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10.2 Solid Waste The different processes within the pulp and paper industry result in the formation of different solid and sludge-like residues. In terms of volume, the largest are those from the deinking of recovered paper and from the treatment of effluents. Wood residues are also produced in large quantities, mainly in the pulp production. Other wastes arising at pulp mills are green liquor, lime sludge and ashes from energy generation and flue gas treatment.
Table 10.1 shows the solid wastes encountered in paper mills. Besides material removed from the pulper or ejected from a drum pulper, solids separated by screening and cleaning are all rejects. Among the sludges encountered there are deinking sludges, solids removed during the mechanical clarification of fresh wa¬ter, process water and wastewater, and sludges from biological effluent treatment plants. Classified as incineration residues are ashes and slags from internal power plants and waste incineration plants including fly ashes. In the category of other waste are chemical residues, used oil, wire and felts and hazardous waste, such as certain laboratory chemicals, batteries or transformer fluids. Wood residues such as bark and sawdust occur where mechanical pulp is also produced at the paper mills concerned.
In Germany the most recent survey on residues from pulp and paper mills dates back to 2001 [10]. According to the data gathered by the German Paper Industry Association a total of 3.6 million tons of residues was produced. This volume re¬fers to the residues in their original state. The calculated mean dry content of all residues was 57 %. Figure 10.5 shows that deinking sludges and sludges from process water and effluent treatment account for the largest proportion of residues with 30 % each. Rejects from recovered paper processing contribute 15 %, followed by incineration residues (11 %) and bark (10 %). Rejects from primary fiber proc-essing and other residues e. g. chemical waste, used wires and felts, and used oil account for only a small share. Solid Waste Composition and Characteristics
As shown in Fig. 10.5 most of the German paper industry’s waste is generated in recovered paper processing. To produce recycled fiber pulp suitable for use in the manufacture of different paper and board grades all substances likely to disturb the processing of recovered paper or the quality of the final product must be re¬moved as far as possible from the disintegrated recycled stock. Depending on the contamination of the recovered paper with non-paper components and the paper grade produced, larger or smaller volumes of waste result. The volume also de¬pends on the amount of effort invested in separation at the different process stages. Table 10.2 shows the percentages of waste amounts related to the recovered paper used and dependent on the paper and board grades produced.
Table 10.3 shows the composition of waste that occurs at different stages of recovered paper processing [11]. The waste from disintegration, cleaning and screening is reject material or rejects. The wastes that occur during flotation deink¬ing and the cleaning of process water from wash deinking are sludges. Because today’s processing methods are selective only to a limited degree, both rejects and deinking sludges continue to contain a certain proportion of fiber and fiber fines.
Sludges also result from process water clarification and biological treatment of wastewater. The sludge from wastewater treatment plants can be divided into pri¬mary sludge from mechanical treatment and bio-sludge from biological treatment. It is technically possible to return these sludges to the production process in man¬ufacturing paper grades as corrugating medium or testliner. Because the in-mill reuse may affect product quality and process runnability the use as raw material in production processes must always be considered on a mill-by-mill basis.
The amount and characteristics of ash resulting from energy generation and flue gas cleaning depend on the fuel and the combustion technology used. In the Ger¬man paper industry the total amount of ashes in 2001 was 400 000 tons. Like all other wastes from pulp and paper processing, the ashes are non-hazardous and were mainly used in the construction material industry. 10.2.1.1 Rejects
The amount of rejects produced during recovered paper processing and their com¬position depends largely on the recovered paper grades. Figure 10.6 shows the composition of a reject sample as a mixture of rejects from six paper mills produc¬ing newsprint. Pulper and drum pulper are used for slushing of the recovered paper (old newspapers and old magazines). The proportion of the reject fractions as dry solids was 4 to 6 % related to air-dried recovered paper.
Due to the high proportion of plastic materials the heating value of rejects is more than 20 GJ t–1 of dry substance. The use of rejects as an energy source such as secondary fuels for cement production can be limited by a rather high chlorine content. It can amount to 3 % by weight of rejects on a dry basis. Polyvinylchloride (PVC)-containing materials such as self-adhesive tapes, foil laminates, carrier han¬dles and PVC products thrown in error into recovered paper containers are the main sources of this chlorine content. 10.2.1.2 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 10.2 Solid Waste
(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.
