Recent studies have demonstrated that sparging CO2 microbubbles directly into the digesters of food waste increased methane yields substantially. Anaerobic digestion produces ammonia from organic nitrogen, and anaerobic digesters can benefit from the presence of ammonia, which acts as a buffer. At higher concentrations of ammonia, however, the digestion process may be inhibited.
This work provides an insight into the application of microbubble stripping mediated by Desai- Zimmerman fluidic oscillators (DZFO) in anaerobic digestion, particularly the ones with microbubbles CO2 sparging to prevent ammonia inhibition. The primary objective is to develop low-cost, low-energy alternatives that can be used to reduce ammonia levels in the anaerobic digestion of food waste with a low C/N value, which was sparged with CO2 microbubbles. Microbubble stripping engendered by an energy-efficient fluidic oscillator, DZFO was utilized to strip ammonia from the digestate. In this way, ammonia inhibition could be alleviated.
Initially, three designs of Desai-Zimmerman (DZ) stripping rigs, including DZ lab-scale, recirculating and continuous DZ rigs, were evaluated for their potential to strip ammonia from highly concentrated liquids. A fluidic oscillator (DZFO) was used to generate microbubbles, and stripping was accomplished within a short period, at a moderate temperature, without adjusting the pH level. It was observed that the KLa values for these rigs were higher than those obtained in previous studies using conventional fine bubbles with a diameter typically ranging from 1to 4mm.
Additionally, the present study revealed that microbubbles of CO2 injected into the AD is unlikely to reduce total ammonia nitrogen (TAN) from the system. By seeding the system with microbubble CO2, the pH was lowered, which decreased the free ammonia nitrogen fraction (FAN). However, the concentration of TAN in the system remained unchanged. Hence, nitrogen gas was chosen as the stripping gas in the simultaneous ammonia removal from anaerobic digester in this work.
In the final experiment, DZ lab-scale stripping rig was coupled with AD using nitrogen as a stripping gas at the flow rate of 1 LPM and temperature of 70 oC. A 25 mL digestate was used for each stripping run with a duration of two minutes. During this process, 2 L of digestate was removed daily from the anaerobic digestion sparged (seeded) with CO2 microbubbles, and the digestate was stripped in a batch operation before returning it to the digester immediately. For an unoptimized process, batch microbubble stripping successfully reduced TAN concentration by 51.4% while increasing methane production by 45.9%.
