Environmental impact assessment of tannin-based coagulants production from chestnut shells

The increasing demand for freshwater faces challenges and assessing the environmental sustainability of wastewater treatment is crucial. The coagulation-flocculation-sedimentation process is common in water treatment plants, with chemical coagulants playing a pivotal role. However, conventional coagulants like aluminum and iron salts present challenges, prompting a search for environmentally friendly alternatives. Natural coagulants from renewable resources are gaining attention for their biodegradable and non-toxic properties. In Portugal, chestnut shell waste provides a sustainable source for tannin-based coagulant production. This study evaluates the environmental impacts caused by producing a natural coagulant derived from chestnut shells using Life Cycle Assessment (LCA). Two functional units were examined. The first (FU₁) focuses on the extraction phase, comparing Solid-Liquid (SLE) and Microwave-Assisted (MAE) extraction methods to produce 100 g of condensed tannins. MAE was identified as the more environmentally friendly option and was used for tannin extraction. The second (FU₂) evaluates the environmental impacts of tannin-based coagulants in treating 1 m³ of river water with spiked kaolin to achieve residual turbidity <1 NTU, comparing its effectiveness with conventional coagulants. LCA scrutinizes critical phases, including sample preparation, tannin extraction, drying, and cationization, identifying freeze-drying as a primary hotspot due to electricity consumption. Natural coagulants offer benefits such as not affecting pH/alkalinity, better cost-benefit ratios, and being non-toxic and non-corrosive.