10.2.2
In-mill Waste Handling
The aim of in-mill waste handling is usually to achieve as high a dry solids content as possible, because all commonly used methods for material and energy recovery benefit from a high solids content.
In a sludge suspension, water exists in the following forms:
. • Free water
. • Capillary water
. • Bound and intercellular water. 10.2 Solid Waste
Free water can be removed simply by gravity settling. Much of the water is re¬moved by allowing the sludge to stand, when solids sink to the bottom and water forms a supernatant. Capillary water can be removed mechanically by filtration or centrifugation. Bound and intercellular water can be removed by drying. 10.2.2.1 Dewatering
For sludge dewatering two-stage processes are often used. An initial step for dewa¬tering using gravity tables or drums and disk thickeners occurs before a high-consistency dewatering with belt filter presses or screw presses. Batch-agitated chamber filter presses are of only minor importance due to their costly sludge conditioning with inorganic precipitation agents (metallic salts and chalk) or or¬ganic polymers, and their high maintenance costs. Centrifuges are universally applicable and generally managed without steps before the dewatering. With pure biological sludges, systems using these devices can achieve the highest dry con¬tents.
Figure 10.9 shows the basic function of a belt filter press. The sludge is trans¬ported between two wire belts where it is subjected to a gradually increasing pres¬sure. Direct pressure forces and shearing forces squeeze water from the sludge. Conditioning with polyelectrolytes is necessary for most types of sludge. Fig¬ure 10.10 shows the basic function of a screw press. The sludge is transported by a slowly rotating screw working against discharge restriction. The water squeezed out is released through openings in the cylindrical housing surrounding the screw. Some screw presses have the optional possibility of heating the sludge by injecting steam through the screw shaft. Conditioning of the sludge with polyelectrolytes is common.
Today, the ragger and the rejects from the pulper disposal system are usually not subjected to any special dewatering. Due to their material composition, “draining off” gives dry contents of 60–80 %. Screen spiral conveyors, vibrating screens, and screw and rake classifiers are useful for dewatering heavy and coarse rejects. De¬watering of light and fine rejects with screens, endless wires or vibrating screens is usually followed by additional dewatering using a reject screw press.
Table 10.5 summarizes the most common dewatering systems for different types of waste. The achievable dry solids contents are also indicated. The differ¬ences in the dry solids contents are due to different pressure levels and times. For example, screw presses operate with pressures up to 8 bar for times up to 10 min. Belt filter presses work with pressures only up to 0.5 bar and short pressing times of only 1–2 min. With increasing proportions of biological sludges from waste¬water treatment plants, the dry solids contents decrease for all techniques. Inde¬pendent of the technique applied, dry contents of only 15–40 % are possible with purely biological sludges.
Table 10.5 Composition and dewatering of rejects and deinking sludges.
Heavy-weight and Light-weight and Deinking sludges coarse rejects fine rejects
composition glass, nails, sand, stones, paper clips, pins
dewatering screens
facilities vibrating screens screw classifiers rake classifiers
achievable dry 60–80 % solids contents sand, textiles, fibers, coating colors, plastic fragments, hot melts, stickies
screens disk thickeners dewatering drums gravity tables screw presses
50–65 % fillers, pigments, fibers, printing ink, stickies
dewatering drums gravity tables belt filter presses screw presses chamber filter presses centrifuges
one-stage operation: < 15 % two-stage operation: < 65 %
10.2 Solid Waste
10.2.2.2 Drying
Thermal drying processes are used for more comprehensive removal of water from mechanically dewatered sludges. Sludge dryers find use primarily when available or potential disposal processes are impossible or uneconomical, e. g. high landfill costs per ton of moist sludge due to insufficient dryness. Besides giving mass reduction, drying of the sludge has other advantages that broaden the waste management options:
. • Sludges at about 95 % dry substance are odorless. The bacteria-free conditions mean that sludges can be stored in the open or in simple silos.
. • Dried sludges can be transported without any problem in the same way as bulk goods in regular transport systems.
. • Dosing and mixing with other waste and combustibles is simpler.
