10 Environmental Aspects of Pulp Production

Detailed data on the typical emissions and the impact of different measures

using BAT, for example, wet scrubbing only or with an electrostatic precipitator,

are available in the European Commission report [2].

10.4

Emissions to the Aquatic Environment

Washing following the pulping and bleaching process requires water, the demand

for which has decreased significantly during the past few decades. This is the

result of more effective washing procedures with improved equipment, and a

higher degree of countercurrent water flow. The process, which is typically

described as “loop closure”, became possible with the elimination of chlorine and

hypochlorite from the bleaching sequences, both of which required a low treatment

temperature and therefore cold dilution water. ECF bleaching allows a

rather constant high temperature range to be maintained from brownstock washing

to the dryer machine. The positive side effect is a lower demand for energy to

heat the process water, while the downside is a higher tendency for scaling, for

example with barium sulfate and calcium oxalate.

Cleaning of the effluent begins with the sedimentation of suspended solids,

which describes mostly the recovery of fiber losses; this step is labeled as “primary

effluent treatment”.

As mentioned previously, wood handling and debarking should be made without

generating a higher volume of effluent. If de-icing or log washing is required,

the effluent must be made nontoxic by a biological treatment. An example of this

Is woodyard effluent (rain water), which must be collected and treated biologically

(unlike other rain water).

The pulping liquor should be recovered very effectively. Screening and brownstock

washing should be conducted in a closed loop mode. Spills and leakages

should be avoided by an optimized process control, but if they do occur enough

temporary storage volume and evaporation capacity must be made available to

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).

1004