How partial nitrification could improve reclaimed wastewater transport in long pipes

Reclaimed wastewater transport is studied in a concrete-lined cast iron pip e, where a nitrification-denitrification process occurs. The pipe is part o f 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 p roduced varies between 0.8 and 4.4 mg/l NOX-N. When DO has disappeared, a d enitrification process begins. The removal of nitrite is complete at the en d 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 k(20) = 0.079 h(-1), for a NOX--N concentration range of 0.8-4.4 mg/l. Fin ally the following temperature dependency for the first order denitrificati on rate constant has been found : k= k(20) . 1 x 15(T-20). Although nitroge n could be used as nutrient in the agricultural reuse, its removal from rec laimed wastewater could be useful in order to diminish the chlorine needs f or reclaimed wastewater disinfection.