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
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.
10.1.2.2.2 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 . 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 .
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.