Chemical Characterisation and Compost-Recycling of a Pulp and Paper Mill Sludge

Australian Newsprint Mills is a pulp and paper mill located in southern Tasmania. This company produces approximately 50000 tonnes of pulp and paper mill sludge (PMS) per year, all of which is currently landfilled. As the current and fill site is nearing full capacity, this study was developed to assess whether compostingcould potentially replace landfilling, with use of the resulting material as a mulch in radiata pine (Pinus radiata) plantation forestry or as a component in horticultural growing media. The suitability of PMS (produced by a primary treatment system) as a substrate for cornposting was assessed. Elemental analyses revealed it to be nutrient poor, indicating that significant quantities of nitrogen, phosphorus and potassium would need to be added to initiate composting. The PMS consisted primarily of cellulose polymorph type I, as in ordinary pulp, with a minor lignin and uronic acid contribution. Since the PMS was highly fibrous and not contaminated with chlorinated organic substances or heavy metals, the material was considered suitable for compost-recycling. Optimal conditions for composting PMS with mineral nutrient supplements were determined in laboratory scale reactors designed to simulate conditions in a large-scale periodically turned windrow. This study showed that composting proceeded faster under thermophilic conditions (relative to mesophilic conditions), and that pH control (to less than 7.5) was necessary to prevent excessive nitrogen loss through volatilisation of ammonia. This could be achieved by substituting some of the mineral nitrogen requirement (as urea) with a non-alkali forming nitrogen source, such as ammonium sulfate or ammonium nitrate. A large scale windrow composting trial was undertaken, based on the optimum conditions determined in laboratory scale composting experiments. Performance of the process, assessment of product quality, and economics of composting relative to landfilling were addressed. The effectiveness of periodic turning as an aeration mechanism was assessed by monitoring spatial and temporal changes in the gas phase. The study showed that periodic turning was ineffective in maintaining aerobic conditions in the PMS windrows, 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. The chemical changes occurring during the composting process, and their relationship to maturity was assessed by solid state carbon-13 nuclear magnetic resonance with cross polarisation and magic angle spinning, and fourier transformed infrared spectroscopy. The mechanism of aluminium retention in PMS and composted PMS was investigated,as the presence of soluble aluminium may have affected the ability of the material to support plant growth. Results provided by solid state aluminium-27 nuclear magnetic resonance with magic angle spinning and chemical extraction techniques showed that aluminium was bound in a monomeric, octahedrally coordinated, non-exchangeable form to the organic matrix. Since aluminium was strongly retained in both the non-composted and composted PMS forms, the potential of this element to deleteriously affect plant growth in the short term, even under low pH conditions (pH < 5), was considered to be low. The effect of PMS compost on the growth, nutrition, water relations and weed suppression in a three year old radiata pine plantation for a twelve month period was studied. Compost applied at rates of 20 to 60 t ha-l (dry matter) without incorporation on a relatively infertile podosol soil markedly improved the growth of radiata pine after twelve months, due to the release of soluble nitrogen from the compost and to decreased water stress. The compost, however, had little effect on soil water availability or weed suppression. The application of PMS compost to young radiata pine was found to be an environmentally acceptable method of improving plantation produ~tivity, capable of utilising large quantities of this material. The germination and growth of various agricultural and horticultural plant species in growing media formulated with PMS compost and perlite without nutrient addition was assessed. Media consisting of between 60 and 90% compost by volume- had excellent levels of air-filled porosity and water holding capacity (at container capacity), however, plant growth could be improved by adding a slow nutrient releasing fertiliser (including trace elements), increasing pH and by reducing the calcium to magnesium ratio. Use of PMS compost as a component in horticultural growing media was considered to be a potentially profitable and an environmentally acceptable method of utilising this organic waste.