Abstract
Septic tanks (STs), up-flow anaerobic filters (UAFs), and horizontal-flow constructed wetlands (HFCWs) are cost-effective wastewater treatment technologies especially efficient in tropical and sub-tropical regions. In this study, the bacterial communities within a decentralized wastewater treatment plant (WWTP) comprising a ST, a UAF, and a HFCW were analyzed using high-throughput sequencing of the V3–V4 region of the 16S rRNA gene. Bacterial diversity and its spatial variation were analyzed at the phylum and family level, and principal component analysis (PCA) was applied to nitrogen- and organic-matter-degrading families. The highest percentage of nitrogen removal was seen in the HFCW (28% of total Kjeldahl nitrogen, TKN, and 31% of NH3-N), and our results suggest that families such as Rhodocyclaceae (denitrifying bacteria), Nitrospiraceae (nitrifying bacteria), and Rhodospirillaceae (sulfur-oxidizing bacteria) contribute to such removal. The highest percentage of organic matter removal was seen in the UAF unit (40% of biological oxygen demand, BOD5, and 37% of chemical oxygen demand, COD), where organic-matter-degrading bacteria such as the Ruminococcaceae, Clostridiaceae, Lachnospiraceae, and Syntrophaceae families were identified. Redundancy analysis demonstrated that bacterial communities in the HFCW were more tolerant to physicochemical changes, while those in the ST and the UAF were highly influenced by dissolved oxygen and temperature. Also, pollutant removal pathways carried out by specific bacterial families and microbial interactions were elucidated. This study provides a detailed description of the bacterial communities present in a decentralized WWTP located in a subtropical region.
Highlights
IntroductionAnaerobic bioreactors (ARs) and constructed wetlands (CWs) are treatment stages that are commonly used in decentralized systems [3]; these units are classified as passive technologies, since they require low energy consumption and maintenance and operational costs [3,4]
These results indicate that efficient anaerobic degradation occurs in the anaerobic stages (ST and Up-flow anaerobic filters (UAFs)), in agreement with the dissolved oxygen (DO) and temperature levels measured in these stages (Table 1), which are optimal for anaerobic reactors [46]
The DO concentrations measured in the horizontal-flow constructed wetlands (HFCWs) were low but were higher than those observed for the anaerobic stages (Table 1)
Summary
Anaerobic bioreactors (ARs) and constructed wetlands (CWs) are treatment stages that are commonly used in decentralized systems [3]; these units are classified as passive technologies, since they require low energy consumption and maintenance and operational costs [3,4]. Systems consisting of ARs followed by CWs are known to be an efficient combination for wastewater treatment [3,12] These configurations present advantages such as low energy requirements, low operational costs, and low sludge generation [13]. By incorporating an AR, such as a ST and UAF, as a pretreatment stage before wastewater is fed into a CW can significantly reduce the solid particles and organic matter levels [3,4], while the CW is useful in reducing nutrient loads in wastewater after anaerobic pre-treatment [14]
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