Metal Detoxification Properties of Phytomass: Physiological and Biochemical Aspects

The paper pulp industry is a source of various kinds of solid, liquid and gaseous wastes. During the recovery of cellulose, several solid residues of inorganic nature (ashes, fly-ashes, dregs and grits) or organic wastes derived by sedimentation of sludge mainly composed by fiber debris (primary sludge and brown stock greening rejects) are generated. The discharges have been significantly reduced during recent years due to new techniques that have been applied both in developed and developing countries. However, the production of residues cannot be completely avoided, and proper techniques for the handling of waste products are required. Although gaseous and liquid residues have greatly decreased, the solid components are still considerable: taking a broad range of mills, an average mill produces about 70.000 hum/tons/year. To date, there is a scarce number of rigorous studies addressing the impact of these wastes on the environment. However, it is likely that some of these residues may be harmful, due to their heavy metals and organochlorides content. At present, waste products are often dried, burnt and disposed off at sites near the mills. This is an expensive strategy excluding consideration of any other possible use. Now, several alternatives to obtain added value from solid residues are being investigated worldwide. They include, among others, anaerobic fermentation, composting, and landspreading. The first two are conducted under controlled conditions — which implies relatively high costs — and are sensitive to the presence of toxic compounds. Therefore, landspreading for the purpose of fertilization and amendment of soils diminishes the environmental impact, and can be used as an adequate alternative.