The present study has been carried out in a completely filled gravity pipe, 61 km long, which transports reclaimed urban wastewater (RUWW), for crop irrigation in the island of Tenerife (Spain), at an average water velocity of 0.4 m/s. Due to the long residence time of the reclaimed wastewater in the pipe (40 hours), anaerobic conditions usually appear, and chemical transformation of reclaimed wastewater occurs during transportation, especially sulfide generation. Field studies have been conducted from November 1994 to January 1997. The variation in sulfide and oxidation-reduction potential (ORP) among other parameters during wastewater transportation have been reported. The object of this work is to study the kinetics of the sulfide generation in a full scale reclaimed wastewater pipe, in order to obtain a forecasting equation of sulfide generation, and to compare this equation with those previously proposed for raw wastewater sewers. Equations obtained from this research differ quite from those previously proposed for sewers. This can be due to the reclaimed wastewater used in this system, which has a very much lower organic matter content than raw wastewater.
During treated wastewater transport, phenomena related to the presence or absence of dissolved oxygen (DO) have been observed. In Tenerife reclaimed urban wastewater (RUWW) transport from the capital, Santa Cruz to the south of the island for agriculture reuse was studied. Anoxic conditions and H2S generation were noted during transport. As RUWW has an electrical conductivity (EC) of about 1600 μS/cm, BALTEN decided to add fresh water (FW) with low EC, saturated or over saturated in DO at an intermediate point in the pipe to reduce the conductivity of water supplied to agriculture, also thus reducing the organic load and consequently H2S generation. However, injection of DO over-saturated fresh water brought on the appearance of nitrification and later denitrification. A study was carried out of H2S generation, nitrification and denitrification during RUWW transport with and without added FW.
Reclaimed wastewater transport is studied in a concrete-lined cast iron pipe, where a nitrification-denitrification process occurs. The pipe is part of the Reuse System of Reclaimed Wastewater of South Tenerife (Spain), 0.6 m in diameter and 61 km long. In order to improve wastewater quality, at 10 km from the inlet there is injection of fresh water saturated in dissolved oxygen (DO), after which a fast nitrification process usually appears (less than two hours of space time). The amount of oxidized nitrogen compounds produced varies between 0.8 and 4.4 mg/l NOx(-)-N. When DO has disappeared, a denitrification process begins. The removal of nitrite is complete at the end of the pipe, whereas the nitrate does not disappear completely, leaving a concentration of about 0.4-0.5 mg/l. For a COD/NOx(-)-N ratio higher than 5, a first order nitrification rate in NOx(-)-N has resulted, with the constant k20 = 0.079 h-1, for a NOx(-)-N concentration range of 0.8-4.4 mg/l. Finally the following temperature dependency for the first order denitrification rate constant has been found: k = k20 x 1 x 15T-20. Although nitrogen could be used as nutrient in the agricultural reuse, its removal from reclaimed wastewater could be useful in order to diminish the chlorine needs for reclaimed wastewater disinfection.
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. Water Environ. Res., 72, 455 (2000).
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