Holik H. (ed.) Handbook of Paper and Board

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Environmental Aspects

Udo Hamm

10.1

Wastewater

The paper industry has a high fresh water demand. At the beginning of the 20th

century, about 500 to 1000 m

of water was required for the production of one

ton of paper. Today the specific fresh water demand is much lower. The German

paper industry, for instance, has succeeded in reducing the specific fresh water

consumption to 13 m

(t paper)

–1

which corresponds to a wastewater volume

of about 11 m

(t paper)

–1

. In an international comparison, the performance

in sustainable use of water resources of the German paper mills is very good.

In the following mainly data from the German pulp and paper industry is pre-

sented, because of the high technical standard of wastewater treatment in this

country.

10.1.1

Characterization of Untreated Wastewater

The pollution in the wastewater of a paper mill depends on the type of raw mate-

rial, the type and amount of fillers and chemical additives applied, and on the

degree of circuit closure. Effluents from mills that largely or exclusively process

chemical pulps have a lower degree of pollution than those from mills in which

mechanical pulp or recovered paper is employed. The use of starch or other or-

ganic additives results in a marked increase in oxidizable effluent compounds,

measured as BOD

(biological oxygen demand) or COD (chemical oxygen de-

mand).

Dyes and fillers can lead to discoloration and/or turbidity of effluents. Effluents

from paper mills have a low content of nitrogen and phosphorus compounds. For

this reason, these elements must be added as nutrient salts (e.g. urea and phos-

phoric acid) to feed micro-organisms during treatment in biological purification

plants. The closure of the water circuit within the mill results in an increase in the

concentration of the effluent components. On the other hand, the inorganic and

Handbook of Paper and Board. H. Holik (Ed.)

ISBN: 3-527-30997-7

422

organic load is reduced owing to its partial elimination via the paper produced. For

this reason, the specific load in kg per t of paper (specific load = concentration V

specific amount of effluent) is a more suitable parameter for quantifying the efflu-

ent pollution. Information on the specific COD and BOD

loads, classified accord-

ing to product groups, is given in [1].

10.1.2

Wastewater Treatment

Most of the wastewater from German pulp and paper mills is treated biologically,

either in municipal treatment plants (18% of production volume) or in in-mill

plants (74% of production volume). 4 % of the annual paper volume is produced in

mills with a totally closed water circuit which means that these mills are absolutely

effluent free [2].

10.1.2.1 Suspended Solids Removal

Effluents from paper mills contain solids and dissolved substances. Solids (fibers,

fillers) are mostly removed from the effluent in a chemo-mechanical clarification

process by the use of flocculants. The degradation of dissolved organic substances

is performed in aerobic and anaerobic biological treatment plants.

Chemo-mechanical clarification of effluents is carried out almost exclusively in

sedimentation plants (round and rectangular basin with bottom sludge removal)

as shown in Fig. 10.1. Only in a few cases it is necessary to neutralize the effluents.

Rakes for the separation of coarse material and sand traps are seldom used. The

clarifying efficiency of sedimentation plants is increased considerably by the use of

flocculants. Undissolved substances are removed with an efficiency exceeding

90%.

Fig. 10.1 Scheme of a sedimentation clarifier.

10.1 Wastewater

423

10.1.2.2 Biological Treatment

Primarily, activated sludge processes and, less often, trickling filter processes are

employed for aerobic biological treatment. In North America and Northern Eu-

rope, effluent purification is frequently carried out in aerated oxidation ponds.

Recently, anaerobic treatment has become established, especially in paper mills

processing recovered paper.

10.1.2.2.1 Aerobic Treatment

The activated sludge processes applied are single-stage processes, systems with

recycled sludge aeration, cascade systems, and two-stage processes. Atmospheric

oxygen transfer is employed, using surface aerators (roll aerator, gyratory aerator)

or pressure aerators. Pure oxygen processes with pressure aeration are also used

occasionally. It is possible to improve the purification efficiency of activated sludge

plants by using pulverized brown coal or foamed plastic carriers in the activated

sludge tank. Biological treatment with a two-stage activated sludge process is

shown schematically in Fig. 10.2. The trickling filter system usually operates in

combination with an activated sludge process. The trickling filter can be either the

first stage (in high-load operation) or the second stage (in low-load operation).

