Water and the Paper Industry ::
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Overview
Water is a key element and much used in the production of paper. It is used in nearly every stage of the pulping and papermaking process. Pulp and paper mills consume 75-225 m³ of water per tonne of paper manufactured.
| Mill size | Water consumption in m³ per tonne of paper |
|---|---|
| Large | 125-200 |
| Medium/Small | 75-210 |
| Recycling based mills | 75-100 |
Figure 2 :: Water consumption in various stages of pulp and paper manufacture
[Source: Ansari 2004]
Cleaning the Water
Inevitably the water used by the paper mills picks up waste (effluents) as a result. To reduce the environmental impact, the effluents from the papermaking process are collected and treated before being returned to surface waters such as rivers, canals or estuaries. The volume of effluents can be reduced by the reuse of processed water.
One method to clean the water is to use something called membrane filtration. The types of membrane filters that can be used are Micro Filtration (MF), Ultra Filtration (UF) and Nano Filtration (NF). There are also some experiments with a new type of membrane, the ceramic membrane. This membrane is used because it is easier to clean this filter with a backflushing principle rather than the normal carbon filter. Backflushing is a process in which the continual water flow is reversed from time to time. This flow reversal, in most cases, improves the flow by removing some of the things than can block the cleaning process from the membrane surface.
Figure 3 :: Member filtration equipment and backflushing (reverse osmosis) process
The pH of the water is also important. For acidic pH lower fluxes and more fouling were observed compared to neutral pH. The pH of the water is also important in controlling the amount of waste build up that covers the cleaning membrane. As a general rule, acidic pH is bad for the membrane and a neutral pH is good.
Interestingly, the water taken from rivers or lakes often has to be purified before it can be used at the mill and, as a result of enhanced treatment techniques and internal changes in the manufacturing process, it will be returned in a cleaner state than when it was taken.
Content of the water
Waste water effluents from pulp and paper mills contain mainly solids, nutrients (nitrogen and phosphorus) and organic substances. The concentration of organic substances in effluent water is expressed as the amount of oxygen it takes to degrade these substances through either biological processes (biological oxygen demand - BOD) or chemical reactions (chemical oxygen demand). Since the mid-1990s there has been a major decrease of over 70% in the discharge of BOD per tonne and this helps to combat the problem of oxygen depletion of surface waters.
Effluents from chemical pulp mills also contain organic chlorine compounds (AOX). Some of these substances are naturally present in wood and some come from the chlorine bleaching process. Chlorine gas was once the primary bleaching agent used by the industry but has now been replaced in favour of more environmentally sound bleaching techniques that use chlorine dioxide and ozone, resulting in a massive reduction of AOX in the effluents.
[See also: www.paperonline.org]
Figure 4 :: Paper Mill near to water source and an Effluent Clarifier used in the water purifying process
Further information
For those with concerns regarding the environmental impact of the paper industry on water sources, the website below provides useful information, if a bit one-sided.
www.foe.co.uk/resource/briefings/consequence_pulp_paper.html
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