Assessment of roughing and slow sand filter modified with slag and clinker ash for removal of microorganisms from secondary effluent

Slow sand filtration has been used for decades for the removal of pathogens from water. A combination of roughing and slow sand filters has been evaluated for the removal of pathogen indicators from secondary effluent. The filter system was modified with the addition of slag and coal clinker ash. The filter system reached up to 100% removal of faecal coliforms, faecal streptococci, Escherichia coli and total coliforms after 17 weeks of treatment. Log reductions ranged between 0.0–4.1, 1.33–2.0, 1.34–2.37 and 1.4–4.5 for E. coli, faecal streptococci, faecal coliforms and total coliforms respectively. There was no strong correlation between pH and the bacterial counts as correlation coefficients were −0.15, 0.15, 0.38 and −0.07 in respect to E. coli, faecal coliforms, faecal streptococci and total coliforms. The correlation coefficients for temperature were −0.18, −0.14, 0.10, and −0.39 for E. coli, faecal coliforms, faecal streptococci and total coliforms respectively, suggesting that temperature did not influence the bacterial removal efficiency. For turbidity, the correlation coefficients were 0.64, 0.70, 0.68 and −0.06 respectively, for E. coli, faecal coliforms, faecal streptococci and total coliforms. Only total coliforms had no correlation with turbidity. There was a significant difference between effluent from holding tank and slow sand filter as p values of 0.008, 0.0006, 0.00000, and 0.00008 were observed for faecal coliforms, faecal streptococci, E. coli and total coliforms counts, respectively. The results obtained indicate that the combination of roughing and slow sand filters can successfully reduce microorganisms from secondary wastewater.