Arrested Anaerobic Digestion and Recovery of Value-added Products Through Vacuum-intensified Alkaline Fermentation

In this proof of concept experimental work, it was demonstrated that vacuum-driven fermentation can enable efficient resource recovery from municipal sludge and provide means to arrest anaerobic treatment to maximize extraction of added value products such as ammonia and volatile fatty acids (VFA). The concept relies on applying low vacuum to strip ammonium from fermented sludge, allowing the recovery of ammonium sulfate by chemisorption. Moreover, volatile fatty acids and soluble phosphorus, highly concentrated in the fermentate, can be separated using a pre-conditioning step based on iron flocculation and hydrogen peroxide (to precipitate phosphates) followed by mechanical filtration. This way, nutrient-free soluble carbon and volatile fatty acids can be recovered in the filtrate, while nutrient-rich solids can be further processed by anaerobic digestion or directly used as soil amendment and fertilizers. In this project, a feasibility study, centered on laboratory tests and model-based analysis, to determine the viability of this new process to accelerate the realization of circular economy in the wastewater industry. The study was designed to confirm the impact of pH on the ability to improve VFA production from fermentation and improve the selective separation of ammonia through stripping compared to conventional fermentation or vacuum assisted fermentation without pH adjustment. It also confirmed whether the value-added products recovered by vacuum technologies can maintain high yields and sufficient purity for circular applications (i.e., supplemental carbon for biological nutrient removal, fertilizers for agriculture, carbon-based precursors to bioplastics, etc.). The study demonstrated the improvement of ammonia recovery by more than double compared to vacuum at neutral pH. In addition, VFA yield was improved by 84% compared to the control. Operating in alkaline conditions ceased the methanogenesis process resulting in accumulation of VFA. On the other hand, the elevated pH reduced the undissociated acids concentrations, which may have resulted in improved hydrolysis and fermentation rates.