10.2.1.2 Paper Mill Deinking Sludges
Deinking sludges consist of fillers and coating pigments, fibers, fiber fines, print¬ing inks (black and colored pigments), and adhesive components. Figure 10.7 shows that more than 55 % of the solids removed by flotation are inorganic com¬pounds. They are primarily fillers and coating pigments such as clay and calcium carbonate. The proportion of fiber material at 7 % is low. Materials extractable with methylene chloride have an average proportion of 8 %. They contain wood compo¬nents from the fibers such as resins, fats and resin acids as well as extractable printing ink and adhesive components and flotation deinking chemicals. The re¬maining 29 % comprises fibers fines, nonextractable ink components (mainly car¬bon black and colored pigments) and nonextractable adhesive components.
Table 10.4 provides a summary of ash contents, heating values, elemental con¬tents and levels of different contaminants of deinking sludges. It shows mini¬mum, maximum and average values for sludges from different German paper mills. A characteristic for deinking sludges is their high ash content, 70 %. The heating value depends on the ash content and is 4.7–8.6 GJ t–1 of dry substance. The sulfur, fluorine, chlorine, bromine, and iodine contents are low. For this rea¬son, no costly flue gas purification systems are necessary when incinerating deink¬ing sludge. Compared with sludges from biological effluent treatment plants, the nitrogen and phosphorus contents are very low. This is something that requires consideration when using deinking sludges for composting and agricultural and land application purposes.
The level of heavy metals in sludges of recovered paper processing is generally low. Figure 10.8 compares the concentrations of heavy metals in deinking sludge with the contents of heavy metals in biological sludge of wastewater treatment plants of paper mills and in municipal sewage sludge. This data shows that sludges of deinking plants have less contamination than those of municipal waste¬water treatment.
The concentration of cadmium and mercury is especially insig¬nificant and sometimes even below the detection limit of the test method applied (atomic absorption spectrometry). Only the concentration of copper has the same order of magnitude as that of municipal sewage sludge. The copper content of deinking sludge is primarily due to blue pigments of printing inks which contain phthalocyano compounds.
Traces of halogenated organic compounds such as polychlorinated biphenyls (PCB), polychlorinated dibenzodioxins (PCDD), and polychlorinated dibenzofur¬ans (PCDF) also require consideration. PCB compounds were used in the manu¬facture of carbonless paper until the early 1970s. The PCB level of deinking sludge has decreased significantly since then. Recent data obtained from several deinking plants confirm that the PCB concentration is below 0.3 mg kg–1 dry solids
(0.3 ppm) using the most relevant seven congeners.
PCDD/PCDF concentrations of deinking sludges show a similar pattern of de¬cline. Due to the continuing change from elemental chlorine bleaching of chem¬ical pulp to chlorine dioxide and oxygen bleaching, the PCDD/PCDF contents of deinking sludges of German paper mills have been decreasing significantly. Today, PCDD/PCDF concentrations of deinking sludge are 25–60 ng I-TE kg–1 dry solids
(I-TE = International toxicity equivalent). These figures are not significantly higher than the average contents of PCDD/PCDF in municipal sewage sludge. As a result of modifications of the bleaching sequences in chemical pulping, dioxin formation does not occur in most pulp producing countries. Consequently, dioxin discharges from recovered paper processing mills are already low and will decrease further.
The parameter AOX (adsorbable organic halogen-containing compounds) plays an essential role in environmental regulations. In Germany for example, the direct application of sewage sludge on agricultural soil is regulated for heavy metal con¬centration, PCB and dioxin concentrations, and AOX level. In many cases, the AOX of deinking sludge is often above the acceptable limit of 500 mg kg–1 dry solids. Investigations have shown that up to 80 % of AOX in deinking sludge is due to chlorinated yellow pigments that are components of printing inks. These pig¬ments are water insoluble and nonbiodegradable.