Effects of Removing Tertiary Alum Treatment on Tualatin River Water Quality

The Tualatin River has had a phosphorus TMDL since 1988 that requires Clean Water Services (CWS) to add tertiary alum at its two advanced water resource recovery facilities (WRRFs) in the summer to meet stringent effluent limits for total phosphorus. Over several decades, management of the river has changed dramatically, especially flow management. U.S. EPA recently finalized new aluminum criteria in Oregon that challenges the use of alum in the tertiary process at current application rates to control phosphorus. CWS conducted a study in which tertiary alum addition at the WRRFs was ceased and the resulting effects on the river were monitored and modeled. While the total phosphorus concentrations in the effluent were higher than they are with alum use, the study demonstrated that the increase in total phosphorus concentration did not negatively impact water quality in the river.