New insight into volatile sulfur compounds conversion in anaerobic digestion of excess sludge: Influence of free ammonia nitrogen and thermal hydrolysis pretreatment

Abstract

Volatile sulfur compounds (VSCs) are the main toxic components generated simultaneously with biogas in anaerobic digestion (AD) of sludge. The differences of chemical and biological processes involved in the VSCs conversion between conventional AD and thermal hydrolysis pretreated high solid AD (THP-HSAD), widely applied in recent years, have yet to be fully elucidated. This study evaluated the effect of solid content, initial pH, and thermal hydrolysis on the VSCs conversion in AD. This study had evidenced that neither the solids content nor the pH adjustment affects the VSCs conversion. It was found that free ammonia nitrogen concentration, which increased (>500 mg/L) in HSAD, directly inhibited the enzymatic activity of cysteine lyase (R2 = 0.87, P < 0.01) and methionine lyase (R2 = 0.89, P < 0.01), thereby affecting the VSCs conversion. Thermal hydrolysis pretreatment (THP) had no consequential impact on the key enzymes involved in the VSCs conversion. Instead, the particle size of excess sludge reduced by 66.05 ± 6.66%. It was postulated that THP may reduce the sludge particle size, increasing the contact of sulfur compounds with invertase, thus favoring VSCs conversion. These findings contribute to a better understanding of the mechanisms and control measures of the VSCs conversion during the AD of sludge.