Comparative Studies of Vegetated and Non–vegetated Submerged–flow Wetlands Treating Primary Lagoon Effluent

The submerged–flow (SF) wetland concept offers high organics and solids removal at relatively low cost for construction, operation, and maintenance. In this comparative study, vegetated and non–vegetated SF wetlands were investigated for their ability to treat primary lagoon effluent. The experimental design was comprised of three vegetated and three non–vegetated SF wetland beds (3 m Ü 1 m Ü 0.5 m) operated in a semi–continuous–flow mode, which were fed every 12 hours with a design 5–day residence time. The wetlands were packed with 19–mm diameter trap rock and planted with bulrush (Scirpus validus). The BOD5 and TSS removals throughout the two–year monitoring period were high (up to 98%) for both vegetated and non–vegetated wetlands and tended to follow seasonal variations. Vegetation slightly enhanced the reduction of biodegradable organic matter and suspended solids within the SF environment, although this enhancement was statistically insignificant (P > 0.05). The annual average mass removal for ammonia nitrogen in vegetated wetland beds was 3.3 kg ha–1 d–1 (up to 95%). The nitrification process in vegetated wetland beds was significantly more pronounced (P < 0.05) than non–vegetated beds. The lack of measurable dissolved oxygen in the non–vegetated wetlands likely restricted the nitrification process. The dissolved phosphorus reduction varied from month to month depending on seasonal variations of plant growth, ranging from 27% to 100% in vegetated wetland beds and from no removal to 66% in non–vegetated beds. The removal of dissolved phosphorus in vegetated beds was significantly higher than in non–vegetated wetland beds (P < 0.05). Based on the results of this study, vegetation significantly contributed to the reduction of nutrients, specifically ammonia nitrogen and dissolved phosphorus, from SF wetlands, but not for BOD5 and TSS.