Combination processes also include activated sludge processes with fixed-bed in-

ternals and submerged disk filters. Activated sludge plants and trickling filter

plants are almost exclusively fed with mechanically pre-clarified wastewater. The

addition of nitrogen and phosphorus compounds is required. The BOD

efficiency

attainable with activated sludge processes is usually in the range of 90–98%, and

the COD efficiency between 80 and 95%. For final clarification after the aeration

tank, horizontal (rectangular and round basin) or vertical-flow (hopper basin) sed-

imentation basins are preferred. Biofilters are occasionally employed as a further

purification stage. Third-stage purification processes for the elimination of nitro-

gen and phosphorus compounds are not usually required.

Fig. 10.2 Scheme of biological effluent treatment with a two-

stage activated process.

10 Environmental Aspects

424

10.1.2.2.2 Anaerobic Treatment

Anaerobic processes have been employed recently for the treatment of more highly

polluted effluents (COD > 2000 mg L

–1

). Effluents from paper mills processing

recovered paper are most commonly treated in anaerobic contact systems with

sludge recycling and in UASB (upflow anaerobic sludge blanket) reactors. Fig-

ure 10.3 shows schematically an UASB-reactor which is used in the German paper

industry for treatment of highly polluted effluents. Information on plants installed

up to now and on operational experience is given in [3]. Fixed-bed reactors, flui-

dized-bed reactors, and other anaerobic reactor concepts are of minor importance.

Anaerobic effluent purification is almost exclusively operated as a preliminary step

to the aerobic treatment.

The sludges generated in effluent treatment must be dewatered and disposed of

if they cannot be returned to the papermaking process. Screen belt presses, cham-

ber presses, vacuum filters, or centrifuges are used for dewatering. The dewatering

behavior can be improved by the addition of chemicals (polymers, trivalent metal

salts, lime). Machines and plants for sludge treatment and their dewatering effi-

ciencies are described in [11].

Most dewatered sludges (solid content 20–60%) are disposed of in landfills.

Thermal utilization (combustion) is becoming increasingly important. The

sludges can also be composted together with bark, applied to agricultural land, and

used as a porousing agent in the production of bricks, as additives for fiberboard,

and in making cat litter.

Fig. 10.3 Upflow anaerobic sludge blanket (UASB) reactor

[11].

10.1 Wastewater

425

10.1.3

Characterization of Treated Wastewater

Biological processes can eliminate nearly 100% of biologically degradable organic

substances in the wastewater of paper mills. Considering the amount of organic

substances determined as COD, the level of elimination is 80 to 95%. A distinction

must be made between a genuine biological degradation and a physico-chemical

elimination, e.g. adsorption to anaerobic biosludge or activated sludge. COD and

BOD concentrations and loads in biologically treated wastewater of paper mills

producing different paper grades are given in [1].

Treated wastewater of paper mills is not normally toxic to water-borne organ-

isms, as evidenced by the results of fish and other biological tests. Due to the low

nitrogen and phosphorus concentration in the treated effluents only a small risk of

extensive growth of water plants in surface waters exists. The wastewater has very

low heavy metal content.

The additives used in paper production generally contribute only slightly to the

load of the treated wastewater. For example, some starches, soaps or surfactants

are easily biologically degradable. Others, and especially higher molecular weight

additives such as retention agents, wet strength agents or coating binders, adsorb

strongly to the biosludge. Detailed research on the subject is described in [4]

and [5].

