Struvite crystallization eliminates ammonia inhibition in thermal hydrolysis of waste activated sludge: Role of Mg/P ratio

Thermal hydrolysis (TH) is a promising sludge pretreatment technology, but the ammonia it produces can be hazardous to subsequent anaerobic digestion (AD). This study assessed the potential of using struvite crystallization to mitigate ammonia inhibition in the AD process and examined the impact of the Mg/P ratio. The Mg/P ratio of 1.0 effectively reduced the ammonia concentration to 409 ± 22 mg/L, although methane production decreased by 14 %. Reducing the Mg/P ratio to 0.6 resulted in an 11 % increase in methane production. The study revealed that an excess of Mg²⁺ hindered sludge solubilization and inhibited the AD process by reducing enzyme activities, which led to a microbial community detrimental to methane production. This study provides a new idea to alleviate the ammonia inhibition of sludge AD, which is of great scientific value and practical significance for applying of struvite crystallization to TH.