Comparison study on ammonia recovery from anaerobic digestion slurry by different biochar: Focusing on the effect of feedstock, pyrolysis temperature, and particle size

Abstract

Treating anaerobic digestion (AD) slurry as industrial wastewater not only consumes a significant amount of energy but also wastes its inherent abundant nutrients, particularly the high ammonium content. Ammonia recovery from AD slurry has attracted attention in recent years and biochar has tentatively been used to adsorb ammonia nitrogen. However, most of the previous studies used pure ammonium chloride solution to simulate ammonia-rich wastewater and ignored the influence of other components. Furthermore, how the physico-chemical properties of biochar influence the adsorption performance of ammonia in AD slurry remains unknown. Therefore, this study focused on the investigation of the adsorption behavior of different types of biochar to ammonia nitrogen in AD slurry. Biochar generated from rice straw, coconut shell, and wood shaving under pyrolysis temperatures of 300 °C, 500 °C, and 700 °C were used to adsorb ammonia from food waste (FW) AD slurry, and five different particle sizes of biochar were also tested. The results showed that biochar derived from rice straw (up to 9.44 mg/g) and coconut shell (up to 8.86 mg/g) had higher ammonium adsorption capacity than biochar derived from wood shaving (up to 5.13 mg/g). Moreover, low pyrolysis temperature resulted in high adsorption capacity, while particle size and surface area of the biochar were not the critical factors determining the adsorption capacity. The correlation results demonstrated that the H/C (aromaticity), O/C (hydrophilicity), pH, electrical conductivity (EC), and ash content influenced the adsorption significantly. Based on the kinetics model results, it seems that physical adsorption was the main adsorption mechanism, while ion exchange and reaction with function groups also contributed to the adsorption. Moreover, the lower adsorption capacity was observed in this study where the real FW AD slurry was used as compared to other studies where pure ammonium chloride solution was used, which implied that microorganisms in FW AD slurry may colonize on the surface or pores of biochar, resulting in a negative effect on the adsorption capacity of biochar. The results derived from this study provided technical support for ammonia recovery of AD slurry.