Section 11.    AGRICULTURAL WASTES
RECOVERY OF PROTEIN FROM SLAUGHTERHOUSE EFFLUENTS
BY DOUBLE ADJUSTMENT OF pH
R. N. Cooper. Section Leader
J. M. Russell, Chemist
Meat Industry Research Institute of New Zealand (Inc.)
Hamilton, New Zealand
J. L. Adam. Engineer
Ruakura Agricultural Research Station
Hamilton, New Zealand
The effluent from a slaughterhouse characteristically contains high concentrations of
fat and organic nitrogen. Primary treatment of slaughterhouse effluents removes only 20%
of the organic nitrogen [ 1 ], reflecting the large proportion that is soluble. Further treatment can therefore be effected only by biological or chemical methods.
Chemical treatment of slaughterhouse effluents has been studied for a long time. However, attention has only recently been focused on recovering potentially valuable protein-
aceous material from these effluents. All recovery techniques acidify the effluent to a pH
between 3 and 5, and these techniques often use protein precipitants such as lignosulfonic
acid [2-6] or sodium hexametaphosphate [71 to recover soluble blood proteins. These
techniques also destabilize other colloidal material and break fat emulsions, enabling proteins, other colloids and fats to be separated from the liquid phase by gravity sedimentation or dissolved air flotation. Aluminum (8) and ferric ions [9] have also been used to
purify slaughterhouse effluents, but the recovered material is contaminated with the metals
which may preclude its use as an animal feed supplement.
The use of sodium hexametaphosphate in conjunction with acidification has been studied
extensively in New Zealand. However, the high cost of this and other protein precipitants
has led to the development of an alternative technique-the double pH adjustment process
[10,11). In this process the effluent pH is reduced to approximately 3 by the addition of
sulfuric acid and then neutralized with either sodium or calcium hydroxide to a pH between
6 and 9.
This chapter reports experiments on the mechanisms of double pH adjustment. It also
describes the performance of a 1 20-m3 /hr double pH adjustment plant of the type described
by Stephenson [111 and reports the results of pig-feeding trials using dried material recovered from this plant.
EXPERIMENTAL
Materials
Fresh blood, collected from a mutton slaughter area, was immediately diluted to 0.1-
1% with water to allow hemolysis to occur. Slaughterhouse effluent, obtained from a large
New Zealand slaughterhouse, was allowed to settle for 1 hour and the middle fraction was
used for subsequent experimental work. Hemoglobin (Calbiochem Bclhring Corp., U.S.),
was dissolved in distilled water to make a 0.1 2% solution.
Analytical Methods
Effluents were analyzed for COD, TKN, NH3N. fat, TSS. TS. VS and total and inorganic
phosphorus (TP and IP) by the procedures described in Standard Methods [12]. Carbo-
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