Effects of inoculation and urea addition on the composting of sewage sludge and rice straw

The annual on-site burning of rice straw gives rise to serious environmental pollution and human health risks in the area surrounding the big city of Valencia (Spain). Its composting together with a sewage sludge suitable for agricultural use may therefore be a sound practice to reduce the problem, recycling at the same time the nutrients and organic matter usually lost during the burning, and producing an organic amendment with better properties than the non-composted sludge. Rice straw is nonetheless a very resistant substrate given its hydrophobicity and high content in silica and lignin. A composting system using rice straw must be carefully planned, and treatments to enhance the degradation rate must be assayed. Rice straw was therefore composted mixed with anaerobically-digested sewage sludge (3:1 v/v ratio) in 20 m piles containing about 1400 kg of material each, aerated by means of manual turning. Four different piles were set up, corresponding to four different treatments: control, addition of 1150 g/pile of urea, inoculation of a MSW-compost extract, and urea plus inoculation. Temperature and moisture were monitored during the process, which lasted 110 days. No significant problems arised during the four composting processes. Temperatures increased slightly faster in the inoculated piles, but the thermophilic phase was also shorter in them. The total sums of temperatures measured in all the piles were nearly the same. No other modifications were found in the composting parameters of the four piles or the final products properties, which showed to be adequated for Agricultural use. It can be thus concluded that the assayed treatments had a nearly-negligible effect on the composting of rice straw and sewage sludge mixtures.