Making waves: Reevaluating iron dosing for carbon recovery in mainstream wastewater treatment system

Wastewater treatment plants face significant challenges in shifting from energy-intensive operations to carbon-neutral, energy-efficient systems. One promising strategy is the iron-enhanced primary treatment process (Fe-CEPT), which focuses on capturing organic carbon for energy recovery rather than biological oxidation. However, while Fe-CEPT has been implemented in wastewater treatment, its potential effects on downstream processes have often been overlooked. This viewpoint takes a comprehensive look at iron dosing for carbon recovery in mainstream wastewater treatment systems. Fe-CEPT has proven effective at capturing particulate organics and phosphorus. However, it is less successful in removing soluble organic carbon. Additionally, the high iron content in sludge, typically between 100 to 200 mg Fe/g SS, has been shown to severely inhibit methane production. This finding contrasts with earlier studies that suggested iron could enhance methane production. It was found the elevated iron levels bind around 20 % of the carbon in the sludge, limiting its bioavailability. These findings indicate that coupling Fe-CEPT with anaerobic digestion may not be an effective method for carbon recovery. A more promising approach that involves limiting iron dosing to less than 10 mg Fe/L in a high-rate activated sludge (HRAS) system is proposed. This strategy combines the benefits of iron dosing and HRAS system, offering a potential pathway to enhance carbon recovery, improve phosphorus management, and reduce the environmental impact of wastewater treatment processes.