The trickling filter/solids contract process has been shown to be an excellent means of upgrading biological filter plants in North America, and about 100 plants have been built since 1979 in both cold and warm climates, and on both large and small works. A variety of effluent requirements has been satisfied consistently, including monthly average BOD and SS concentrations of 10 mg/l and 10 mg/l respectively. Key design features are described including the use of aerated solids contact tank to flocculate dispersed solids. Also, a flocculator-clarifies with special design features allows the use of high overflow rates and smaller sedimentation tanks.