The aim of the study was to investigate biodegradability of fat industry waste, characterized by very high contents of fatty substances (57%) and the re-sulting greasy consistency. The composting process was run under aerobic conditions using the compost heap method in three independent, simultaneously prepared compost heaps. A necessary pre-condition to ensure the effectiveness of the conducted process was to improve physical properties of the above mentioned waste by selecting an appropriate structure-forming material, providing aerobic conditions for the adequate functioning of microorganisms responsible for de-composition of fatty substances. In the conducted investigations it was essential to determine adequate moisture content of compost heaps at the beginning of the process (ranging from 60 to 75%) in order to provide appropriate water activity for the development of microorganisms and initiation of metabolite production by mesophilous bacteria, which when oxidized initiated generation of large amounts of heat, and thus a rapid increase in temperature in heaps already at the third day to 40oC, and next reaching the maximum temperature (60oC) at day 12, and the maintenance of the temperature range of 50–57oC for the next 15 days. De-termined optimal moisture contents and proportions of component materials for compost heaps contributed additionally to the determination of appropriate pH inside compost heaps to be within the range of 6.5 to 7.5, which at the adverse value pH = 3 of the biodegradable waste, i.e. oiled fuller's earth, constituted a serious problem at the beginning of the process. The intensive mineralization process, manifested in an increase in temperature, resulted in quantitative changes in carbon and total nitrogen, leading in turn to a regular decrease in the values of the C:N ratio ration from 29:1 to 15:1 was reduced. The content of fat substances was 9,1 g•kg-1d.m in mature compost. That concentration of fat not reduce or-ganic value of prepared compost.
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