Public law 92-500 in 1972 set a national goal of secondary treatment for all domestic wastewater discharges. Secondary treatment was subsequently defined by the Environmental Protection Agency (EPA) as an effluent containing 30 mg/l or less of BOD5 and suspended solids. Recognizing that this level of treatment is not adequate for all environmental conditions, EPA has defined two further degrees of treatment, “advanced waste treatment” and “advanced secondary treatment.” Advanced waste treatment produces an effluent with BOD5 and suspended solids of 10 mg/l or less and/or total nitrogen removal greater than 50 percent. Advanced secondary treatment is defined as a degree of treatment less stringent than advanced treatment but more stringent than conventional secondary. The general trend in construction and operating costs for wastewater treatment plants has shown the cost per unit of contaminant removed increasing exponentially for degrees of treatment more advanced than secondary. Running counter to that trend, recent plant-scale operating experience at the Corvallis, Oregon, wastewater treatment plant has shown that trickling filters, for years the economical work-horse of the wastewater treatment industry, can be designed to achieve BOD5 and suspended solids removals equivalent to advanced waste treatment.