Towards a better understanding concerning carbon to nitrogen ratio and the carbon source in aerobic granular sludge

This study investigated the effects of the carbon-to-nitrogen (C/N) ratio on aerobic granular sludge (AGS) using two identical biological reactors (C/N = 5 and C/N = 10). After 84 days of operation, the reactors' organic substrate was switched from a simple synthetic carbon source (acetate and propionate) to a more complex carbon source (50% volatile fatty acids and 50% fermentable substrate). The C/N = 10 resulted in smaller, uniform granules (200 < Diameter(D)(mu m) < 500) with improved settling characteristics (sludge volumetric index - SVI5 < 30 mL gTSS L-1). The nitrogen compounds in the influent influenced the protein/polysaccharide ratio (PN/PS), with higher PS content observed under C/N = 5 and lower PS content under C/N = 10. Both C/N ratios achieved high carbon removal rates (similar to 90%), but ammonia oxidation was lower under C/N = 5 (up to 45%) due to alkalinity deficiency. Insufficient alkalinity and higher nitrogen compounds in the influent negatively affected phosphorus removal by reducing the activity of phosphorus-accumulating organisms and promoting the growth of glycogen-accumulating organisms. The complexity of the organic substrate hindered microbial activities through intermediate products from glucose and amino acid metabolic pathways. EPS-producing denitrifying organisms (Thauera and Xanthomonas) were favored in C/N = 5, while at C/N = 10, organisms associated with granular biomass stability (Zoogloea and Flavobacterium) were selected. The addition of glucose in the effluent led to a significant increase in fermentative organisms (Aeromonadaceae). This study emphasizes the crucial role of the C/N ratio and organic substrate complexity in achieving optimal performance in AGS systems..