Rice hull degradation by co-composting with dairy cattle slurry

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Abstract
A liquid dairy cattle slurry was composted using rice hulls as absorbing matrix and bulking agent. The main results obtained were as follows: 1) The structure and size of the rice hulls as well as their absorption capacity, enables to obtain a good composting mixture (moisture: 714.2 g kg−1; C/N ratio: 28.80; bulk density: 0.429 Mg m−3). 2) Compost stability was reached after 58 d of composting, with the following characteristic: (on a dry matter basis), volatile solids 730.0 g kg−1, C/N 18.36, TKN 19.8 g kg−1, K 18.4 g kg−1, P 11.3 g kg−1, humfied carbon 46 J g kg−1, respirometric index 0.88 mg O2 g−1 VS h−1, dry matter yield 573.4 g kg−1, organic matter yield 500.5 g kg−1, water losses 569.9 g kg−1 and airlow-rate (m3 kg−1 d.m. d−1): maximum 2.96, minimum 0.28, mean 0.75. 3) The curing phase was determined when a high degree of compost maturity was reached. After 254 d of composting the data obtained (on a dry matter basis) were as follow: volatile solids 658.9 g kg−1, C/N 13.52, TKN 25.1 g kg−1, K 20.0 g kg−1, P 12.7 g kg−1, humified carbon 108.1 g kg−1, dry matter yield 463.6 g kg−1, organic matter yield 369.1 g kg−1, and water losses 639.9 g kg{−1}. 4) The kinetic approach showed that the active composting phase could be represented by a first order reaction. High reaction rate constant (k = 0.0281 d−1) due mainly to cellulose and hemicellulose degradation was determined. 5) The compost data obtained suggest that two marketable products could be developed: the first after 56 d of composting, could be utilized for field application and the second after 254 d could be used as a substitute for organic substrate (e.g., peat) in greenhouses and for sensitive plants.