9 research outputs found
Modelling lead and cadmium uptake by star grass under irrigation with treated wastewater
This study was conducted to investigate the capacity of Cynodon nlemfuensis (star grass) to accumulate lead (Pb) and cadmium (Cd) and develop metal uptake models for sandy soils receiving treated sewage from domestic and industrial sources. The study area comprised a non-polluted area and an adjacent area that received treated sewage from Harare’s Firle Wastewater Treatment Plant for over 30 years. Measured soil properties, total Pb and Cd in soils and grass and past records of Pb and Cd in treated sewage were analysed. Growing grass in a greenhouse in pots with previously non-polluted soils amended by single and mixed Pb and Cd salts and irrigated with treated sewage tested the uptake capacity of star grass. Yields, soil bio-available and grass Pb and Cd levels were measured and used to develop models for estimating critical soil and grass concentrations at which productivity declines. In the field, star grass grown in 10m x 10m plots in the non-irrigated and irrigated areas, received varying amounts of treated sewage over 11 months. Soil bio-available and grass metal contents were measured and used to develop field-based models to predict Pb and Cd content in star grass. Star grass had a high Pb and Cd extraction capacity, making it unsuitable for pasture if grown on polluted soils. Correlation between total Pb and Cd in soils and grass was insignificant (p<0.05). Logarithm-based models of log10 bio-available soil levels and log10 grass metal levels provided the best-fit regression models for Pb and Cd predictions in grass. Toxicity levels of Pb and Cd that were derived for star grass from pot-based models were higher than levels recommended for pasture. Toxicity occurred without visible signs on grass, making it difficult to recognise toxicity without testing. The field-based uptake models predicted safe bio-available limits for pasture on sandy soils. The co-presence of Pb and Cd resulted in increased Cd uptake but did not significantly affect Pb uptake. Star grass can accumulate more than 1 mg/kg of Cd at total soil Cd levels of less than 1 mg/kg, suggesting that the soil limit may be too high for a sandy soil.Thesis (PhD (Chemical Engineering))--University of Pretoria, 2007.Chemical Engineeringunrestricte
Heavy metal content of vegetables irrigated with mixtures of wastewater and sewage sludge in Zimbabwe: Implications for human health
There is growing public concern in Zimbabwe over the illegal cultivation of vegetables on soils amended with sewage sludge or irrigated with admixtures of sewage and sewage sludge. Excessive accumulation of heavy metals in agricultural soils may not only result in environmental contamination, but lead to elevated heavy metal uptake by crops, which may affect food quality and safety. The work reported here studied heavy metal concentrations in crops irrigated with sewage sludge and sewage/sewage sludge admixtures at Firle Municipal Farm in Harare. The crops analysed in this study are heavily contaminated with the four regulated elements Cd, Cu, Pb and Zn. This contamination is at its highest in two of the staple dietary crops maize and tsunga. Tsunga leaves contained 3.68 mg kg-1 Cd, over 18 times the permissible level by the EU standards (0.2 mg kg-1); Cu concentrations were 111 mg kg-1, 5 times the EU Standard (20 mg kg-1); concentrations of Pb were 6.77 mg kg-1, over 22 times the permissible levels allowed by both EU standards and UK guidelines (0.3 mg kg-1); Zn concentrations were 221 mg kg-1, over 4 times the guideline value (50 mg kg-1). The other plants (beans, maize, peppers and sugarcane) also contained concentrations of heavy metals above the permissible levels. Furthermore the concentrations observed in this study were higher than those reported by other workers who have examined vegetation from other contaminated sites. This study highlights the potential risks involved in the cultivation and consumption of vegetables on plots irrigated with sewage sludge, a practice which may place at risk the health of the urban population who consume these vegetables
Bulk water suppliers in the City of Harare – An endogenous form of privatisation of urban domestic water supply in Zimbabwe?
This paper investigates the phenomenon of bulk water suppliers in the city of Harare, Zimbabwe’s largest urban metropolis and capital. Bulk water suppliers began in 2005 to sell domestic water to middle- and high-income suburbs because of shortcomings in the city’s water delivery system without state regulation, and have since become a permanent feature of the Zimbabwean urban waterscape. The study was conducted between 2012 and 2013 in three up-market suburbs of Harare, which were known to depend on bulk water suppliers. State regulation of bulk water suppliers was introduced in 2013, close to a decade after the start of operations, indicating a reactive and reluctant acknowledgement that bulk water suppliers were now significant players in water service provision. The regulation was, however, poorly conceptualised, based on potable water standards, which proved to be cumbersome and placed onerous demands on the suppliers. The paper concludes that bulk water suppliers are playing a critical role in water service provision in Zimbabwe’s largest metropolis and represent a spontaneous injection of local private capital in the urban domestic water supply sector. They can therefore be seen as a viable endogenous form of privatisation of urban domestic water service (as contrasted to multinational companies) but should be viewed as complementing rather than replacing functional urban water supply systems. The operations of bulk water suppliers can be enhanced if a regulatory regime, informed by realities on the ground is crafted