Smart and sustainable production of fertilizer by electrochromic self-perception recovery of ammonium from wastewater

Ammonium-containing wastewater represents both an environmental burden and a wasted nitrogen resource. Here we develop an electrochromic self-perception recovery (ECSPR) system that integrates selective intercalation, intrinsic electrochromism and data-driven control for recovering ammonium from wastewater to produce value-added fertilizer in a smart manner. The system employs a copper hexacyanoferrate (CuHCF) electrode capable of selective NH4+ intercalation (selectivity coefficient of 5–28 versus other cations) with high capacity (2.2–2.7 mmol g−1). Fe/Cu redox transitions couple charge transfer with reversible color switching, which enables in situ visible perception of adsorption-desorption states. By embedding image recognition and machine learning algorithms, the captured electrode images are converted into quantitative adsorption states, which the controller uses as the decision variable to schedule adsorption-desorption and route flows in real time. In continuous-flow operation with actual swine wastewater, the ECSPR system achieves cumulative recovery efficiency of 90.2% over 10 cycles with second-scale robust self-feedback regulation, accounting for a low energy consumption of 4.9 kWh kg−1 N. Liquid ammonium sulfate fertilizer can be produced directly with comparable agronomic performance to commercial products, indicated by the similar shoot biomass, leaf development, and leaf nitrogen content in tatsoi and bok choy in pot experiments. Furthermore, the environmental benefits and economic feasibility are evidenced by life-cycle and techno-economic analysis. Overall, this study provides a novel strategy for efficient ammonium recovery and liquid fertilizer production from ammonium-containing wastewater, making wastewater treatment more economic and more sustainable.