Chemical Pretreatment of Spent Coffee Grounds for Biomethane Production: A Review of Biochemical Methane Potential Assays

Abstract

Spent coffee grounds (SCG) constitute a rapidly growing organic waste stream with a complex lignocellulosic matrix that restricts microbial degradation during anaerobic digestion (AD). This review evaluates the effects of oxidative, alkaline, and acidic pretreatments on SCG solubilization and biomethanation performance. The methodology involved a comparative synthesis of reported batch mesophilic biochemical methane potential (BMP) assays employing distinct substrate-conditioning strategies. Particular attention was given to operational conditions, soluble chemical oxygen demand (SCOD), biogas yield, and methane yield. The synthesized evidence indicates that ambient alkaline treatment using 8% w/w NaOH produced a robust methane yield of 392 mL CH₄/g VS. In comparison, severe thermo-acidic treatment with 1.5% w/w H₂SO₄ at 121°C generated 381.12 mL CH₄/g VS. Treating SCG with 4% w/w H₂O₂ for 24 hours at room temperature increased SCOD by 713.02% relative to untreated SCG, indicating substantial solubilization of previously inaccessible organic matter. This condition produced 642.69 mL biogas/g VS and 345.53 mL CH₄/g VS without elevated temperature or pressure. Overall, pretreatment effectiveness cannot be assessed solely from solubilization rates; chemical dosage, energy use, and inhibitor formation must also be evaluated. Standardized comparative BMP testing and routine toxicity assessments are essential to forecast digester performance and improve sustainable waste valorization accurately.