Influence of the bulking agent on the degradation of olive-mill wastewater sludge during composting

Two different piles prepared with olive-mill wastewater (OMW) sludge and either maize straw or cotton waste as bulking agents were composted by the Rutgers static pile system in a pilot plant, with the aim of ascertaining the most suitable conditions for degrading the OMW sludges through composting. The use of maize straw, instead of cotton waste, as a bulking agent led to the following effects on the composting process of the OMW sludges: (1) a lower mineralization of the organic matter at the end of the active phase of the process; (2) lower total nitrogen losses by NH3-volatilization; (3) a higher biological nitrogen fixation; and (4) production of a stabilized organic matter with less humic characteristics. The phytotoxic effects in the pile with maize straw lasted for a longer time, due probably to its slower rate of organic matter mineralization. However, no phytotoxic effects were observed in both mature composts.