Dual application of Azolla- and Wolffia-based floating constructed wetland in sewage treatment and phytomass production under the circular economy model

There is growing concern over developing a low-cost and energy-efficient on-site wastewater treatment system with multiple benefits. In this regard, floating construction wetlands (FCWs) could serve as a sustainable solution. This study assesses the suitability of FCWs planted with Wolffia and Azolla sp. for treating domestic sewage within a meso-scale treatment setup. Sewage was introduced into these setups, and changes in wastewater characteristics (pH, nitrate N, NO 3 - ; ammonium N, NH 4 + , phosphate, PO 4 -3 ; sulphate, SO 4 -2 ; COD, chemical oxygen demand; and BOD, biochemical oxygen demand) were examined after each 7-day interval until 28 -day period. Results revealed a significant ( p < 0.05, p <0.001) reduction (%) in COD (70.49 – 81.04), BOD (85.45 – 89.09), NH 4 + -N (80.41 – 86.56), NO 3 - -N (83.53 – 91.18), PO 4 -3 (79.54 – 82.08), and SO 4 -2 (77.16 – 83.96). Overall, Wolffia -based systems exhibited high nutrient removals, while the Azolla -based setup showed efficient COD reduction. These differences are attributed to variations in frond morphology, root zone structures, surface area, and metabolic traits of the wetland plants studied. The mechanisms likely involved in sewage treatment by these FCWs include plant uptake, nitrification-denitrification, sedimentation, adsorption, and precipitation. No toxic effect of sewage on the photopigments of the studied species was observed in the current investigation. Harvested phytomass displayed high content of protein (35.10 – 52.40% DM), total carbohydrates (31.12 – 35.80% DM), and lipids (5.88 – 6.45% DM), along with other valuable plant nutrients such as organic C, total N, phosphorus, potassium, and micronutrients (Fe, Zn, Mg, and Ca). These findings suggest its potential use as a feedstock for various industrial applications within a waste-to-wealth approach. In conclusion, FCWs can be effectively utilized for a dual purpose: low-cost wastewater treatment and phytomass cultivation for other industrial uses. Future research could explore the impact of toxic chemicals and seasonal variations on the performance of Wolffia and Azolla in FCWs. • The potential of floating constructed wetlands (FCWs) planted with Wolffia and Azolla in sewage treatment was investigated. • FCWs showed significant removal (%) of COD (70.49 – 81.04), BOD (85.45 – 89.09), NH 4 + -N (80.41 – 86.56). • Phytomass from FCWs exhibited the maximum content (% DM) of protein (52.40), carbohydrates (35.80), and lipid (6.45).