Performance Evaluation of a Full‐Scale Anaerobic Internal Circulation Reactor Treating Calcium Rich Dairy Industry Wastewater

ABSTRACT Calcium, an often‐overlooked divalent cation, plays a crucial but complex role in anaerobic digestion (AD). Moderate calcium concentrations positively influence microbial activity, biomass structure, the granulation process and overall process stability. While excessive calcium can precipitate as calcium carbonate, affecting reactor performance and sludge settling. This study monitored and evaluated the performance of a large‐scale internal circulation (IC) reactor, treating calcium‐rich (400–600 mg L −1 ) dairy wastewater, under seasonal variability. It was found that biogas production and methane yield in the second year of operation were 17% and 25% lower, respectively, compared with the first year. This decline was attributed to the inactivation of biomass due to calcification and calcium scaling buildup on reactor walls and pipes, despite routine sludge replacement. Elemental analysis and microscopic observations confirmed the progressive accumulation of calcium phosphate and calcium carbonate precipitates in the anaerobic sludge and on reactor walls.