Removal of nutrients in constructed wetlands for wastewater treatment through plant harvesting – Biomass and load matter the most

Abstract

Uptake of nutrients as a removal process in constructed wetlands with horizontal subsurface flow (HF CWs) treating municipal wastewater is usually considered negligible. However, there is very low number of results which would confirm this assumption. The amount of nutrients sequestered in the aboveground biomass, and thus available for harvesting, is called standing stock and it is determined by both nutrient concentration in the biomass and biomass itself. The common values of standing stock vary in the range of 30–80 g N m−2 and 2–6 g P m−2 while inflow loadings of municipal wastewater are around 950 g N m−2 yr−1 and 350 g P m−2 yr−1. Under these conditions, the standing stock represents less than 10% of the nitrogen load and 5% of phosphorus load. Removal of nitrogen and phosphorus in HF CWs is usually low and therefore, the percentage of removed load by plant uptake is higher. Under the conditions of low loadings, i.e., less that 500 (200) g N m−2 yr−1 or 100 (50) g P m−2 yr−1, the portion of removed nitrogen or phosphorus could be substantially higher. In our study, the combination of high standing stocks (> 100 g N m−2 and 10 g P m−2) supported primarily by high plant biomass and low inflow loadings was responsible for removal of up to 62% of nitrogen and 58% of phosphorus. The results of our study carried out in four HF CWs in the Czech Republic indicate that the direct role of plants in nutrient removal in HF CWs may be underestimated especially under conditions of low inflow loading and high plant biomass.