Polishing wastewater effluent using plants: floating plants perform better than submerged plants in both nutrient removal and reduction of greenhouse gas emission

https://doi.org/10.2166/wst.2023.203

By Lisanne Hendriks, Alfons J. P. Smolders, Thom van den Brink, Leon P. M. Lamers & Annelies J. Veraart

This blog post provides an introduction to the recently published paper from Water Science & Technology, and highlights some of the key features of the research.

According to the European Water Framework Directive, the wastewater we discharge into the environment today contains excessive concentrations of pollutants like phosphorus and nitrogen. In fact, wastewater effluent contributes substantially to eutrophication of, and high greenhouse gas emissions from natural waterbodies. Wastewater treatment facilities need to address this, but the conventional techniques are either too expensive or inadequate.

Purifying wastewater by plants is well-known, and is already happening in nature. Yet how to use these plants as efficient effluent polishers while ensuring low greenhouse gas emissions remained to be studied. In particular, the role of plant growth strategies – either floating or submerged – remained to be tested. That is why we performed an experiment with two types of aquatic plants: submerged plants that cover the whole water column, and floating plants that only cover the top layer of the water. Both plant types have different growth rates, and different characteristics in how they change the conditions in the water column – such as oxygen concentration – and we therefore expected differences in how they can polish effluent and alter greenhouse gas fluxes.

We grew two submerged plant species and two floating plant species on wastewater effluent in the greenhouse facility at Radboud University, The Netherlands, and measured nutrient concentrations, greenhouse gas fluxes and their growth over time, for a period of two weeks. We found that the floating plants performed best on all three aspects. They removed all phosphorus from the water column, and almost all nitrogen as well. Also, instead of emitting greenhouse gasses, they took up a lot of CO2 (up to 1000 mg/m2/day), which is an important finding, because CO2 reduction is a key objective of the Climate Agreement, to which water authorities are required to contribute.

An additional benefit of plant-mediated effluent polishing is that the phosphorus and other nutrients taken up by the plants, which need these nutrients for their growth, can be recovered for re-use after harvesting. Because phosphorus is a finite nutrient and is needed for products such as fertiliser, we would benefit from recovering it from wastewater, which contains plenty of phosphorus. We need more research on the applicability of these plants in new products, such as potting soil, but are hopeful that the plants could be used in a way that could contribute to a circular economy.

The research is not yet complete, but the first results are promising. Some floating plants are better in taking up nitrogen, others take up more phosphorus. We are now studying the best combination of plants and the right order in which to place them to maximize effluent polishing with the lowest greenhouse gas emissions, or even preferably with the highest greenhouse gas uptake. The next challenge is to learn how the plants can be integrated in the wastewater treatment process. Since this effluent polishing technique takes up quite some space, we could think about vertical farming. However, this is something for the future; let’s first expand our research on a smaller scale.