradable dissolved organic carbon, and 10.5 µg/ℓ as assimilable organic carbon) using a granular activated carbon (GAC) filter. Biofilm density was monitored using biofilm bacteria counts and analysis of the physiological substrate utilisation profiles in Biolog gram-negative (GN) micro-plates. The monochloramine residuals were maintained stable in the low-nutrient water pipes, which contributed to the inhibition of biofilm density. Increasing the monochloramine residual from 0.5 to 2.0 mg/ℓ suppressed the total cells and heterotrophic plate count (HPC) bacteria in the biofilms by about 1 and 2 log units, respectively. The biofilm HPC densities were more sensitive to monochloramine residual, and the reduction in biofilm HPC densities expressed as log CFU/cm² showed an exponential relationship with the increase in monochloramine residual. The Biolog micro-plate-based community-level assay showed that the biofilm communities occurring at 3 levels of chloramination were distinguished by the differences in their substrate utilisation potentials. The functional/metabolic potential of the biofilm community's ability to utilise specific substrates was much lower at higher monochloramine concentration. Results suggest that the maintenance of a consistently high-level monochloramine residual in the low-nutrient water system led not only to a reduction in biofilm density on pipe surfaces but also depressed potential functional/metabolic ability of the biofilm community. <![CDATA[<b>An improved area-based guideline for domestic water demand estimation in South Africa</b>]]> Increased infrastructural development and potable water consumption have highlighted the importance of accurate water-demand estimates for effective municipal water services infrastructure planning and design. In the light of evolving water consumption trends, the current guideline for municipal water demand estimation, published in 1983, needs to be revised. This study investigated, using regression analyses, the combined effect of various socio-economic and climatic parameters on municipal water consumption with the objective of determining the dominant influencing parameters and suggesting a new guideline for water-demand estimation. To this end, an initial database comprising more than 2.5 x10(6) metered water consumption records extracted from 48 municipal treasury databases, which are located within 5 out of the 7 South African water regions was analysed. Each of the 48 municipal treasury databases spanned a period of at least 12 months. The final amalgamated database, after rigorous cleaning and filtering, comprised 1 091 685 consumption records. Single variable and stepwise multiple variable regression analyses were utilised. Results show that stand area, stand value and geographical location are the dominant parameters influencing municipal water consumption, with stand area and stand value positively correlated to water consumption. In suggesting a new municipal water-demand estimation guideline, these three parameters were considered. Stand value, however, fell away as a reliable parameter for estimating water consumption because of the inconsistent basis for predicting stand values due to the constant fluctuations in the value of property, and municipal valuations that often become outdated. Inland and coastal geographical locations exhibited different consumption patterns, with coastal stands of the same stand area and stand value consistently consuming less water than inland stands. These should therefore be treated separately in any design guideline. Stand area then became the best parameter on which to base water-demand estimations. A single guideline curve is therefore proposed which gives various confidence limits for estimating water demand in South Africa, based on stand area. <![CDATA[<b>Water temperature and riverine ecosystems: An overview of knowledge and approaches for assessing biotic responses, with special reference to South Africa</b>]]> Available information pertaining to water temperature in riverine ecosystems is examined and consolidated into an overview that describes the spatial and temporal variation in water temperature, the importance of water temperature in lotic ecosystems, the measurement and modelling of water temperature, anthropogenic factors that modify water temperature, the effects of temperature changes on the physical and chemical characteristics of water; and on aquatic organisms and ecosystems. Methods for assessing the effects of temperature changes on aquatic organisms are discussed and current water temperature guidelines for the protection of aquatic ecosystems are outlined. This paper highlights the complexity of water temperature in the aquatic environment and the importance of understanding the spatio-temporal variability in water temperature and the variable responses of aquatic organisms to thermal stress. Anthropogenic modifiers of the thermal regime, which include heated discharges, flow