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10.1 Wastewater The paper industry has a high fresh water demand. At the beginning of the 20th century, about 500 to 1000 m3 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 m3 (t paper)–1 which corresponds to a wastewater volume of about 11 m3 (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 BOD5 (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 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 BOD5 loads, classified accord¬ing to product groups, is given in [1]. 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 %.
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 BOD5 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.
10.1 Wastewater
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.
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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 m3 t–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].
10.2 Solid Waste
.(a) Paper machine, (b) stock preparation plant,
.(c) storage, (d) save-all, (e) fresh water, (f ) high density stock, (g) clarified water.
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 CaCO3, which might cause problems when the biologically treated water is led back into the papermaking process
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