Comparative evaluation of different pre-treatment alternatives for granular media filters treating greywater and their ranking using analytical hierarchy process

In this study, the performance of four different pre-treatment alternatives for granular media filtration, namely, settling, aeration, coarse media filtration, and chemical coagulation were compared experimentally. Further, the analytical hierarchy process (AHP) was used to compare their performance based on economic, environmental, technical, and performance criteria. Performance of settling and aeration were evaluated up to 24 h duration. The coarse media filter was intermittently operated with 10 L of greywater in downflow mode, while alum was used for chemical coagulation. Experimental results showed that settling up to 6 h did not show significant removal of different pollutants, whereas 24 h settling resulted in moderate removal of turbidity and organic content but was not efficient in the removal of nutrients and faecal coliforms. Chemical coagulation reduced 93, 66, 48, and 97% of turbidity, chemical oxygen demand (COD), NH4-N, and faecal coliforms, respectively, from greywater but resulted in excessive sludge generation and is difficult to adopt onsite and requires skilled supervision. Coarse filtration of greywater resulted in 61, 41, 36, and 35% removal of turbidity, COD, PO4-P, and faecal coliforms, respectively. Considering different criteria AHP gave coarse filtration as the best pre-treatment option to the granular media filters treating greywater.