Paper 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.
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.
Heavy-weight and Light-weight and Deinking sludges coarse rejects fine rejects
composition glass, nails, sand, stones, paper clips, pins
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 %