modifications, riparian vegetation removal and global climate change; present ongoing threats to aquatic ecosystems. Whilst Northern Hemisphere information on water temperature is plentiful, this overview has identified the huge gap that exists in temperature-related data in South Africa. Without baseline data on water temperature and the thermal requirements of aquatic organisms, it is extremely difficult to adequately manage aquatic ecosystems. The risk of increasingly harsh conditions caused by greater water demands and climate change accentuates the need for a greater understanding of the thermal conditions in aquatic ecosystems in South Africa and the requirements and triggers of the associated aquatic biota. Future directions for thermal research are described. <![CDATA[<b>Supercritical fluid extraction of pesticides in sediment from the Okavango Delta, Botswana, and determination by gas chromatography with electron capture detection (GC-ECD) and mass spectrometry (GC-MS)</b>]]> The study reports supercritical fluid extraction (SFE) of pesticides in sediment samples from the Okavango Delta, the headwaters of which originate from Angola and traverse through Namibia into Botswana. An acetone modifier and fractional extraction by pressure ramping gave recoveries ranging from 55 to 86% for the 15 pesticides studied. Hexachlorobenzene (HCB), aldrin and 4, 4-dichlorodiphenyltrichloroethane (4, 4'-DDT) were identified at concentration ranges of 1.1 to 30.3, 0.5 to 15.2 and 1.4 to 55.4 µg/g, respectively by gas chromatography with electron capture detection (GC-ECD) and were unequivocally confirmed by gas chromatography time-of-flight mass spectrometry (GC-ToF-MS). The study indicated an increase of pesticide concentrations in the direction of water flow from the Panhandle (point of entry) to the lower delta. The results show that there are ramifications associated with activities either upstream or downstream; hence close monitoring is required for the long-term preservation of the delta. <![CDATA[<b>Water services with independent providers in peri-urban Maputo: Challenges and opportunities for long-term development</b>]]> Water service delivery to most residents of peri-urban areas of greater Maputo depends largely on alternative service providers, mostly in the form of small-scale independent providers (SSIPs). This paper discusses the present and long-term challenges facing SSIPs in supplying quality water of sufficient quantity in peri-urban Maputo and possible human health risks associated with the consumption of water provided by SSIPs. Extensive water sampling and analyses were conducted to evaluate the physicochemical and bacteriological quality of water provided by independent providers and the associated human health risks. Borehole pumping tests, the results of which were interpreted using the graphical method of Jacob, were used to evaluate the regional aquifer potential, the long-term impacts of its exploitation and the aquifer vulnerability to external contamination. From the results of borehole pumping tests it was concluded that the present yields are in average 33% lower than estimated safe yields and that larger than present yields therefore can be exploited. The aquifer vulnerability to external contamination (e.g. by E. coli and nitrates) is low, mainly because of low hydraulic loads and the existence of a rather thick (10 to 30 m) sandy unsaturated stratum where bacteria die-off and biological denitrification probably occurs. However, the aquifer vulnerability to sea sea-water intrusion is high. Currently, the health risks posed to consumers relying on services provided by SSIPs are small; even so, 13 out of 35 controlled boreholes had either total coliform or faecal coliform levels higher than the WHO standard. In the long run SSIPs may face more serious water quality problems due either to over-exploitation of the aquifer system or increased hydraulic loads resulting from increased population density. <![CDATA[<b>Sample preparation using liquid membrane extraction techniques</b>]]> A brief review is given of membrane extraction techniques that are seen as suitable for the extraction of various chemicals in water samples. Membrane-based extraction methods have now gained popularity as methods of choice in the extraction of both ionisable and non-ionisable molecules from different samples. The main attractive features for these techniques include the use of minimal organic solvents, high selectivity and clean-up efficiency, with high enrichment factors. In most cases the overall cost involved is low due to the simplicity of the techniques which normally involve relatively fewer steps and handling procedures as compared to many other sample-preparation techniques. The various forms and the configurations of membrane-based techniques are another attractive feature which allows the possibility of hyphenation with separation instruments such as gas/liquid chromatographs and even capillary electrophoresis.