Deal with these problems. Brownstock washing following an oxygen delignification

Step must remove dissolved organic compounds and inorganic pulping

Chemicals effectively. Typically, the level of washing efficiency asked for is recovery

of 99% of the dissolved organic material. Similarly, the lime mud generated in the

Green liquor clarification requires efficient washing.

Evaporation condensates must be stripped and appropriately reused. The condensates

Of acidic pulping processes (e.g., sulfite pulping) contains compounds

Such as methanol, acetic acid and furfural. These compounds may be separated, and

Represent a valuable byproduct; alternatively, they can be used as an energy source in a

Boiler. Water can be saved by recycling all clean cooling and sealing water.

The typical secondary effluent treatment unit for kraft pulp mills uses aerated

Lagoons in which the biomass is degraded by bacteria. Zones with a low oxygen

Content allow anaerobic fermentation and sludge decomposition, which decreases

The generation of excess biomass (sludge).

Emissions to the Aquatic Environment

The energy demand for oxygen addition is rather high [11], with about

1 kWh kg–1 BOD5 being required for the aeration. About half of the organic material

Is converted into carbon dioxide, while the other half is converted into biomass

And must be separated as sludge. Sludge dewatering and disposal, followed by

Combustion of the sludge, leads to additional costs. Anaerobic fermentation may

be an attractive alternative, as conversion of the organic matter leaves only 5% of

The input carbon as residual for disposal or combustion. The process requires energy,

Predominantly for pumping, but also generates biogas consisting of methane

containing a largeamount of carbon dioxide.The typical process has four steps: (a) the

Fermentation and dissolution of insoluble residuals (fibers); (b) acidic degradation

Into alcohols and aldehydes; (c) conversion of these intermediates into acetic acid and

Acetates by micro-organisms; and (d) conversion into methane. For an effective process,

Acetogenic and methanogenic bacteriamust exist in a close symbiosis.However,

The surplus of energy generated by the conversion of waste sludge from the sustainable

Source wood into biogas does make anaerobic treatment very attractive.

A tertiary treatment is the final precipitation of remaining suspended or dissolved

Compounds by chemical coagulation. Usually, compounds used for such

treatments include aluminum salts, ferric salts (Fe3+), and lime slurries, while

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