Refinement of Purified Sewage for Unrestricted Industrial Use
633
pressure control valve. The inadequate alum supply pump was replaced by a Bran and
Lubbe variable stroke piston feed pump, capable of working against the pressure in the
feed line. This arrangement resulted in a substantial improvement in floe formation, but
there was still some carry-over of fine floe into the clarified water.
To further improve floe formation, various polyelectrolytes were tested as a coagulant
aid. The results were promising enough to warrant the erection of permanent dosing
equipment for continuous addition of polyelectrolytes. In March 1971 the chemical
dosage was changed to —
Alum as [A1 2 (S0 4 ) 3 .14H 2 0]:
70-75 mg/1
Davco polyfloc 84A
:
1.2-1.6 mg/1
NaOH
:
0 to 10 mg/1
Cl 2 (after leaving the flotation tank,
before sand filtration)
:
3 mg/1
The addition of polyelectrolyte resulted in a spectacular improvement, with floe
carry-over being reduced; the suspended solids content of the clarified water dropped to
between nil and 1 mg/1.
Addition of up to 10 mg/1 NaOH is necessary as calculations of the stability index
show that the treated water is slightly corrosive.
A Wallace and Tiernan chlorinator was installed for the prevention of unwanted slime
and algal growth on the sand filters. Addition of chlorine on an intermittent basis
averages 3 mg/1.
Practical operational results as far as water quality is concerned are shown in Table 3.
The untreated purified sewage water sometimes has a very slight pinkish colour which
is due to dye containing effluent from a polony factory. This colour is difficult to remove
and is still present in the treated sewage water. Experiments, however, showed that the
effect of this colour on paper brightness is insignificant.
DISCUSSION
When comparing the practical results obtained with alum dosage and those obtained
on a pilot plant with lime treatment it will be noted that with alum the removal of heavy
metal ions is not as efficient. Manganese in particular is hardly reduced with alum, but the
removal of the iron is good. However, the brightness attained with alum treated sewage
water is practically as good as with lime treated sewage water. From experience it appears
that, amongst the heavy metals, the iron concentration is more detrimental as far as pulp
and paper brightness is concerned.
The success of this water reclamation scheme is largely ascribed to the removal of
organic matter which would otherwise be precipitated with alum in the subsequent
process of papermaking. On the other hand the effect of residual heavy metals on paper
brightness seems to be negligible, which is not quite in line with various statements made
in literature. At SAPPI it has been found that pulp leaving the digesters contains on the
average about 100 mg manganese per kg of bone dry pulp, thought to be mainly
originating from the unslaked lime used for re-causticizing the cooking liquor after the
soda-recovery process. Similarly, the pulp leaving the digester contains 30-50 mg iron per
kg. It can thus be seen that the concentration of iron and manganese in the water is
insignificant in comparison with the contamination from other sources.
633
pressure control valve. The inadequate alum supply pump was replaced by a Bran and
Lubbe variable stroke piston feed pump, capable of working against the pressure in the
feed line. This arrangement resulted in a substantial improvement in floe formation, but
there was still some carry-over of fine floe into the clarified water.
To further improve floe formation, various polyelectrolytes were tested as a coagulant
aid. The results were promising enough to warrant the erection of permanent dosing
equipment for continuous addition of polyelectrolytes. In March 1971 the chemical
dosage was changed to —
Alum as [A1 2 (S0 4 ) 3 .14H 2 0]:
70-75 mg/1
Davco polyfloc 84A
:
1.2-1.6 mg/1
NaOH
:
0 to 10 mg/1
Cl 2 (after leaving the flotation tank,
before sand filtration)
:
3 mg/1
The addition of polyelectrolyte resulted in a spectacular improvement, with floe
carry-over being reduced; the suspended solids content of the clarified water dropped to
between nil and 1 mg/1.
Addition of up to 10 mg/1 NaOH is necessary as calculations of the stability index
show that the treated water is slightly corrosive.
A Wallace and Tiernan chlorinator was installed for the prevention of unwanted slime
and algal growth on the sand filters. Addition of chlorine on an intermittent basis
averages 3 mg/1.
Practical operational results as far as water quality is concerned are shown in Table 3.
The untreated purified sewage water sometimes has a very slight pinkish colour which
is due to dye containing effluent from a polony factory. This colour is difficult to remove
and is still present in the treated sewage water. Experiments, however, showed that the
effect of this colour on paper brightness is insignificant.
DISCUSSION
When comparing the practical results obtained with alum dosage and those obtained
on a pilot plant with lime treatment it will be noted that with alum the removal of heavy
metal ions is not as efficient. Manganese in particular is hardly reduced with alum, but the
removal of the iron is good. However, the brightness attained with alum treated sewage
water is practically as good as with lime treated sewage water. From experience it appears
that, amongst the heavy metals, the iron concentration is more detrimental as far as pulp
and paper brightness is concerned.
The success of this water reclamation scheme is largely ascribed to the removal of
organic matter which would otherwise be precipitated with alum in the subsequent
process of papermaking. On the other hand the effect of residual heavy metals on paper
brightness seems to be negligible, which is not quite in line with various statements made
in literature. At SAPPI it has been found that pulp leaving the digesters contains on the
average about 100 mg manganese per kg of bone dry pulp, thought to be mainly
originating from the unslaked lime used for re-causticizing the cooking liquor after the
soda-recovery process. Similarly, the pulp leaving the digester contains 30-50 mg iron per
kg. It can thus be seen that the concentration of iron and manganese in the water is
insignificant in comparison with the contamination from other sources.