Juan Rodríguez-Sevilla

Reclaimed wastewater quality enhancement by oxygen injection during transportation

In-sewer treatments have been studied in sewer systems, but few have been carried out on reclaimed wastewater systems. A study of oxygen injection has been performed in a completely filled gravity pipe, 0.6 m in diameter and 62 km long, in cast iron with concrete inside coating, which is part of the reclaimed wastewater reuse scheme of Tenerife (Spain). A high pressure oxygen injection system was installed at 16.0 km from pipe inlet and a constant dosage of 30 mg/L O(2) has been injected during six months, under three different operational modes (low COD, 63 mg/L; high COD, 91 mg/L; and partially nitrified water). Oxygen has been consumed in nitrification and organic matter reduction. Generally, nitrification is clearly favored instead of the organic matter oxidation. Nitrification occurs, in general, with nitrite accumulation due to the presence of free ammonia above 1 mg/L. Denitrification is in all cases incomplete due to a limitation of easily biodegradable organic matter content, inhibiting the appearance of anaerobic conditions and sulfide generation. A notable reduction of organic matter parameters is achieved (TSS below 10 mg/L), which is significantly higher than that observed under the ordinary transport conditions without oxygen. This leads to a final cost reduction, and the oxygen injection system helps water reuse managers to maintain a final good water quality in the case of a treatment plant malfunction.

Nitrogen transformation of reclaimed wastewater in a pipeline by oxygen injection.

A study of oxygen injection was performed in a completely filled gravity pipe, which is part of the South Tenerife reclaimed wastewater reuse scheme (Spain), in order to inhibit the appearance of anaerobic conditions by a nitrification‐denitrification process. The pipe was 0.6 m in diameter and 62 km long and made of cast iron with a concrete inner coating, A high‐pressure oxygen injection system was installed at 16 km from the pipe inlet, where severe anaerobic conditions appear. Experiments on oxygen injection were carried out with three different concentrations (7, 15 and 30 mg l−1 O2). In all experiments, oxygen dissolved properly after injection, and no gas escapes were detected during water transportation. Most oxygen was consumed in the nitrification process, due to the low COD/NH4‐N ratio, leading to a maximum production of oxidized nitrogen compounds of 7.5 mg l−1 NOX‐N with the 30 mg l−1 O2 dose. Nitrification occured with nitrite accumulation, attributed to the presence of free ammonia within the range 1.2–1.4 mg l−1. Once the oxygen had been consumed, an apparent half‐order denitrification took place, with limitation of biodegradable organic matter. The anoxic conditions led to a complete inhibition of sulphide generation.