Evolution of microbiological and chemical parameters during manure and straw co-composting in a drum composting system

Dairy cattle and swine manure mixed with barley straw was composted in four successive series in a continuously working horizontal drum composting system. Changes in microbiological and chemical parameters were monitored during the active composting process in the drum and during three months curing in indoor heaps. Hygienization, as determined by disappearance of fecal coliforms and fecal streptococci, was obtained within seven days of composting in the drum. During the active composting, the populations of heterotrophic mesophilic bacteria, enumerated by the dilution plate method, decreased compared with the populations in feeding material, and the densities increased during early curing, being highest after two months of curing. Thermophilic heterotrophic bacteria were most abundant in the early stages of curing. The ratio thermophilic/mesophilic heterotrophic populations was highest inside the drum during the early thermophilic phase of composting. Microbial activity, measured as dehydrogenase activity, was typically high in the feeding material, and decreased slowly during the active stage of composting in the drum and the early stages of curing in heaps. In all experimental series, dehydrogenase activities decreased in mature composts to the final constant level of 4.44–5.43 μM TPF g −1 (dw) 3 h −1. Water-soluble mineral N decreased during drum composting and was at its minimum after one to two months curing. In the outfeeds hardly any NO 3N was observed, but during curing, the NH 4N NO 3N ratio decreased to less than one. Water-soluble P was at its minimum in outfeeds and increased during curing. During composting, the C N ratio decreased from 22.6–28.5 to 12.7–13.6. The ratio C N final to C N initial decreased to 0.65–0.75 and to 0.49–0.59 after one and two months of curing, respectively. Approximately 11–27% of the total C was lost during seven days active composting and 62–66% during the whole composting time; the respective gaseous losses of N were 13–23% and 23–37% of the total N.