Abstract
Four subsurface flow constructed wetlands (SFCWs) filled with different substrates including ceramsite, ceramsite+pyrite, ceramsite+ferrous sulfide, and ceramsite+pyrite+ferrous sulfide (labeled as SFCW-S1, SFCW-S2, SFCW-S3, and SFCW-S4) were constructed, and the removal of nitrogen and phosphorus by these SFCWs coupled with intermittent aeration in the front section was discussed. The key findings from different substrate analyses, including nitrification and denitrification rate, enzyme activity, microbial community structure, and the X-ray diffraction, revealed the nitrogen and phosphorus removal mechanism. The results showed that the nitrogen and phosphorus removal efficiency for SFCW-S1 always remained the lowest, and the phosphorus removal efficiency for SFCW-S4 was recorded as the highest one. However, after controlling the dissolved oxygen by intermittent aeration in the front section of SFCWs, the nitrogen and phosphorus removal efficiencies of SFCWs-S2 and S4 became higher than those of SFCW-S1, and SFCW-S3. It was noticed that the pollutants were removed mainly in the front section of the SFCWs. Both precipitation and adsorption on the substrate were the main mechanisms for phosphorus removal. A minute difference of nitrification rate and ammonia monooxygenase activity was observed in the SFCWs' aeration zone. The denitrification rates, nitrate reductase, nitrite reductase, and electron transport system activity for SFCW-S2 and SFCW-S4 were higher than those detected for SFCW-S1 and SFCW-S3 in the non-aerated zone. Proteobacteria was the largest phyla found in the SFCWs. Moreover, Thiobacillus occupied a large proportion found in SFCW-S2, and SFCW-S4, and it played a crucial role in pyrite-driven autotrophic denitrification.
Highlights
Urban inland rivers in China are mostly seasonal rivers, the water quality of these rivers is often affected by the sewage treatment plants effluent intrusion and rainstorm runoff mixing
The determination methods of ammonia monooxygenase (AMO), nitrite reductase (NIR), and nitrate reductase (NAR) activities has been described in the previous study (Yang et al 2020)
As-obtained results from one-way ANOVA showed that there was no significant difference found for CODCr removal efficiency for all subsurface flow constructed wetlands (SFCWs) before and after aeration (p>0.05).This endorsed that the substrates and aeration have little effect on the CODCr removal by SFCWs series
Summary
Urban inland rivers in China are mostly seasonal rivers, the water quality of these rivers is often affected by the sewage treatment plants effluent intrusion and rainstorm runoff mixing. As a result, these seasonal rivers gradually form black and smelly water bodies, called low pollution water, which is a great threat to ecological environment sustainability and human water safety concerns. In the case of application of the CWs for low pollution water purification, the C/N ratio for most low pollution water is lower than the value reported above, which makes it challenging to meet the carbon source requirements of heterotrophic denitrification process. Excessive organic carbon availability in the influent will lead to the increase of unused carbon sources in the denitrification process, in which become a cause of secondary pollution spreading, reactor blockage, and high effluent turbidity
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