 The city began treating sewage to remove nitrogen in 1988, and the 2005 upgrade increased the amount of nitrogen removed from sewage from 65 percent to 90 percent.The upgrade also included three odor-control machines at a total cost of $6 million.The machines use massive fans to suck odiferous fumes into chambers where chemicals are used to remove 99.9 percent of the hydrogen sulfide that causes the odor.