Wastewater polishing index: a tool for a rapid quality assessment of reclaimed wastewater

A new index, the Wastewater Polishing Index (WWPI), has been defined for the rapid assessment of the quality achieved by different polishing treatments for water discharged into surface water bodies and for reuse purposes. The index is defined by a weighted average of six parameters (SS, BOD5 COD, ammonia, total phosphorus, and E. scherichia coli), each transformed onto a sub-index scaled from 0 to 100. E. coli has been assigned a greater weight than the other indicators. The index is equal to 0 if none of the six pollutants are present in the effluent and to 100 when all six parameters equal their corresponding Italian legal limits for discharge into surface water bodies. When all six of them equal their corresponding Italian legal limits for reuse, the WWPI is 36. The index has been validated and tested on a pilot plant including a rapid sand filtration, a slow filtration through a horizontal subsurface flow system and a lagooning, in addition to their combinations. The experimental investigation showed that the index is a good tool for (1) rapidly comparing the water quality achieved by different treatment sequences, particularly natural systems; (2) rapidly evaluating whether the proposed sequence is able to produce an effluent adequate for reuse; and (3) rapidly evaluating the water quality improvement achieved by different systems. The proposed index could be of great help for managers and decision makers when planning for water resources, in particular, for comparing the quality level achieved by different wastewater treatment sequences.