Page 136 - Membranes for Industrial Wastewater Recovery and Re-Use

110 Membranes for Industrial Wastewater Recovery and Re-use

Table 3.11 Specific freshwater consumption in modern paper mills (Sundholm, 2000; Weise
et al., 2000)
Paper grade in general Typical example Water consumption (m3/t)

Newsprint 5-1 5
Pulp mill and newsprint line 9
Wood-free fine paper 5-10
Supercalandered (SC) paper 10-1 5
Lightweight coated (LWC) paper 10-20
Pulp mill and LWC line 11
Tissue 5-1 5
Paper machine andprocess 12
Liner and fluting 5-1 5
Pulpand testliner mill 5
Multiple board 8-1 5
Board line 10

Table 3.12 Freshwater consumption in papermaking (Edelmann, 1999a; Haasanlammi,
1999)
Unit operation water use, m3/t Printing paper Newspaper
grades grades
Total fresh water consumption 16-2 6 27
Paper machine showers 6-7 5
Dilution/preparation of chemicals 2-3 3
Makeup waters in pulp production 2-3
Cooling of process equipment 3-10 10
Sealing waters 1 6
Power plant 2-3
Miscellaneous (washing, sealing etc.) 3

suspended solids from the water. The modern disc filters produce three filtrates;
cloudy, clear and superclear filtrate. The cloudy filtrate is usually directed back to
the stock preparation, but the clear and superclear filtrates are either reused -
e.g. as wire section shower water - or discarded as effluents, depending on their
suspended solids content and on the paper grade produced. In the worst case the
clear filtrate can constitute more than a half of the total effluent load of a paper
mill. The main prerequisite for recycling the clear or the superclear filtrate in the
process is that they are practically free of suspended solids.
The clear filtrate from the paper machine consists of suspended solids,
dissolved and colloidal substances originating from wood, and salts. In addition,
the clear filtrate also contains traces of all the paper making chemicals added
to the process. If the paper machine produces coated paper grades, the traces of
the coating colour ingredients are also found in the clear filtrate. These include
various pigments and latices. It should be noted that white water quality varies
significantly between machines and, in some cases, no significant difference is
seen between the clear and superclear filtrates.