10.1.4

Closed Water Circuit

A closed water circuit is achieved when no wastewater leaves the paper mill. The

process water is then almost 100% utilized. Only evaporation and vaporization

losses and the water content in the paper and in the residual matters must be

replaced by fresh water (1–2 L (kg paper)

–1

). A closed water circuit is shown sche-

matically in Fig. 10.4. The essential requirement for reusing circulating water in

the tertiary circuit is adequate save-all clarification. The quality of the clarified

water must be suitable for the operation of sensitive systems (e.g. showers, trim

showers). There must be enough storage capacity to compensate for variations in

quality caused by disturbances and interruptions in production. Until now, closed

circuits have been realized only in paper mills processing recovered paper. In

Germany about ten recovered paper processing mills have closed the process water

circuit totally. Most are small paper mills producing corrugating medium and test-

liner [6]. In the middle of the 1990s a German paper mill applied a biological in-

line treatment plant to control the demanding conditions of the effluent-free proc-

ess water loop. The biological treatment uses two UASB reactors as anaerobic

stages followed by two activated sludge basins, a sedimentation basin and a sand

filter. The process water volume treated in this plant corresponds to about 4 m

–1

of paper produced. Before installation of the process water treatment plant, the

COD concentration of the process water amounted to 35 000 mg L

–1

. After the

start-up of the treatment plant the COD decreased to 8000 mg L

–1

[7].

10 Environmental Aspects426

In a new development the aerobic activated sludge plant was replaced by a com-

pact aeration reactor, in order to reduce the odor of the anaerobic treated process

water and to eliminate CaCO

, which might cause problems when the biologically

treated water is led back into the papermaking process [8, 9].

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

Fig. 10.4 Scheme of a closed water circuit [6].

(a) Paper machine, (b) stock preparation plant,

(f) high density stock, (g) clarified water.

10.2 Solid Waste

427

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-

Fig. 10.5 Wastes of the German pulp and paper industry in

2001.

Table 10.1 Solid wastes of paper mills.

Wood residues Stock preparation

rejects

Sludges Incineration

waste

Other waste

Bark Ragger tails Deinking sludges Ashes Chemicals

Sawdust etc. Drum rejects Clarifier sludges Cinders (slags) Used oils

Screening rejects Biological sludges Flue ashes Wires

Rejects of cleaner Gypsum etc. Felts

Hazardous waste etc.

10 Environmental Aspects428

essing and other residues e.g. chemical waste, used wires and felts, and used oil

account for only a small share.

10.2.1

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

Table 10.2 Amount of rejects and sludges dependent on the

recovered paper grade and the paper produced [11].

Produced

paper

Recovered paper Amount of waste

[% by dry weight]

Total Rejects Sludges

Rejects and

sludges

Heavy-weight

and coarse

Light-weight

and fine

Flotation

deinking

White water

clarification

graphic

paper

news, magazines

high grades

15–20

10–25

1–2

3–5

) 3

8–13

7–16

2–5

1–5

hygienic

paper

files, office paper,

ordinary, medium

grades

28–40 1–2 3–5 8–13 15–25

market DIP office paper 32–40 < 1 4–5 12–15 15–25

liner,

fluting

sorted mixed

recovered paper,

supermarket corrugat-

ed paper and board

4–9 1–2 3–6 – 0-(1)

board sorted mixed

recovered paper,

supermarket corrugat-

ed paper and board

4–9 1–2 3–6 – 0-(1)

10.2 Solid Waste 429

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 400000 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

Table 10.3 Solid waste from different stages in recovered paper

processing.

Source of waste Composition of waste

pulper, drum pulper larger objects such as plastic bags, bookbinding backs, textiles,

bottles, shoes, strings, tools, wires, wood pieces, wet strength

paper…

high-density cleaning glass, nails, paper clips, textiles, pins, staples

pre-screening long, thin and wide contaminants

flotation deinking fillers, fibers, fines, printing ink, stickies

low-density cleaning small compact particles with high density such as sand, shives,

hard particles from UV-colours, coating colours, varnish …

fine screening plastic fragments, light-weight contaminants, hot melts, stickies

process water clarification colloidal material, fillers, fibers, fines, ink particles

10 Environmental Aspects430

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

Fig. 10.6 Composition of a mixture of reject samples from the

production of newsprint based on recycled fibers related to dry

substances (Germany) [12].

10.2 Solid Waste

431