Roles of vegetation, flow type and filled depth on livestock wastewater treatment through multi-level mineralized refuse-based constructed wetlands

Abstract

The treatment efficiencies of mineralized refuse-based constructed wetlands (MRCWs) for livestock wastewater have been investigated through a series of laboratory-scale experiments over a period of 180 days from summer to winter. Four wetland plant species ( Cyperus alternifolius, Phragmites spp., Canna indica and Acorus calamus) were grown as monocultures in individual MRCWs and their effect on wastewater treatment by mineralized refuse particles was monitored. It was found that the presence of vegetation caused only minor variations in the efficiency of removing chemical oxygen demand (COD Cr), suspended solids (SS), ammonia nitrogen (NH 3-N), total phosphorus (TP) and total nitrogen (TN) from livestock wastewater. Single-stage MRCWs were integrated to organize combined CWs system to investigate the inlet flow type on pollutants removal efficiency. Three-level/stage MRCWs were then designed to be used without vegetation to treat livestock wastewater by the horizontal-flow (ladder-type system) and vertical-flow (tower-type system). The removal rates of pollutants were observed to be directly proportional to the number of levels or stages in the MRCWs. The average removal rates for three-level tower-type MRCWs were 62.4 ± 11.2% ( P < 0.01) for COD Cr, 86.5 ± 6.7% for SS, and >95% for NH 3-N and for TP under vertical flow that are slightly higher than the removal rates for the three-stage ladder-type horizontal flow MRCWs in long-term operation. The high removal rate of COD Cr and strong nitrification ability of MRCWs result in a much lower stoichiometric ratio of COD Cr:nitrate nitrogen (NO 3 −-N) at the outlets, which caused a shortage of carbon for subsequent denitrification. A leaching column experiment was conducted with 150 cm-deep mineralized refuse to evaluate its denitrification effect on a synthetic solution having a low ratio of COD Cr:NO 3 −-N. More than 95% of the NO 3 −-N content was removed for column depths between 120 cm and 150 cm, the mineralized refuse particles themselves serving as a carbon source. The integrated system of anaerobic storage tank, multi-level/stage MRCWs and an MR-filled column in series is therefore recommended for livestock wastewater treatment, with C. alternifolius and/or C. indica being planted.