Efficiency of Azolla pinnata in Purifying Treated Wastewater in Lebanon via Phytoremediation as a Nature-Based Solution

Abstract

Reusing non-conventional water resources such as treated wastewater (TWW) is being considered in Lebanon, given the increasing water scarcity. Irresponsible wastewater management has caused water pollution to increase to alarming levels in Lebanon. This is compounded by the economic hardships that have forced several wastewater treatment plants (WWTPs) to either shut down their operations or be inconsistent with the treatment level. Given the unsustainable performance of centralized WWTPs and their vulnerability to economic shocks, integrating nature-based solutions such as phytoremediation has become essential. Macrophytes (ferns) have the potential to improve wastewater quality by absorbing nutrients at a low to moderate cost. As such, this study evaluates the potential of growing Azolla pinnata, a floating fern, for the purification of primary, secondary and tertiary TWW in Lebanon through phytoremediation. Three seasons of experiments were conducted to study the temporal variation in the physicochemical properties of water, nutrient removal efficiency, sediment composition, biomass composition and economic feasibility. All nutrients that were considered in this study were reduced in the presence of A. pinnata in TWW, except for nitrates and sodium. The highest nutrient removal efficiencies were observed in the primary TWW, with an average of 97% for ammonium, 88% for orthophosphates and 90% for potassium. Additionally, chemical oxygen demand decreased between 66-86% in the three TWW types. This reduction has been negatively correlated with dissolved oxygen (R= -0.683, p-value=0.000). The results of the phosphorus (P) mass balance has shown that 74% of the P was fixed by Azolla in primary TWW, out of 84% P removal efficiency. In contrast, only an average of 60% and 64% P was absorbed by Azolla in STWW and TTWW out of 100% and 95% P removal efficiency, respectively. This shows that a portion of the P was removed from the water column by sedimentation, rather than being fixed by Azolla, particularly when P concentration in the water is low. The proximate analysis of Azolla revealed that Azolla cultivated in PTWW had a higher % crude protein and % P (24% and 3.4%) as compared to that grown in freshwater (14% and 0.63%). This indicated that the nutritional value of Azolla varies with the variation in the chemical composition of water, along with luxury consumption of P. Although Azolla has a rich nutritional value, the economic assessment has shown little economic savings from its use in animal feed. The thesis revealed that phytoremediation of wastewater with Azolla pinnata is a promising method to improve decentralized wastewater management, and established the groundwork for a more in-depth investigation for nature-based innovations in the wastewater management sector in Lebanon. Further studies on the expansion of this technique, microbial and heavy metals contamination, palatability by different animals, disposal of sediments and the utilization of the Azolla biomass are needed.