Refinement of Purified Sewage for Unrestricted Industrial Use
629
was made by filtering the dispersion rapidly on a 15 cm Bόchner funnel followed by
pressing and drying under standard conditions. These sheets were tested in reflectance
(brightness) against sheets made with distilled water.
Laboratory-scale Investigations
The purified sewage was subjected to various treatments with chlorine compounds and
the aforementioned test on pulp brightness applied.
Chlorine gas at dosages up to 100 mg/1 did not give reliable improved results. The
required dosage appeared to vary with the fluctuation in organic content of the purified
sewage. Sodium hypochlorite (NaOCl) was even less promising than chlorine gas, with
dosage requirements between 200 and 400 mg/1. Calcium hypochlorite Ca(OCl) 2 showed
the same disadvantages and was even more expensive.
Other treatments which were investigated were:
Filtration through activated carbon removed the soluble colouring matter, including
Fe and Mn, but the costs were prohibitive. Filtration through a bed of adsorbents was
tested with bauxite, gas works coke and various types of manganese dioxide such as
pyrolusite and 'battery grade'; results proved to be erratic and inadequate.
Flocculation with 300 mg/l of lime gave the most promising results. In the pH range of
10.8 to 11.1 not only was sterilization achieved, but in addition, an excellent removal of
heavy metals was obtained. Organic matter was also reduced by about fifty per cent. Pulp
brightness came very close to that with distilled water and was at least equal to the
brightness obtained when Rand Water Board water was used. Flocculation with alum was
also tested with promising results.
Pilot plant investigations
(a) Excess lime treatment
The favourable results obtained in the laboratory led to pilot plant-scale investigations
(4.5 m
3 /hr or 1000 gal/hr), using the same dosage of 300 mg/1 of hydrated lime.
Difficulty was encountered with the occurrence of scale when the pH was adjusted by
recarbonation with flue gas; this is thought to be due to calcium carbonate being both in
solution and supersaturated solution at pH values in excess of 9.4. This scale formation
could not satisfactorily be controlled. The best sequence of application was found to
include carbonation in two stages; the sequence of the treatment steps is shown below:
(i)
addition of 300 mg/1 Ca(OH) 2 to reach a pH of 10.8 - 11.1 followed by
settling or flotation;
(ii)
solids contact stabilization, consisting of carbonation with C0 2 and settling
to produce an effluent of pH 9.5;
(iii)
recarbonation to the desired end pH; and
(iv)
sand filtration.
Chemical costs for lime treatment are 0.43 c/m
3 .
Typical results obtained with this type of treatment on pilot plant scale are shown on
Table 2.
As can be seen from Table 2, colour, organic matter, magnesium and heavy metals
were effectively coagulated and removed, iron and manganese were reduced to below the
limit specified by SIC 26 or TAPPI E603 Specification. It is interesting to note that
although hardness was not reduced to the level specified, the brightness was not adversely
affected. Experience has shown that it is possible to produce good paper with hard water
by counteracting hardness through increasing the alum addition in papermaking.
629
was made by filtering the dispersion rapidly on a 15 cm Bόchner funnel followed by
pressing and drying under standard conditions. These sheets were tested in reflectance
(brightness) against sheets made with distilled water.
Laboratory-scale Investigations
The purified sewage was subjected to various treatments with chlorine compounds and
the aforementioned test on pulp brightness applied.
Chlorine gas at dosages up to 100 mg/1 did not give reliable improved results. The
required dosage appeared to vary with the fluctuation in organic content of the purified
sewage. Sodium hypochlorite (NaOCl) was even less promising than chlorine gas, with
dosage requirements between 200 and 400 mg/1. Calcium hypochlorite Ca(OCl) 2 showed
the same disadvantages and was even more expensive.
Other treatments which were investigated were:
Filtration through activated carbon removed the soluble colouring matter, including
Fe and Mn, but the costs were prohibitive. Filtration through a bed of adsorbents was
tested with bauxite, gas works coke and various types of manganese dioxide such as
pyrolusite and 'battery grade'; results proved to be erratic and inadequate.
Flocculation with 300 mg/l of lime gave the most promising results. In the pH range of
10.8 to 11.1 not only was sterilization achieved, but in addition, an excellent removal of
heavy metals was obtained. Organic matter was also reduced by about fifty per cent. Pulp
brightness came very close to that with distilled water and was at least equal to the
brightness obtained when Rand Water Board water was used. Flocculation with alum was
also tested with promising results.
Pilot plant investigations
(a) Excess lime treatment
The favourable results obtained in the laboratory led to pilot plant-scale investigations
(4.5 m
3 /hr or 1000 gal/hr), using the same dosage of 300 mg/1 of hydrated lime.
Difficulty was encountered with the occurrence of scale when the pH was adjusted by
recarbonation with flue gas; this is thought to be due to calcium carbonate being both in
solution and supersaturated solution at pH values in excess of 9.4. This scale formation
could not satisfactorily be controlled. The best sequence of application was found to
include carbonation in two stages; the sequence of the treatment steps is shown below:
(i)
addition of 300 mg/1 Ca(OH) 2 to reach a pH of 10.8 - 11.1 followed by
settling or flotation;
(ii)
solids contact stabilization, consisting of carbonation with C0 2 and settling
to produce an effluent of pH 9.5;
(iii)
recarbonation to the desired end pH; and
(iv)
sand filtration.
Chemical costs for lime treatment are 0.43 c/m
3 .
Typical results obtained with this type of treatment on pilot plant scale are shown on
Table 2.
As can be seen from Table 2, colour, organic matter, magnesium and heavy metals
were effectively coagulated and removed, iron and manganese were reduced to below the
limit specified by SIC 26 or TAPPI E603 Specification. It is interesting to note that
although hardness was not reduced to the level specified, the brightness was not adversely
affected. Experience has shown that it is possible to produce good paper with hard water
by counteracting hardness through increasing the alum addition in papermaking.