RICE HULL DEGRADATION BY CO-COMPOSTING WITH DAIRY-CATTLE SLURRY

A liquid dairy cattle slurry was composted using rice hulls as absorbi ng matrix and bulking agent. The main results obtained were as follows : 1) The structure and size of the rice hulls as well as their absorpt ion capacity, enables to obtain a good composting mixture (moisture: 7 14.2 g kg(-1); C/N ratio: 28.80; bulk density: 0.429 Mg m(-3)). 2) Com post stability was reached after 58 d of composting, with the followin g 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), h umified carbon 46.0 g kg(-1), respirometric index 0.88 mg O-2 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 airflow-rate (m(3) kg(-1) d.m. d(-1)): maximum 2.96, minimum 0.28, mean 0.75. 3) The curing phase wa s 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 obt ained suggest that two marketable products could be developed: the fir st after 56 d of composting, could be utilized for field application a nd the second after 254 d could be used as a substitute for organic su bstrate (e.g., peat) in greenhouses and for sensitive plants.