The development of bleaching sequences without the use of chlorine gas has made it technically possible to eliminate the discharge of the highly chlorinated organic substances, which are a threat to the environment. Once the remaining - and considerably less serious - problems have been solved, well-designed and well-operated pulp mills may be wholly environmentally compatible. However development of the effluent-free pulp mill has not proved to be feasible. In stead, developing effluent external treatment has proved to be more successful in terms of cost efficiency, pulp quality and environmental impact.
Chlorate
When chlorine dioxide is used in the bleaching process, chlorate is formed in the bleach plant's effluent. Chlorate is acutely toxic to bladder wrack (sea kelp) but is quickly broken down into sodium chloride or salt. The chlorate in the effluent water can be easily eliminated through reduction in an anaerobic environment or through the addition of chemicals.
Other compounds in the mill effluent
In addition to the chlorinated organics produced in the bleach plant, the water emanating from other sources contains resin acids, alcohols and various other compounds. These discharges need to be taken care of in the external treatment plant before being let out to the recipient.
Biological treatment
In large amounts, organic substances and other oxygen-demanding compounds (BOD and COD) can deplete the receiving water of oxygen if the renewal rate of the recipient is low. This may render conditions in seas and waterways hostile to life. This development is prevented by treating the effluent discharged from the mill in a wastewater treatment plant designed to break down organic compounds by forced aeration. Such plants take the form of large aerated lagoons, biofilm reactors, biologically activated sludge plants and/or plants for the treatment of wastewater by chemical precipitation. Compounds other than oxygen-consuming substances can also be reduced by this form of treatment.
Heavy metals
Among the naturally occurring constituents of wood are small quantities of heavy metals. Examples of measured values of metal per ton of wood include
0.3 g chrome, 1.4 g copper and 30 g iron. In the manufacturing of pulp, some of the metals are collected in the plant's recycling process, some end up in the product, and some in the plant's wastewater. The concentration of metals in the wastewater is low, and does not constitute an acute environmental problem.
Chelating agents
In some hydrogen peroxide bleaching methods, chelating agents are used to regulate the effects of metals on the hydrogen peroxide. Some concern has been expressed that chelating agents may increase the effects of heavy metals on the environment. That they do so has not been scientifically confirmed, however. Chelating agents are organic substances. In the bleaching of pulp, EDTA (ethylene diamine tetra-acetic acid) is often used. EDTA is broken down primarily by the effects of light. Without light, the process is much slower. It is non-toxic in the concentrations in which it occurs in mill wastewater. Chelating agents are very soluble in water and are not considered to be bio-accumulating substances.