. • Combustion is easier and produces more heat per unit of mass of dry mate¬rial. Direct and indirect drying systems are in operation. Direct systems are convection dryers, and indirect systems are contact dryers. With convection dryers, heat trans¬fer occurs by direct contact between hot air and the sludge. The hot air also pro¬vides pneumatic transport through the dryer. Convection dryers can be heated directly or indirectly. Compared with the contact dryers, the contamination of the resulting condensed exhaust air is low, but the volume of exhaust air is consider¬ably higher.
Among the contact drying processes in the paper industry are steam heated centrifugal dryers and disk dryers. The types of convection dryers used are drum dryers, belt dryers or rack dryers.
In all drying processes, flue gas and dry goods control have primary importance due to the danger of combustion and explosion. In this respect, indirect drying processes are less problematical. Depending on its structure in the moist state, the dried sludge takes on the form of fine granules or pellets. For direct and indirect drying, the heat requirement is 4.0–4.5 GJ t–1 of evaporated water.
Utilization and Final Disposal of Solid Waste
The current possibilities for material and energy recovery and final disposal of the different types of paper mill waste are shown in Figure 10.11. The dominant prac¬tice in the past for disposal was landfilling. Today, and in the future, energy recov¬ery and material use of waste as in other branches of industry is gaining increasing importance.
Figure 10.12 shows how wastes of the German paper industry were utilized and disposed of in 2001. The largest share, at 36 %, was utilized in the building materi¬als sector, mainly in the brick and cement industry. The energetically utilized share was 35 %. Internal power plants at paper mills had a utilization level of 26 % and 9 % was accounted for by external energetic utilization in power plants or waste incineration plants. 18 % of residues were directed to biological utilization, of
10.2 Solid Waste
which two-thirds were composted. The share of residues disposed of by landfilling had already dropped to just 6 %.
10.2.3.1 Energy Recovery
In the pulp and paper industry there is a long tradition of waste incineration. For example, consider the combustion of bark and wood residues. Recent years have seen a growing interest in the use of other types of waste such as sludges and rejects for energy production purposes. The reasons are:
. • increased costs of fossil fuels and purchased power
. • reduced landfill capacities and increased landfill costs
. • more stringent environmental regulations governing the use of this waste in agriculture
. • development of new combustion technologies with highly effective flue gas cleaning technologies. Due to their heating values and low content of harmful substances, most types of waste from paper mills are suitable for energy recovery. Sludges and rejects are burned mainly in grate and fluidized bed combustion facilities. Burning of sludges is also carried out in multiple hearth incineration plants.
10.2.3.1.1 Grate Combustion
Grate combustion systems are especially suitable for lumpy fuels but not for most, paste-like, and finely grained fuels. With moist sludges for example, the retention times on the grate can be insufficient for complete combustion. Nevertheless, grate combustion systems are used in the paper industry primarily for co-firing of sludges with coal, bark or wood residues. Grate combustion with a stationary grate is shown in Fig. 10.13. Long reaction times for control processes and the difficult combustion control on the grate by the addition of air presuppose that the mois¬ture content and ash content of the sludge vary only within narrow tolerances.
10.2.3.1.2 Fluidized Bed Combustion
Fluidized bed combustion results in good heat transmission and intimate mixing of air and fuel. Combustion is, on the whole, better than in grate furnaces, and emissions are therefore lower. Figure 10.14 shows the scheme of a stationary flui¬dized bed furnace. In fluidized bed boilers the combustion conditions can be con¬trolled through regular temperature and pressure measurements, which is not the case with grates. Stationary fluidized bed furnaces as well as circulating fluidized bed furnaces offer very good operating conditions for low emissions of air-borne pollutants. The emissions of nitrogen oxides are especially low compared with grate combustion. Direct desulfurization is possible by the addition of basic sor¬bents such as limestone or dolomite into the body of the furnace.
10.2 Solid Waste
10.2.3.1.3 Multiple Hearth Combustion
Multiple hearth combustion plants, as shown schematically in Fig. 10.15, are espe¬cially suitable for the combustion of moist and paste-like waste. They have been used for decades in the paper industry for energy production from primary and biological sludges, often together with bark. The flue gas purification plant down¬stream from the multiple hearth furnaces usually consists of wet scrubbers to remove dust and sulfur compounds. The flue gas finally enters the stack via a mist eliminator.
