The study examined the effects of mixing sewage sludges with calcium oxide or brown coal ash and composting the mixtures on the dry matter, organic matter, organic carbon and total nitrogen contents of the materials and their pH values. The parameters under study, except for pH and dry matter, had lower values for mixtures than for fresh sludges, with the effect being stronger for the addition of CaO than ash. Adding CaO resulted also in a stronger alkalisation of sludges and in greater losses of nitrogen. Composting decreased the organic matter, organic carbon and nitrogen contents both of sludges and their mixtures with CaO and ash. The fall in the rate of organic matter mineralisation in the composting process was larger for the CaO addition.

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