Performance of constructed wetlands with different substrates for the treated ef ﬂ uent from municipal sewage plants

Constructed wetlands (CWs) are effective as an advanced treatment process for the treated ef ﬂ uent of municipal wastewater plants. An appropriate substrate, suitable macrophytes, and proper operation are crucial for pollutant abatement. In this research, three subsurface ﬂ ow CWs with various substrates were investigated. Pollutants abatement ef ﬁ ciency under various operational schemes were analyzed. The results showed that the satisfactory hydraulic loading rate was 0.25 m 3 /(m 2 ·d). When the C/N ratio of in ﬂ uent was adjusted to 5.87 by adding a carbon source, the denitri ﬁ cation and dephosphorization ef ﬁ ciency would be improved, with 7 – 8 mg/L for total nitrogen (TN) and 0.4 mg/L for total phosphorus (TP) in the ef ﬂ uent, which can achieve the Class 1A Discharge Standard for discharge to natural waterways in China. A greater depth of submersion for the substrate layer resulted in a more conducive environment for the abatement of nitrogen substances. However, a 40-cm depth of submersion in CWs results in better removal ef ﬁ ciency of TN and TP. A plastic ring substrate (PRS) contains biological enzyme promoter formula, which was conducive to nitrifying and denitrifying bacteria. The bio ﬁ lm af ﬁ nity and coordination with plants made the PRS more effective than the other two substrates, especially for NO 3 – -N and TN abatement ef ﬁ ciency.


INTRODUCTION
The treated effluent from a municipal sewage plant may contain pollutants that contribute to eutrophication of waterbodies when the effluent is directly discharged. Different countries have different requirements for treated effluent discharged into natural rivers, as shown in Table 1 Figure 1).

Substrate selection
The substrate was an important part of CWs, which could abate pollutants by physical filtration, chemical reaction, and biodegradation. Substrates had a great impact on the treatment effect because of its features (Herrera-Melián et al. ). Three substrates were selected to explore the influence for the pollutant abatement efficiency in CWs, including a combinatorial substrate.
(1) Plastic ring substrate (PRS) in CW No.1: PRS is a kind of biofilm carrier that is widely applied in biological Ammonium-nitrogen (NH 4 -N) 5 8 10 n/a n/a n/a n/a contact oxidation processes for wastewater (Zhao et al. ). PRS is made of polymer materials with stable properties, large specific surface area, and excellent hydrophilicity, which was beneficial for colonization of a biofilm. As shown in Figure 2(  of 8.67% carbon elements that remained after firing and had an internal aperture of 22.0 nm. Zeolite was no longer used as an auxiliary because of its huge pore area. According to the effluent discharge standard, the CW with a proper substrate and corresponding operational procedures will be demonstrated.

Research methods
According to the factors that may influence the pollutant abatement efficiency, the parameters investigated in the research were as follows:

Hydraulic loading rate
Hydraulic loading rate (HLR) was an important parameter to characterize the unit area treatment efficiency of CW Thus, it was necessary to test the carbon-nitrogen (C/N) ratio of the influent and assess whether it could satisfy the denitrification and dephosphorization process before further treatment through the CWs. If C/N was too low, a carbon source was usually added to improve the C/N ratio and ensure denitrification and dephosphorization efficiency.
However, the appropriate C/N ratio adjusted is crucial for the efficiency of the removal process.

Submerged depth of substrate
The design of the submerged layer followed the non- ning was stable. The optimal HLR corresponding to each system was determined. Among the four-level HLR, the removal rate (Figure 3) was highest when the HLR was 0. In this study, three substrates were carriers of microorganisms and resulted in COD abatement efficiency of 95%.
The efficiency of the third device with fly ash substrate (FLAS) was slightly higher than that of the other two for the abatement of NH 4 þ -N, NO 3 --N, and TN. For the nitrogen in the water, the removal process was relatively complex.
The organic nitrogen in the water must be transformed into NH 4 þ -N in the aerobic condition by ammoniating bacteria, and then NH 4 þ -N could be oxidized into NO 2 --N by nitrous acid bacteria, followed by further oxidation to CW with PRS for the treatment of NH 4 þ -N was slightly worse than the other two treatments in terms of nitrogen abatement, but for the treatment effect of NO 3 --N it was slightly better than the other two and overall advantageous for TN abatement. More denitrifying bacteria produced higher removal efficiency for TN.
The CW with FLAS performed the worst for pollutant abatement efficiency. Even when the C/N ratio was adjusted Compared with a 20-cm depth, the removal rate of NH 4 þ -N was 10% greater than removal with a 60-cm depth. The

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In conclusion, the denitrogen efficiency trend and the dephosphorization efficiency contradicted each other in terms of the submerged depth of substrate in CWs. In practice, a submerged depth of 40 cm in height ensured high removal efficiency for both nitrogen and phosphorus.

CONCLUSIONS
Three CWs with PRS, FLAZS and FLAS as substrates were investigated separately to explore the performance of the treating effluent from municipal plants. The pollutants abatement efficiency under different HLR and various C/N ratios of the influent and diverse submerged depth of substrate were discussed to determine the optimal substrate and corresponding operational conditions. The results showed that (1) Three CWs achieved a better pollutant efficiency when the HLR was 0.25 m 3 /(m 2 ·d).
(2) When the C/N ratio of influent was adjusted to 5.87 by additional carbon source, the denitrification and dephosphorization efficiency would improve, with 7-8 mg/L for TN and 0.4 mg/L for TP in the effluent, which can satisfy the Class 1A Discharge Standard for natural waterways in China.
(3) A greater depth of submergence for a substrate layer in CWs resulted in the greater abatement of nitrogen substances. However, removal efficiency of phosphorus was optimal with submerging depth of 40 cm.
(4) Because of the biological enzyme promoter formula contained in PRS, it was the substrate that better promoted habitats for bacteria that participated in the pollutant abatement processes. Biofilm affinity of PRS and coordination with plants was better than the other two substrates, especially for the abatement efficiency of