Effect of variations in the nitrogen loading rate and seasonality on the operation of a free water surface constructed wetland for treatment of swine wastewater

Abstract

The aim of this study was to evaluate the effects of variations in the nitrogen loading rate (NLR) and seasonality on the operational efficiency of a free-water surface constructed wetland (FWS) and on the processes involved in total nitrogen (TN) removal in treating swine wastewater. The system, which operated for 550 days, consisted of a FWS mesocosm inoculated with Typha angustifolia L., using swine wastewater from a storage lagoon as an influent. After operating with nitrogen loading rates (NLRs) of 2.0 to 30.2 kg TN ha⁻¹·d⁻¹, the FWS reduced total nitrogen (TN) concentration by between 21.6 and 51.0%, achieving maximum removal (48.2 ± 3.0%) when the system operated at a NLR below 15.0 kg TN ha⁻¹·d⁻¹. Moreover, operations over 25.0 kg TN ha⁻¹·d⁻¹ resulted in a 50.6% decrease in the maximum FWS efficiency, which may have been related to increased anoxic conditions (< 0.5 mg O₂ L⁻¹; −169.8 ± 70.3 mV) resulting from the high concentration of organic matter in the system (12.3 ± 10.5 g TCOD L⁻¹), which hindered nitrification. Ammonia volatilization is considered the main method to remove TN, with an average value of 14.4 ± 6.5% (3.1–26.2%). Maximum volatilization occurred during the summer (21.5 ± 2.4°C) at an NLR higher than 25 kg TN ha⁻¹·d⁻¹ (26.6%), favored by higher temperatures (17.3–19.7°C), and high NH₄⁺−N (>600.0 9 mg NH₄⁺−N L⁻¹) and pH levels (7.1–7.9). Uptake by plants accounted for 14.9% of the TN removed, with the vegetative peak in summer (height: 105.3 cm; diameter: 2.1 cm) at an NLR of 25.3 ± 0.3 kg TN ha⁻¹·d⁻¹. However, growth decreased to 94.4% at an NLR of over 25.3 ± 0.3 kg TN ha⁻¹·d⁻¹ (>379.9 mg NH₄⁺−N L⁻¹) in autumn (17.4 ± 2.4°C). This was associated with the period of plant senescence and the effects of ammonium phytotoxicity (379.9–624.2 mg NH₄⁺−N L⁻¹) and continued to the end of the study with the complete loss of macrophyte species. Finally, 1.5% of the TN removed was incorporated into the sediments where NH₄⁺−N is the main form of nitrogen, with an accumulative value of 2.6 g m⁻².

Keywords:

• Free water surface constructed wetlands
• swine wastewater
• nitrogen removal
• Typha angustifolia L

Acknowledgments

The authors thank César Contreras of the Yanine Succession for the use of the facilities and also thank M.Sc. Guillermo Pozo for his valuable support.

Funding

This work was supported by CONICYT (Grant 21080123 and 24110086), CONICYT/FONDAP/15130015 and Doctoral Network REDOC.CTA, MINEDUC Grant UCO1202 at University of Concepción.