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Several chemical and microbiological parameters were tested during a 140 day period to characterize the stabilization process of a paper processing wastewater sludge composted with straw. The parameters indicate two months as a suitable period for the establishment of an equilibrium in the decay process, although some parameters show that compost reaches an acceptable level of stability in 30 days. Evaluation of stability in the end-product is difficult to define by only one parameter, whether it be chemical or biological.

Carbon mineralization and microbial biomass content of wheat straw (WS), pig slurry (PS) and their mixture (WSPS), either intact or with extraction of soluble substances (-SS) or soluble substances plus hemicellulose (-SSH), added to soil, were monitored over 230 days in a laboratory incubation experiment. The WSPS showed a CO2 release of up to 23% above that predicted by summing the CO2 evolved from WS and PS. Of the several kinetic models tested to describe the mineralization process, a double exponential model best described the C mineralization of all the materials, both intact and with extractions. The extraction of the labile substances from WS, PS and WSPS lowered the values of the rapidly mineralizable C and of the amount of microbial biomass. The organic fraction of WS was found to be almost completely represented by mineralizable carbon, while PS and WSPS showed only 62% of mineralizable carbon. In spite of this, after 8 months, about half of the initial amount of the organic C in the intact residues still remained unmineralized