Use of Molecular Oxygen in Treating
Semi-Chemical Pulp Mill Wastes
MILLARD H. ROBBINS, JR., Resident Engineer
National Council for Stream Improvement
Blacksburg, Virginia
This investigation was conducted by the National Council's Virginia
Polytechnic Institute project, under the direction of the writer. It involved
the determination of the treatment that could be obtained by biological
oxidation of neutral semi-chemical pulping waste by use of modifications of
the activated sludge process.
The past decade has seen the increasing use of the activated sludge
method to treat industrial wastes. The application of activated sludge treatment forkraft paper mill wastes is well established and several large activated
sludge plants have been installed by kraft mills in the past few years.
Because of the successful results obtained by these mills, a study was
initiated at the National Council's Virginia Polytechnic Institute Project to
study the feasibility of treating neutral sulfite semi-chemical mill wastes by
the activated sludge technique. This study was conducted in a bench-scale
pilot plant with a total aeration tank capacity of 1.5 cu ft. Neutral sulfite
semi-chemical spent liquor of 15 to 30 per cent solids and BOD ranging from
30, 000 ppm to 70, 000 ppm, were diluted to give a BOD concentration range
that would exist in the total mill effluent. Nitrogen and phosphorus supplements were added to the waste feed to correct deficiencies in these elements
needed for efficient biological oxidation.
The results of the study were encouraging. At a loading of 0.46 lbs of
BOD per lb of suspended solids, up to 91 per cent BOD removal was effected.
The waste feed BOD concentration was 9, 600 ppm, giving aerator loadings as
high as 330 lbs of BOD per 1, 000 cu ft of aerator capacity per day for short
periods of time. The suspended solids concentrations as high as 12, 000 ppm
were carried in the mixed liquor.
As might be expected, considerable difficulty was encountered at the
high suspended solids concentrations in trying to supply sufficient oxygen to
the system to maintain aerobic conditions. Several different aeration methods
were tried with little success. Based on these results, it was felt that some
aeration method other than diffused air or mechanical aeration was necessary.
Okun introduced the use of molecular oxygen for aerobic biological waste
treatment in a modified activated sludge plant, which he termed a bio-
precipitation system. The process consisted of oxygenation of the waste with
molecular oxygen prior to its introduction into an upflow unit containing a
high concentration of activated sludge. He concluded that the results were of
sufficient promise to warrant consideration of the process as a means for
sewage treatment.
Budd and Lambeth conducted an extensive pilot plant study on the bio-
precipitation system proposed by Okun. They concluded that the use of pure
oxygen in the treatment of sewage by the bioprecipitation process is practical
and can be economically justified,  particularly for large plants in confined
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