^aInstitute of Microbiology and Biotechnology, University of Latvia, Latvia

^bInstitute of Environmental Engineering, Chiao Tung University, Taiwan

^cState Forensic Science Bureau, Ministry of Justice of the Republic of Latvia, Latvia

The appearance, shape and intensity of the nitrate band at 1384 cm^? 1 was well pronounced and evident for a sewage sludge-based compost maturity. An increase of the peak ratio 1384 / 2925 and decrease of 2925 / 1034, 1034 / 1384 correlated with the degree of decomposition. For the composting mixture under study the peak ratios 1034 / 1384 and 1384 / 2925 were more demonstrative. Considering the influence of the composting mixture (components and their ratio) on the shape of the FT-IR spectra the nitrate band at 1384 cm^? 1 can be overlapped by other absorption bands (e.g. lignin bands in 1300–1400 cm^? 1 region) thus appearing in the spectra as a shoulder, and therefore the ratios 1384 / 2925 and 1034 / 1384 become unusable to evaluate the maturity.

FT-IR spectroscopy is a quick and useful method to monitor the composting process; however any particular composting mixture needs preliminary studies of the spectra. Consequently, the most appropriate criteria describing the process (e.g. band ratios, shape and intensity of nitrate band) should be selected and the time / peak ratio curves recorded for a compost mixture under study. Afterwards monitoring of the composting process can be based on the comparison of the controls and definite samples.