Study on water extract of sewage sludge composts

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Abstract
Quantitative changes in the components of the water extracts from the mixtures of sludge and organic residues during the composting and maturation processes were studied in order to identify parameters that can be used as indicators of compost maturity. The pH of the water extracts increased during the first 40 days of composting and then decreased. The EC increased in almost all the samples during composting. There was a significant decrease in the water soluble organic matter, water soluble C, organic N, and carbohydrate contents during composting and maturation. These processes were also responsible for a decrease in the amount of phytotoxic substances such as ammonium, phenols, and organic acids of low molecular weight. The organic matter content of the water extract, the water soluble carbon/water soluble nitrogen ratio, the sugar content and the biodegradability index can all be used as indices of maturity since, in addition to the decrease of the values with time, similar values were obtained in the four cases studied at the end of the composting and maturation processes. The water soluble carbon/organic nitrogen ratio in the extracts can also be considered as a good index of compost maturity since this ratio reached similar values ( < 5) at the end of the processes in all composts. Spectroscopic measurements of the alkaline extracts were not suitable for characterizing alkali soluble organic matter.