Wastewater treatment by slow sand filters using uncoated and iron-coated fine sand: impact of hydraulic loading rate and media depth.

Two lab-scale slow sand filters (SSFs), packed with uncoated fine sand (SSFu) and iron-coated fine sand (SSFco), were operated to study their efficiency in treating municipal wastewater. The effects of sand coating; hydraulic loading rates (HLRs) (0.56, 0.85, and 1.12 cm/h); and filter depths (22, 32, and 42 cm) were evaluated. Sand coating did not have any significant effect on wastewater treatment by the SSF at all depths (p > 0.05). The removals of total suspended solids (TSS), chemical oxygen demand (COD), and phosphate decreased with increase in HLR. On the other hand, media depth had positive effects on the removal of turbidity, TSS, COD, and total coliforms (TC). At HLR of 0.56 cm/h, the average removals of each studied parameter, i.e., turbidity, TSS, and COD, at filter depth d42 in SSFu and SSFco were 94.3, 90.1, and 56% and 92.7, 93, and 30.95%, respectively. Both filters efficiently removed the total coliforms (> 90%) and fecal coliform (up to 99%) but inefficient in nitrate removal. Frequent clogging was observed in SSFu due to the colonization of microorganisms on the sand surface, which was confirmed by SEM images. Biofilm formation or microbial colonization was absent in SSFco, which might be responsible for uninterrupted operation of SSFco. Overall, the sand coating is beneficial for long-term operation of SSF.