Abstract
The aim of this 15-month study was to evaluate and compare two partially saturated (PS) vertical flow (VF) wetlands for total nitrogen (TN) removal. The PS VF wetlands, evaluated in duplicate, were added with corncob (CC) in two different heights of the free-drainage zone (FDZ). The FDZ had a height of 40 cm and the saturated zone (SZ) had a height of 30 cm. The configuration of the system I (SI) was a 20 cm-corncob bed above the SZ followed by a 20 cm-tezontle bed; in system II (SII) the order of the beds were inverted. The SZ was added with tezontle with a size of 1−2 cm. Weekly measurements of water quality parameters including oxygen demand (BOD5), chemical oxygen demand (COD), color, total suspended solids (TSS), organic nitrogen (Org-N), ammonium (NH4+), nitrate (NO3−) and nitrite (NO2−), were taken in the influent and effluents, and interfaces (nitrate and nitrite). Measurements of pH, dissolved oxygen (DO) and oxidation-reduction potential (ORP) were taken in the SZ. The addition of CC in the FDZ did not interfere with the capacity of the PS VF wetlands for BOD5, COD, TSS and true color removal, reaching mass removal efficiencies of 91.9% and 92.2%, 66.6% and 75%, 89.8% and 92%, 63.3% and 66.0%, for SI and SII, respectively; without significant difference between the systems (p > 0.05). The CC in the FDZ neither interfered with the PS VF wetlands nitrification capacity. The removal of TN was similar in SI and SII (p > 0.05), attaining average mass removal efficiencies of 68.2% and 66.0%, respectively. These efficiencies were not sufficiently high due to the limited denitrification process in the SZ as a result of the absence of biodegradable carbon, generated and consumed in the FDZ.
Highlights
Nitrogen compounds are responsible for the deterioration of surface waters, resulting in eutrophication, toxicity for aquatic creatures and risks to human health [1,2]
In a previous study we evaluated the use of corncob (CC), an abundant agricultural lignocellulosic residue in Mexico which was placed in the saturated zone (SZ) of partially saturated (PS) vertical flow (VF) wetlands for the removal of total nitrogen (TN) and obtained an average efficiency of 73.0% for seven months that decreased in 11% for the seven months [26]
According to [29], the wastewater fed to the PS VF wetlands could be classified as a weak domestic wastewater due to its BDO5 and chemical oxygen demand (COD) concentrations the ammonium concentration was higher of that commonly found in domestic wastewater
Summary
Nitrogen compounds are responsible for the deterioration of surface waters, resulting in eutrophication, toxicity for aquatic creatures and risks to human health [1,2]. Nitrification is a two-step process by which autotrophic ammonia-oxidizing bacteria (AOB) and archaea (AOA) oxidize. Recently it was discovered that some bacteria, members of the genus Nitrospira, are capable to perform a complete nitrification; this kind of bacteria are currently called COMAMMOX (complete ammonia oxidizer) [8,9]. Denitrification is the reduction of NO3 − until nitrous oxide (N2 O) or molecular nitrogen (N2 ) under anoxic/anaerobic conditions by heterotrophic bacteria; for denitrification, organic carbon becomes a limiting factor when the wastewater has a low C/N ratio [12,13]. Denitrification is performed by AOB and AOA due to the fact that N2 O is generated as by-product of ammonia oxidation [14]. Autotrophic denitrification takes place by those groups of bacteria capable of heterotrophic nitrification [11].
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