For the past several years, the Municipality of Metropolitan Seattle (Metro) has been engaged in facility planning. From this effort a comprehensive sewerage plan will emerge spelling out, among other things, those particular waste treatment processes best suited to meet the federal government’s Best Practicable Treatment Requirements at various Metro plant sites. An ambitious two-and-a-half-year pilot testing operation was conducted as part of this program. Waste treatment processes designed to achieve certain water quality objectives were tested side-by-side to determine which best satisfied Metro requirements as regards treatment effectiveness, space, cost, energy demands, and sludge production. Biological processes (activated sludge and fixed growth systems) were tested as well as strictly physical-chemical schemes (alum and lime treatment). This paper summarizes results obtained from the fixed-growth reactor (FGR) system studies. Full study details are summarized in reference 3. The FGR system (also known as the trickling filter system) was recommended as a candidate process because of its general simplicity and historical reliability, because designs incorporating recent technological advances (plastic media and tub settlers) were perceived as being able to satisfy effluent quality criteria while still fitting within severely space-constrained available treatment plant sites, and because such systems were believed to require less energy than activated sludge systems treating equivalent flows.