To determine total nitrogen, samples drawn from the activated sludge containing keratin were added with 40 g L−1 of potassium persulfate, 15 g L−1 of sodium hydroxide and the samples were digested at 120 °C for 40 min. The digestion was terminated by adding 1 ml HCl and the absorbance of the solution at 220 and 275 nm were measured on a spectrophotometer.