Upgrading slow sand filtration: exploring the practicality of traditional technology for the 21st century

Water stress is intensifying globally due to rising demand and declining quality. The goals of this research were two-fold: to unlock previously unusable wastewater effluent for re-use and improve the standard of water post-treatment. Slow sand filtration (SSF), an established approach to wastewater treatment, is still effectively used worldwide, particularly in resource-poor regions. A traditional SSF reactor was built in this study, then later reconstructed replacing a section with sawdust, a novel and sustainable material; both were asynchronously investigated for their capacity to remediate water of antibiotics and common surface water nutrients, nitrate and phosphate. Each filter was operated in a recirculating setup over four weeks to enable the development of the biological component known as “Schmutzdecke”. Standard water-quality testing indicated that the hybrid sawdust filter could buffer the impact of shock events (e.g., turbidity spikes) while still supporting a healthy Schmutzdecke. Furthermore, the sawdust layer facilitated greater microbial activity, which is associated with adsorption and biodegradation of various contaminants and pathogenic microorganisms. However, upon exposure to simulated wastewater effluent spiked with selected antibiotics (1 mg.L-1 each), sulfamethoxazole (SMX) and trimethoprim (TMP), negligible differences in removal efficiency were observed between the traditional and hybrid SSFs. SMX removal was low in both traditional and hybrid filters; median removals (n=3) were 3.7% (1.2 - 38.6%) and 2.0% (1.4 - 3.4%), respectively. However, TMP removal exceeded 93% in each case. Further analysis of the mechanisms of interaction between contaminants and the filter material revealed that SMX removal is highly pH-dependent and independent of contact time. We can conclude that even given a greater residence time, it is unlikely that the more persistent compounds like SMX will be removed from aqueous solutions by SSF. This
underscores the need to control and prevent these compounds from entering water bodies from the outset.