Pulp Mill Waste Water
Stripping by Air and Steam
H. KIRK WILLARD, Sanitary Engineer
Paper and Forest Industries Research
National Environmental Research Center
Environmental Protection Agency
Corvallis, Oregon
INTRODUCTION
Most kraft and sulfite pulp mills now practice chemical recovery as a means of
economical operation and to reduce pollution to the environment. During this recovery
process, effluent solids from the pulping process are applied to a 5-7 effect evaporator, and
concentrated to approximately 50 percent solids. During this process product condensate
water is produced which generally has relatively low total solids and quite high volatile
organic contaminants. These contaminants range up to 40 pounds BOD per air dry ton
(ADT) and can add a significant load to receiving streams.
Currently much of this material is used within the plant itself wherever the
contaminants do not affect the quality of product. In particular the brown stock washers
have received considerable amounts of evaporator condensate. The new Ticonderoga Mill
in New York has incorporated this use into its system as have many other mills at
Springfield, OR, and Macon, GA. Other uses of this condensate are for washwaters,
causticizing, stack scrubbers for the lime kilns and liquor makeup (1,2). Vapors from these
streams are often collected in vaporspheres and burned in the lime kiln.
Unfortunately volatile material which is used in counter current brown stock washers
will be vented as malodorous gases or be collected and will eventually end up back in the
evaporators during the liquor concentration step. This recycle from liquor to condensate to
washers to liquor again is repeated. There is no method for moving these materials out of
this cycle, without employing either stream stripping or adsorption of some type. It has also
become apparent, due to the recent 1970 Clean Air Act, that there will be more limited use
of these sulphur-bearing condensates throughout the mill process waters (3). Therefore,
a process is needed whereby volatile materials can be effectively removed from both the
mill effluent wastewater and the system itself. The basis system for an evaporator-di-
gestor process line is shown in shcematic diagram, Figure 1.
Leonard (2) and Bergkvist and Foss (4) have shown that the concentration of BOD
materials from each effect of evaporation (Figure 2) increases as one goes from the first to
last effect. Although the flow from each effect is somewhat similar, the volatile materials
removed are more concentrated in the last effect. Thus, correct selection of these streams
can contain most of the condensates in a small volume of water.
Other volatile waste streams come from the pulping process. These are normally of
similar characteristics to that of the evaporator condensates but come from the pulping
process itself as blow and relief condensate materials. In addition, turpentine decanter
underflows, accumulator hot water, and decker filtrates also contain similar organic
chemicals which are quite volatile in nature and exert high BODs.
Along with the alteration of sulfite pulping from calcium base to magnesium or
ammonium, has come the collection and evaporation of cooking chemicals. Evaporator
condensates from these units respond to stripping and recently have been studied more
intensely (5). The evaporation of hardboard process waters produces condensates similar to
that of the sulfite process and have not had stripping applied yet.
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