Abstract
The assimilative capacity of water bodies is an important factor in the integrated management of surface water resources. The current study examined the auto-recovery processes of the Apies River from wastewater discharged into it from a municipal wastewater treatment facility, using a series of equations, including the modified Streeter-Phelps equation. Field data obtained include dissolved oxygen (DO), temperature, stream velocity, depth, and width. Water samples were also obtained at 10 sampling stations for the determination of biochemical oxygen demand (BOD) using standard methods. It was observed that the DO and BOD level (5.59 mg/L and 8.5 mg/L respectively) of the effluent from the wastewater treatment facility indicated better water quality than the Apies River background DO level (5.42 mg/L) and BOD level (13 mg/L). Also, at 270 m downstream of the effluent discharge point, another effluent stream (Skinnerspruit) adversely impacted on the Apies River with DO and BOD levels of 6.5 mg/L and 9.0 mg/L, respectively, compared to the Apies River background values of 6.81 mg/L and 8.0 mg/L, respectively. The stream, however, recovered well from both the background and imposed pollution sources as it had a computed positive auto-recovery factor of 1.74. Furthermore, the measured DO deficit was plotted against predicted DO deficit. The plot revealed a close match between the measured and predicted DO deficit, indicating that the model could be used for predicting DO deficit along other segments of the river. To further improve on the natural auto-recovery processes of the Apies River, it was recommended that flow along the Skinnerspruit should be enhanced by clearing the observed aquatic plants growing within the channel. Also, suspected pollution activities taking place further upstream on the Apies River should be investigated and appropriately addressed.
Highlights
The data showed that the depth and cross-sectional width of the river segment generally increased as it progresses downstream of the effluent discharge point
It was demonstrated that the Apies River recovered within a 150 m distance after the confluence point with the treated wastewater being disposed into it from Daspoort Wastewater Treatment Works (DWTW) (Fig. 6)
The auto-recovery process of the river was enhanced, largely because the effluent discharge from DWTW had been treated to recommended standards specified by DWAF (1996)
Summary
Surface water is a vital natural resource for South Africa as 70% of the country’s GDP, 70% of the country’s population, and 74% of all water demand are supported by this resource (NWRS, 2004). When compared to the global average of 860 mm/annum, Gauteng can be the law specifies that all effluent must be treated before being discharged into nearby receiving streams, there have been reports of increased pollution level in dams situated downstream of urban areas as well as outbreaks of waterrelated diseases among the populace (Oelofse and Strydom, 2010; Swatuk, 2010; Bamuza and Abiye, 2012). One of the management tools that could be employed in the monitoring and management of surface water bodies around the country is re-aeration (Jha et al, 2007; Omole et al, 2013) This is based on the principle that imposed biological waste loads deplete the dissolved oxygen (DO) content of natural water systems in the process of breaking down the wastes.
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