During wastewater transportation, chemical and biological transformations may occur. Aerobic, anoxic, and anaerobic conditions exist depending on the presence or absence of dissolved oxygen and oxidized nitrogen compounds. The occurrence of anaerobic conditions has a negative effect
on wastewater transportation because of the usual subsequent sulfide generation process. In Tenerife, Spain, reclaimed wastewater used for agricultural purposes is transported via a completely filled gravity pipe, 0.6 m in diameter and 61 km long. Because of the long residence time (approximately
30 to 40 hours), anaerobic conditions typically occur after 5 to 10 hours of flow. Field studies during transportation were conducted from November 1994 to July 1997. This paper describes a study of the evolution of septic conditions (aerobic, anoxic, and anaerobic) and the relationship between
septicity and oxidation–reduction potential (ORP), which can predict the occurrence of anaerobic conditions.
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