10 Environmental Aspects of Pulp Production

The Tasmanian mill guidelines exclude any discharge of effluent to a river. It is

obviously assumed that rapid dilution of the effluent with sea water reduces the

environmental impact. This limits the potential sites for a pulp mill to the coastline.

The data listed in Tab. 10.3 show the comparability of the emission limits

with the values described by the European Commission.

Tab. 10.3 Selected emission limits to the marine environment

described in 2004 for a potential pulp mill in Tasmania using

“accepted modern technology” (AMT) [3].

Parameter Unit Monthly average maximum Daily maximum

TSS kg adt–1 2.6 4.5

BOD5 kg adt–1 2.1 3.6

COD kg adt–1 20 34

AOX kg adt–1 0.2 0.4

In North America, effluent color is a parameter which must be monitored.

Color is more of aesthetic value than a parameter of toxicity. The presence of small

amounts of humic acids represent a natural source for the brown color of water.

However, in the past black liquor spills and the extraction stage effluent following

a chlorination stage may have been responsible for the dark brown color of a pulp

mill’s effluent. Therefore, color still can be an effluent control parameter, and is

analyzed in color units (CU) using different dilutions of a mixture of a platinum

complex and a cobalt salt.

10.5

Solid Waste

Solid waste results from a variety of sources, including inorganic sludge from the

chemical recovery consisting of dregs and lime mud, bark and wood residues, as

well as sand from wood handling. Sludge results from the effluent treatment, and

consists of inorganic and organic material. In addition, there are dust and ashes

from boilers and furnaces. Most of the organic waste products are burned for energy

generation. The ash resulting from bark burning typically contains nutrients

taken with the wood from the forest, and may be suitable as a fertilizer as long as

It is not contaminated with other trace elements such as mercury, lead, or cadmium.

The metal traces that occur naturally in wood are in part collected in the

lime sludge and in the wastewater treatment sludge. These are then partly discharged

with the effluent.

Sludge from the wastewater treatment constitutes the main group of waste, and

large amounts may be generated in the primary and secondary treatment stages.

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10.6 Outlook

The amounts of sludge become smaller with anaerobic treatment conditions.

Because aerobic sludge can be difficult to dewater, primary sludge (with fibers) or

bark is frequently added to improve results. The amount of inorganic material in

sludge may be rather high; a few ppm of transition metals in the wood and bark

will translate, together with the lime mud and other waste, into over 40 kg adt–1 of

solid waste [2]. It should be noted that this value is the sum of all waste, consisting

of dewatered wastewater treatment sludge, wood and other ash and wood waste.

Lime mud and green liquor clarification dregs can be disposed of in landfills.

However, they must be washed very well to avoid the emission of H2S during

mud drying, and to avoid the leaching of caustic soda residual. Sludge with a high

content of combustible material may be dried and incinerated.

10.6

Outlook

Hopefully, this short summary on the environmental impact of pulp production

has highlighted the point that there is no simple solution to all negative impacts.

The close interrelation of the different process steps makes it almost impossible to

avoid one problem without raising or increasing the size of another one. To name

a few examples: An effluent-free mill will need to dispose of more solid waste and

consequently may require more fossil fuel or more energy in general. Very tight

water loops might require more chemical for more frequent cleaning steps, or the

application of poorly biodegradable chelants. They might at the same time