Characterization of denitrifying granular sludge with and without the addition of external carbon source

► The denitrification rate was five times lower without external electron donor. ► The nitrite removal rate was similar with or without external electron donor. ► Nitrate without external electron donor led to a slight drop in microprofiles pH. ► Nitrite without external electron donor led to an increase in microprofiles pH. ► Large increases in pH were observed when acetate was used with nitrate or nitrite.