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Comparative assessment of microplastic removal in two wastewater treatment plants in Nairobi, Kenya
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Two wastewater treatment plants in Nairobi removed 78-89% of microplastics (tiny plastic bits) from sewage before releasing treated water back into rivers—which sounds good, but they were still pumping out over 130 million plastic particles combined every single day. This matters because even "well-treated" wastewater can be a major source of plastic pollution flowing into waterways that people and ecosystems depend on, showing that current treatment plants aren't fully equipped to catch these particles, especially in fast-growing cities.
ABSTRACT Graphical abstract comparing microplastic removal in two wastewater treatment plants in Nairobi. At the Ruai WWTP (lagoon-based), domestic and industrial wastewater enters with an average microplastic abundance of 9.99 ± 0.50 particles/L, leaving effluent with 1.03 ± 0.08 particles/L (89% ± 1.91% removal efficiency). The Kariobangi WWTP (biological filtration) receives domestic wastewater with an average microplastic abundance of 4.70 ± 0.28 particles/L, leaving effluent with 1.08 ± 0.18 particles/L (78% ± 3.16% removal efficiency). Wastewater treatment plants (WWTPs) are recognised as key pathways through which microplastics enter the environment. Therefore, effectively detecting these particles and understanding their presence and fate in these plants is crucial for controlling their release. However, data are scarce in Sub-Saharan Africa. This study evaluated the effectiveness of two major WWTPs in Nairobi, Kenya, in removing microplastics, each utilising different treatment methods. Samples were collected during the dry season, representing low-flow conditions from influent and effluent streams. On average, the concentration of microplastics in the Ruai and Kariobangi WWTPs was found to be 9.99 ± 0.50 particles/L and 4.7 ± 0.28 particles/L at the influent, respectively. These concentrations significantly decreased in the final effluent to 1.08 ± 0.18 particles/L and 1.03 ± 0.08 particles/L, corresponding to removal efficiency of 78 and 89% respectively. Despite these reductions, the WWTPs were releasing approximately 123.1 million and 10.3 million microplastics into the receiving water each day. The Ruai WWTP demonstrated a significantly higher removal efficacy compared to the Kariobangi WWTp. This is likely due to longer hydraulic retention times and more stable treatment conditions. These findings suggest that conventional WWTPs may act as important secondary sources of microplastics particularly in rapid urbanising cities.
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