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

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Abstract
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 O2 consump tion and CO2 accumulation in situ. Gas exchange during the static phase was found to be limited to the outer periphery of the windrow with interstitial O2 being reduced to 0 to 2% in the pile centre between 2 and 6 hours after turning, indicat ing that the piles were oxygen starved for most of the trial, however, at no time was methane detected. The effectiveness of periodic turning in replenishmg interstitial O2 and elimi nating CO2 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 CO2 produced to O2 con sumed in a given interstitial gas sample was found to be a bet ter indicator of whether aerobic or anaerobic conditions were present, than simply considering the level of interstitial O2. An upward convective flow of gas existing in the PMS wind row during the static phase was not sufficient to maintain aer obic conditions withm 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.