Secondary Treatment

Harnessing the power of decomposers to break down organic matter in wastewater is at the heart of a treatment process now known as secondary treatment. Two distinct methods of this treatment type evolved around the turn of the twentieth century. The Lawrence Experiment Station in Massachusetts pioneered the first method in 1892. Called the trickling filter method, it involves spraying wastewater onto a column of crushed stone on which a community of bacteria, fungi, protozoa, and insects resides. The organisms take in a portion of the organic matter and break it down. Some of the breakdown products, such as carbon dioxide, escape to the atmosphere. Others, like nitrate, remain in solution. Still other products are absorbed into the organisms themselves. This latter material is eventually collected in settling tanks as sludge after the organisms die or is otherwise detached from the stone.

A second method of secondary treatment was advanced around 1913 by the Lawrence Experiment Station and Edward Ardern and W. T. Lockett (1914) in England. Known as activated sludge treatment, it follows the same principles as the trickling filter, but instead of cultivating decomposers on the surface of rocks, organisms are simply suspended in a tank by a continuous flow of wastewater. Both methods of secondary treatment result in discharges with substantially less organic matter than is produced by primary treatment. City officials having problems with litigious neighbors downstream were especially eager to adopt this new technology into their urban water cycles. One of the first trickling filter facilities in the nation was constructed in the city of Gloversville, New York, in 1907. The motivation was not so much citizen demands in Gloversville for a cleaner river as it was the need to respond to a riparian rights suit filed by the downstream city of Johnstown. Chicago officials also grew tired of their ongoing battle with St. Louis, and in 1916, they constructed the first activated sludge treatment plant in the nation (Metcalf and Eddy, 1991).

Officials in most other U.S. cities, however, did not have neighbors like Johnstown or St. Louis forcing them to upgrade their wastewater treatment capabilities. Consequently, they were content to embrace a theme reflected in a leading textbook of the time, Sewage Disposal. In this text, Kinnicutt, Winslow, and Pratt (1913) argued that under certain circumstances, dilution of raw sewage by disposal into lakes, rivers, and tidal waters, rather than treatment of sewage, was an economical and proper method of purification.

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