Pulp and paper industry is a major source of industrial pollution worldwide. The characteristics of the wastewater generated from various processes of the pulp and paper industry depend upon the types of process and wood materials, process technology applied, management practices, internal recirculation of the effluent for recovery, and the amount of water to be used in the particular process. The conventional pollutant parameters as present in pulp and paper industry wastewaters are characterised as biochemical oxygen demand (BOD), total suspended solids (TSS), and pH.
The priority and nonconventional pollutants that have been determined for the pulp and paper industry are chlorinated dioxins and furans, chlorinated phenoliccompounds, volatile organic compounds (VOCs), adsorbable organic halides (AOX), chemical oxygen demand (COD), colour and other compounds such as resin and fatty acids, metals, semi-volatile compounds, and pesticides/herbicide. The major sources of pollutant released in pulp and paper industry occur at the pulping and bleaching stages.
Pulp, paper and paperboard mills that use chemical pulp and bleached wood with chlorine and chlorine derivatives generate significant discharges of toxic pollutants from the pulping and bleaching processes. Such toxic pollutants include chlorinated dioxins and furans, particularly 2,3,7,8-tetrachlorodibenzofuran (2,3,7,8-TCDF) and 2,3,7,8-tetrachlorodibenzo-p-dioxin (2,3,7,8-TCDD) and the other chlorinated dibenzo-p-dioxins (CDDs), and chlorinated dibenzofurans (CDFs) (US EPA 1993a). VOCs (such as terpenes, alcohols, phenols, methanol, acetone, chloroform, methyl ethyl ketone (MEK), chloromethane, trichloroethane)and chlorinated phenolic compounds are also formed from the same processes (US EPA 1993a; US EPA 2002). Chlorinated phenolic compounds found in pulp and paper wastewaters cover chlorinated phenols, chlorinated guaiacols, chlorinated catechols and chlorinated vanillins. AOX is a measure of the halogenated organic compounds that adsorb onto granular activated carbon. For pulp and paper industry wastewaters, essentially all of the halogenated organic substances measured as AOX are chlorinated compounds which result from the bleaching of pulps with chlorine and chlorine derivatives.
In pulp and paper industry wastewaters, refractory contaminants such as wood resin, dioxins, furans, catechols, guaiacol, lignin, and other chlorinated phenolic compounds, and color are so recalcitrant that they cannot be completely degraded by conventional biological treatment systems. The wastewater components causing color (lignin and its derivatives) are difficult to degrade biologically because of strong linkages in their molecular structure (especially the biphenyl type of carbon-carbon bond). Besides an important group of compounds present in pulp and paper industry wastewaters is responsible for the toxicity of the wastewater and may adversely affect the microbial systems in the biological treatment systems. Therefore, these non-biodegradable toxic contaminants have to be eliminated before biological treatment. As described before, in certain cases, efficient treatment technologies such as chemical oxidation for the treatment of pulp and paper industry wastewaters are necessary. In recent years, various chemical oxidation technologies have been applied as a pretreatment or post-treatment option to facilitate the removal of chlorine based organic compounds, color and toxicity from pulp and paper industry wastewaters.
Implementation of chemical oxidation technologies for the treatment of pulp and paper industry wastewaters can be summarized as follows:
• Removal of toxic and bio-recalcitrant organic compounds before biological treatment in order to reduce the toxic effects and increase the biodegradability.
• Removal of residual organic compounds after biological treatment.
• Removal of highly resistant color.
Ozonation is efficient in removing COD, TOC and colour as well as increasing the biodegradability of the wastewater in many cases.