Page 275 - Membranes for Industrial Wastewater Recovery and Re-Use

244 Membranesfor Industrial Wastewater Recovery and Re-use

of 40 000 m3. The internal heating loop has reduced gas consumption by 20%
from 660 m3 tor1 to 520 m3 ton-l producing an overall saving in gas of
700000 m3. Total wastewater discharged from the sites has halved to a
population equivalent of 5000 p.e. which generates a saving of €22 7 000 per
year ($226 000).

5.5 Kronospan Ltd: medium-density fibreboard wash water
recycling (UK)

5.5.1 Background
The production of medium-density fibreboard (MDF) involves a number of simple
but large-scale operations. Forest thinning and sawmill residues are debarked
and chipped before being washed to remove residual dirt and grit from the wood.
The fibres are then steam-softened, cooked and refined between two flat plates
and finally mixed with resin to produce the fibreboard.
An example facility is at Chirk in North Wales where Kronospan UK
manufacture chipboard, MDF, melamine-faced boards and sawn timber. The
plant produces 47800 m3 of MDF effluent per year of which the majority is
associated with the washing and refining stages of production. The quantity of
excess water generated during the process depends on the moisture content of
the wood and can range from 400 1 per tonne of bone dry wood processed in the
summer to up to 600 1 in the winter. The characteristics of the effluent are high
COD and high suspended solids of which cellulose, lignins and resin acids are key
components.
Prior to 1995 the effluent was tankered off site at a cost of €9.8 tonne-l
($14.72 tonne-'). The company was not only concerned about the cost of
this option but also the risk to production if the tankers failed to arrive as the
site could only store one day's effluent. A decision was made to incorporate
on-site treatment at the production facility which should meet the following
aims:

0 Low capital cost/rapid investment payback
0 Optimum productfresource recovery
0 Effluent reduction, recycling and reuse
0 Long-term environmental compliance
0 Easy to upgrade modular system

The site contained an existing conventional biological treatment train
comprising a dissolved air flotation unit followed by an activated sludge plant.
Problems with effluent reliability and limited resource recovery due to the poor
effluent quality led to the search for an alternative solution. Ultimately, a
treatment train containing membranes was installed in June 1995, which
offered the potential for zero discharge of the wash water effluent.