10.2.3.1.4 Choice of Combustion Technology
The choice of combustion technology depends on the operating conditions and the nature of the waste. According to today’s knowledge, fluidized bed combustion is the most suitable for burning sludges and rejects. In fluidized bed combustion neither fluctuations of heating value of the fuel nor changing proportions of non¬combustible components such as sand, metals or fillers have a negative influence on the combustion efficiency.
For fuels with a low heating value, such as deinking sludges, the stationary fluidizing is probably a more suitable option. Fuels with a higher heating value such as wood and bark are incinerated more effectively in a circulating fluidized bed. Due to the mechanical flow requirements of the fuel, producing a consistent grain size distribution by mechanical processing such as extraction of metals, shredding and grinding may be necessary. This requires consideration, especially when co-burning rejects with sludges.
For sludge and reject incineration in paper mills the following conclusions con¬cerning flue gas emissions are possible:
. • Emissions of solid particles, sulfur dioxide, hydrogen chloride, hydrogen fluo¬ride and carbon monoxide are low compared with legal standards. Due to the high calcium carbonate content of deinking sludge this sludge should not re¬lease all sulfur as sulfur dioxide. State-of-the-art flue gas purification guarantees that no significant environmental impact arises.
. • The emission of heavy metals is very low compared with the legal standards, especially for emission of cadmium and mercury.
. • The emission standard of nitrogen oxides can usually be met by improved com¬bustion control (temperature and oxygen level). Sometimes, the application of specific measures to remove NOx could be necessary.
. • The stringent emission standard of polychlorinated dioxins and furans can be met without any additional measures in flue gas purification. 10.2.3.2 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 sub¬
10.2 Solid Waste
stance 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.
10.2.3.3 Use in Other Industries
Rejects, deinking sludges, and combustion ashes of paper mills are used in many branches of industry for material and energy production purposes. The material use possibilities for deinking sludges and combustion ashes depend primarily on the composition of the inorganic components. The inorganic part of the deinking sludges consists mainly of calcium carbonate and clay. In the combustion ashes of deinking sludges, calcium oxide and sintered clay are primarily present. The afore¬mentioned inorganic components also have use as raw materials in the construc¬tion industry. The primary use possibilities are therefore the following:
. • cement production
. • brick manufacturing
. • concrete production
. • mortar and sand lime brick production
. • road construction. 10.2.3.4 Landfilling
Landfilling is the most commonly applied method worldwide for the final disposal of paper mill wastes. This situation will not change in many more remote regions of the world in the medium term. In contrast, landfill capacities have already become very limited in densely populated countries and it is becoming increas¬ingly difficult to obtain authorization to open new landfill sites. Further develop¬ment of landfilling techniques has dramatically increased costs for construction and operation of landfills. With the current technical state-of-the-art, operating a landfill site such that any annoyance to the population living nearby or environ¬mental risk due to leakage, odors, fire and explosion risk can be largely elimi¬nated.
Theoretically, the following possibilities exist for landfilling waste from paper mills:
. • landfilling in monofill dumps for certain types of waste such as ash, sludge, and bark
. • works-owned landfills where all in-plant waste is stored
. • landfilling in public dumps where waste from other sources is also stored. Depending on local circumstances and the types of waste concerned, pulp and paper mills sometimes use all three disposal methods. Because the waste from paper mills is not harmful, using municipal landfills is possible. To minimize leakage and maintain landfill stability, an extensive dewatering of sludges and rejects is necessary. Dry solid contents should be above 40 %. The landfill of ashes from reject and sludge incineration does not require any measures beyond those governing the dumping of household waste.
10.2.3.5 New Developments
Besides extension of the processes to use waste material described so far, further possibilities exist for the use of waste from paper mills. These mainly concern fibrous sludges and deinking sludges. Only a small number of paper mills use these possibilities, or they have been limited so far to laboratory scale tests and pilot installations. New developments in the area of sludge use include:
. • wet oxidation processes
. • fermentation processes
. • hydrolysis processes
. • production of cat litter and adsorbent materials. For ash use, filler recovery processes are under development that should permit the reuse of recovered fillers in the paper industry or other branches of industry.
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