Paper Sludge Composting and Agricultural Utilization
Nearly all types of paper industry waste are suitable for composting. These include fiber-containing sludges, deinking sludges, bark, wood residues, and biological sludges from effluent treatment plants. Rejects from recovered paper pulping and screening operations are not suitable or suitable only to a limited degree. With this type of waste, the content of plastic and other nonpaper components such as glass or stones has a detrimental influence.
In all cases, composting of residues from paper manufacturing requires the application of additives. With the exception of biological sludges, the sludges have an unfavorable carbon/nitrogen (C/N) ratio for microbial decomposition. They also have a dense structure that is unfavorable for composting. This requires the addition of structure-improving materials that are ideally also nitrogen carriers. Biological waste from households, garden waste, cuttings from trees and plants, straw, bark, and waste from animal husbandry are suitable as such components. The composting of fiber sludges and biological sludges, usually with bark, has been practised on an industrial scale for a long time.
The paper industry also has a long tradition of using sludges in agriculture. This is true especially for virgin fiber sludges and for biological sludges since the first biological effluent treatment plants started operation. With the exception of sludges from biological effluent treatment plants, sludges from the paper industry have a high carbon/nitrogen ratio and therefore contain only a small proportion of nitrogen. For this reason, they have only a limited fertilizing effect. The advantage of their use in agriculture relates to their soil enhancing properties. They not only contribute to covering the requirements of a humus forming organic substance but also improve the aeration and cultivation of the soil, increase the water reten¬tion capacity and prevent erosion.
Use of deinking sludges in agriculture is still controversial. In North America, no objections exist concerning their use as a soil improving material provided the material does not exceed defined contaminant concentrations or harmful substance loads. The situation in Germany is different. In general, the contaminant concentrations of deinking sludges are well below the valid threshold limits for municipal sewage sludge defined by legislation governing sewage sludge. The Ger¬man Environmental Protection Agency still considers the agricultural use as un¬justifiable for reasons of soil protection, including the irreversibility of numerous soil contaminants. The agency claims the ecological risk potential of deinking sludges is not sufficiently known. In the Biowaste Ordinance issued in 1998, the use of deinking sludges is also not allowable on soil intended for agricultural, forestry, or horticultural purposes.