Towards enhanced treatment of piggery biogas slurry in tidal flow constructed wetlands by regulation of hydraulic retention time

Livestock farming wastewater is complex in composition, containing a large number of pathogenic microorganisms and ammonia nitrogen (NH4+-N) with high concentration, which makes biological treatment difficult. In this study, tidal flow constructed wetlands were applied to treat piggery biogas slurry. Compared with a continuous flow constructed wetland, two tidal influent modes (hydraulic retention time: idling time for 12 h:12 h and 20 h:4 h, namely TFCW1 and TFCW2) were set up to explore treatment performances on chromaticity, turbidity, chemical oxygen demand (COD), total nitrogen (TN), NH4+-N and total phosphorus (TP), and determine the relationship between nitrogen transfer and dissolved oxygen (DO) contents. The fluorescence spectra of dissolved organic matter (DOM) in constructed wetlands were analyzed by excitation-emission matrix spectroscopy-parallel factor analysis to reveal its composition and content of humus. The results showed that tidal flow constructed wetlands obviously improved the removal of chromaticity and turbidity by 44.6% and 10.1%, in comparison to continuous flow constructed wetland. TFCW2 had enhanced reduction on pollutants, with removal efficiencies of COD, NH4+-N and TP at 78.6%, 82.1% and 84.0%, respectively. Furthermore, three fluorescent components were detected in TFCW2, and the value of Fmax was increased in the effluent, among which the Fmax of C3 (Humic-like substances) increased obviously from 0.320 to 9.39. The ratio of C2 (Aquatic fulvic acids) to C3 showed a slow increasing trend in the daily outflow of TFCW2, while C1 (dark-incubation substances) remained relatively constant. Fluorescence index (FI), humification index (HIX) and biological index (BIX) increased from 1.35 to 2.09, 0.210–1.51, and 0.240–1.05, respectively, showing that the effluent from the tidal flow constructed wetland had a high microbial activity. This study provided a new insight for the advanced treatment of livestock biogas slurry with a high NH4+-N content.