With the help of the Infrared (IR) spectroscopy, the presence of organic and inorganic components was established by the absorption bands within the IR region in the activated sludge after wastewater treatment. Its IR spectra contained absorption bands of functional groups of aromatic compounds, polysaccharides, peptides, proteins, lipids, aliphatic chains, ester compounds, nitrates, nitramines, nitroso compounds, nitrosamines, nitro compounds, amine salts, ammonium salts, silicates, phosphates PO43-, HPO42-, H2PO4-. When comparing samples of activated sludge obtained by different methods, a difference was revealed in the intensities of the bands within the range of 1500-1400 cm-1, 1236 cm-1, related to the characteristic absorption bands of components containing nitrogen, as well as within the range of 1100-900 cm-1, characteristic for absorption bands of components containing phosphate ions and polysaccharides. The Spectrum 100 Fourier transform spectrometer (Perkin Elmer, USA, 2007) was used to study the composition of activated sludge. The spectra were recorded using an ATR (Attenuated Total Reflectance) accessory in the form of a fine powder in the range 4000-650 cm-1. The Figure 1 shows the IR spectra of two sludge samples 1 and 2. Their spectra have differences in the intensities of the absorption bands. In spectrum 1 the intensity of the peak at 1236 cm-1 (nitrites NO2-) is several times more pronounced than that for sample 2. There is an increase in the intensity of peaks 1450 and 1406 in sample 1, which is perhaps associated with an increase in the content of organic components containing nitrogen by a factor of 2 (Fig. 1) Fig. 1. IR spectra of activated sludge samples 1 and 2 obtained under different conditions.