Polluted runoff from densely populated and poorly serviced informal settlements is a growing issue in South Africa that leads to various health risks and environmental degradation. Surface waters affected by informal settlements are known to display high nutrient (NH3, NO2 - , NO3 - and PO4 3- ) concentrations. These nutrient concentrations are highly variable due to the fluctuations in the type and frequency of human activities occurring in the informal settlements. A decentralised, non-invasive and easy to maintain surface water rehabilitation system that is capable of handling variable inlet nutrient concentrations is therefore necessary in these areas. Engineered nature-based solutions such as horizontally orientated subsurface flow biofilters are a potential suitable remediation measure, as they are cost-effective, scalable and easy to maintain. However, the variable nutrient levels in surface waters affected by informal settlements pose a challenge to achieving consistent nutrient reduction in a system. The nutrient degradation potential of a microbially colonised horizontally orientated subsurface flow pilot-scale biofilter (length: 2 m; width: 0.44 m; depth: 0.7 m) that was packed with stones of 8-11 mm in diameter was assessed in this study. The purpose of performing controlled experiments on the pilot-scale biofilter was to enable data collection for the design of optimum full scale biofilters. Pulse tracer studies at varying flow rates (0.5 -3 L/min) determined that the flow behaviour in the pilot-scale biofilter approximated plug flow; with the extent of plug flow behaviour and degree of mixing in the radial direction increasing with a decrease in flow rate Surface runoff from the Stiebeuel River, contaminated by an upstream informal settlement called Langrug, was used as the polluted water source for the pilot-scale biofilter. Six nutrient degradation studies were carried out on the microbially colonised pilot biofilter with each study occurring over a 10-day period (228 hours). Three of the nutrient degradation studies were carried out at a flow rate of 0.5 L/min and three were carried out at a flow rate of 1.5 L/min. Water from the Stiebeuel River (200 L) was circulated through the system during the batch nutrient degradation studies with samples being taken every six hours. The inlet concentration varied in each study with an inlet concentration range of 8.41 - 24.2 mg/L NH3, 1.06 - 2.30 mg/L NO3 - and 1.45 - 6.82 mg/L PO4 3- being observed. An NH3 reduction of up to 91.8%, total nitrogen reduction of up to 82.4% and a PO4 3- reduction of up to 88.3% was observed. The main biological processes occurring within the pilot biofilter were nitrification and denitrification. An extent of nitrification of up to 91.7% and an extent of denitrification of up to 95.6% were observed in the nutrient degradation studies. Ammonia degradation and orthophosphate removal in the system was described using the double first-order in parallel reaction kinetic model which expresses the reaction kinetics as the sum of first two order reactions. The results of the nutrient degradation studies show that the microbial community in the pilot-scale biofilter system is able to metabolise moderate pulses of nutrients when fresh contaminated water is introduced to the system at varying inlet concentrations. The microbial community is able to survive under nutrient limited conditions. These findings indicate the effectiveness of stone biofilters at degrading nutrients in polluted runoff from informal settlements in a controlled batch experiment.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:uct/oai:localhost:11427/38049 |
| Date | 07 July 2023 |
| Creators | Maraj, Kalpana |
| Contributors | Harrison, Susan, Winter Kevin |
| Publisher | Faculty of Engineering and the Built Environment, Department of Chemical Engineering |
| Source Sets | South African National ETD Portal |
| Language | English |
| Detected Language | English |
| Type | Master Thesis, Masters, MSc |
| Format | application/pdf |
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