Assessment of periodic turning as an aeration mechanism for pulp and paper mill sludge composting

The effectiveness of periodic turning as a method of aerating a pulp and paper mill sludge (PMS) produced by Australian Newsprint Mills during windrow composting was determined by monitoring spatial and temporal changes in O 2 consumption and CO 2 accumulation in situ. Gas exchange during the static phase was found to be limited to the outer periphery of the windrow with interstitial O 2 being reduced to 0 to 2% in the pile centre between 2 and 6 hours after turning, indicating that the piles were oxygen starved for most of the trial, however, at no time was methane detected. The effectiveness of periodic turning in replenishing interstitial O 2 and eliminating CO 2 decreased as composting progressed, due to an increase in bulk density which reduced the volume of voids participating in gas exchange. This was particularly evident when the bulk density of PMS increased to more than 550 kg m -3. The volumetric quotient of CO 2 produced to O 2 consumed in a given interstitial gas sample was found to be a better indicator of whether aerobic or anaerobic conditions were present, than simply considering the level of interstitial O 2. An upward convective flow of gas existing in the PMS windrow during the static phase was not sufficient to maintain aerobic conditions within the pile. Periodic turning of PMS in static windrows was found to be ineffective in maintaining aerobic conditions, suggesting that a reduction in pile height, addition of a bulking agent to improve porosity and/or the installation of open-ended perforated plastic pipes could improve aeration during the static phase. Such measures are relatively inexpensive and would significantly reduce the time required to produce a stable compost.