Paper Mill Waste 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 which two-thirds were composted. The share of residues disposed of by landfilling had already dropped to just 6 %.