The proportion of highly polluted water in all city waterways is set to drop from 18 percent to 12 percent by the end of the year. The city launched a three-year plan last year aimed at eliminating water rated 5 by 2020. Water rated 5 refers to malodorous black water that is so polluted it can’t be used and is yucky to look at. More than 3,150 waterways were improved last year, thanks to dredging, pipeline renovation and a crackdown on illegal constructions.