Spatial Analysis and Quantification of Microplastics in the Nairobi Rivers and Wastewater Treatment Plants

Abstract

Microplastics have raised significant concerns due to their potential threats to human health and aquatic species. The accumulation of microplastics in aquatic environments has become increasingly alarming in recent years. While microplastics have been widely detected in various water bodies over the past few decades, studies in many developing countries, including those in Africa, remain limited. This study aimed to determine the abundance and composition of microplastic particles in selected rivers and in two wastewater treatment plants in Nairobi, Kenya. Water samples were collected from both the rivers and the wastewater treatment plants, using grab sampling technique. The microplastic particles, ranging in size from 4.75 mm to 300 µm, were categorised through visual identification with a dissecting microscope and characterised using Fourier Transform Infrared Spectroscopy (FTIR). Pearson Product-Moment Correlation analysis was then employed to investigate the relationship between microplastic concentration and land use. The abundance of microplastics in the rivers ranged from 0.22±0.026 to 0.917±0.061 particles/litre. Fibres (51.15%) and white particles (40.08%) were the most common types of microplastics detected, with polyethylene (PE) and polystyrene (PS) being the predominant polymers. Higher concentrations were noted near industrial, commercial, and residential areas, particularly in informal settlements along the rivers. A positive correlation was observed between microplastic abundance and built-up areas, while a negative correlation was found with forested areas and wetlands. In the studied wastewater treatment plants, the average abundance of microplastics in the influent at Ruai was 9.99 ± 0.50 particles per litre, and 4.7 ± 0.28 particles per litre in Kariobangi. This abundance significantly decreased in the final effluent to 1.08 ± 0.18 particles/litre in Ruai and 1.03 ± 0.08 particles/litre in Kariobangi, corresponding to removal efficiencies of 89 ± 1.91% and 78 ± 3.16%, respectively. Despite these reductions, the wastewater treatment plants were releasing approximately 123.1 ± 20 million and 10.3 ± 0.91 million microplastics per day into the receiving river. These findings highlight the influence of anthropogenic activities and wastewater treatment plants on the abundance of microplastics in the Nairobi River Basin. This study represents one of the first detailed assessments of microplastic distribution in Nairobi’s freshwater systems and provides critical baseline data on microplastic pollution in urban rivers of Sub-Saharan Africa. It underscores the urgent need for improved wastewater management and pollution control strategies, as the presence of microplastics poses a serious ecological risk and potential implications for human health.