Discussion by Menahem Rebhun
The authors' evaluation of water quality criteria was not based on commonly accepted standards
for papermaking expressed in terms of limited concentrations of certain constituents. Their direct
evaluation method of testing the effect of the processed water on the quality of the paper was a
practical approach to the situation. It is surprising that water of relatively high manganese content had
no adverse effect on fine paper production.
The proper selection of chemical process for wastewater treatment should be based on the following factors. 1. The desired quality of the product, 2. the quality of the wastewater or effluent to be
treated and 3. economics.
Of the two processes considered both alum and lime treatment, when properly appplied, give good
phosphate and turbidity removal and considerable (and comparable) color and COD reduction. The
lime process should be selected as the treatment method if a) ammonia stripping follows chemical
treatment, b) high bacteria removals are desired, and, c) high pH is required for precipitation of certain
cations such as softening and heavy metals precipitation.
For best turbidity, color, and COD reduction lime dosages to pH>10.5 should be used to obtain
magnesium hydroxide precipitation, the latter having good flocculating properties due to its
amorphous structure, high surface area and positive surface charge. The disadvantage of lime treatment
is the necessity for neutralization of the final effluent and the difficulties encountered with scaling and
post-precipitation when recarbonation is used for pH reduction.
The treatment process described in the paper has two groups of objectives. 1. Turbidity, color,
phosphate and organics reduction, and 2. Softening and heavy metals precipitation. While the first
group of objectives could be accomplished by either alum or lime treatment the second requires lime
treatment. The authors report that lime treatment on a pilot plant scale met with operational difficulties due to scaling and that because the residual hardness was also high the alum process was
selected. The water used for the pilot trials (Table 2) had a very low alkalinity:hardness ratio of
93:338 which is unfavorable for lime softening (to high pH); this is the reason for the high residual
hardness in the form of calcium hydroxide and consequent difficulties in recarbonation and scaling.
The wastewater used in the full-scale plant, shown in Table 3, was more favorable for lime softening. Its
alkalinity to hardness ratio was 210:240 and therefore, a low residual hardness could be expected,
with little difficulties with scaling. These could be further reduced by applying solid contact recirculation reactors (such as sludge blanket type) characterized by higher precipitation kinetics.
Our experience with a 2 mgd lime process plant treating the secondary effluent from Greater Haifa
indicates that good results can be obtained with proper operation and control. The effluent from the
lime treatment plant operated by Haifa Refineries Ltd. has been successfully utilized for almost two
years as make-up water for the recirculating cooling system.
l
The better flotation performance of alum floe than that produced by lime treatment is probably
the result of the higher density of the latter.
The poor removals of most heavy metals by alum treatment as opposed to lime are not surprising.
Calculations of solubilities of most of these metals, whether controlled by hydroxide or by carbonate,
show that at a pH of 7.0 little precipitation can be expected except for Fe(III) and possible copper
hydroxocarbonate. In continuous flow reactors equilibrium solubility values are rarely reached, but
rather steady state concentrations are maintained. These values are determined for a given reactor and
operation condition by reaction kinetics. The rate of percipitation of the heavy metals increases
considerably with increase in pH. Rates of reactions which precede precipitation such as oxidation of
Mn (II) to Mn(IV) are also pH dependent. A pH ^ 9.0 is necessary for appreciable precipitation of
most metals from springs wastewater. One should be aware that heavy metals precipitation in sewage
involves more than interaction between simple ions. At least a part of each metal is present in the form
of complexes with various organic ligands. Little is known about the nature and stability of these
complexes under conditions of treatment, or about the interaction and fate of the stable complex
form in the various processes, such as adsorption on alum, ferric or magnesium floe, or on calcium
carbonate precipitates.
The authors assume that similar organics are removed by the lime and alum processes. Although
the nature of organics remaining after chemical coagulation has not been yet reported, some conclusions in this regard can be made by knowing the nature of organics in secondary effluents. We have
attempted to classify these organics and have determined the percentage of each fraction in secondary
effluents.
2 Based on this study and on mechanisms of the flocculation processes one could expect that
high molecular weight proteinaceous matter and humic acid type organics will be removed by alum or
lime processes, while the low (volatile) acids, carbohydrates and most of fulvic type fractions having"
high hydrophilicity will remain in solution after such treatment.
REFERENCES
1. REBHUN, M., ENGEL, G. "Tertiary Chemical Treatment of Haifa Wastewater and Its Utilization
for Cooling Water Make-up", Report Haifa Refineries Ltd. (1970).
637
The authors' evaluation of water quality criteria was not based on commonly accepted standards
for papermaking expressed in terms of limited concentrations of certain constituents. Their direct
evaluation method of testing the effect of the processed water on the quality of the paper was a
practical approach to the situation. It is surprising that water of relatively high manganese content had
no adverse effect on fine paper production.
The proper selection of chemical process for wastewater treatment should be based on the following factors. 1. The desired quality of the product, 2. the quality of the wastewater or effluent to be
treated and 3. economics.
Of the two processes considered both alum and lime treatment, when properly appplied, give good
phosphate and turbidity removal and considerable (and comparable) color and COD reduction. The
lime process should be selected as the treatment method if a) ammonia stripping follows chemical
treatment, b) high bacteria removals are desired, and, c) high pH is required for precipitation of certain
cations such as softening and heavy metals precipitation.
For best turbidity, color, and COD reduction lime dosages to pH>10.5 should be used to obtain
magnesium hydroxide precipitation, the latter having good flocculating properties due to its
amorphous structure, high surface area and positive surface charge. The disadvantage of lime treatment
is the necessity for neutralization of the final effluent and the difficulties encountered with scaling and
post-precipitation when recarbonation is used for pH reduction.
The treatment process described in the paper has two groups of objectives. 1. Turbidity, color,
phosphate and organics reduction, and 2. Softening and heavy metals precipitation. While the first
group of objectives could be accomplished by either alum or lime treatment the second requires lime
treatment. The authors report that lime treatment on a pilot plant scale met with operational difficulties due to scaling and that because the residual hardness was also high the alum process was
selected. The water used for the pilot trials (Table 2) had a very low alkalinity:hardness ratio of
93:338 which is unfavorable for lime softening (to high pH); this is the reason for the high residual
hardness in the form of calcium hydroxide and consequent difficulties in recarbonation and scaling.
The wastewater used in the full-scale plant, shown in Table 3, was more favorable for lime softening. Its
alkalinity to hardness ratio was 210:240 and therefore, a low residual hardness could be expected,
with little difficulties with scaling. These could be further reduced by applying solid contact recirculation reactors (such as sludge blanket type) characterized by higher precipitation kinetics.
Our experience with a 2 mgd lime process plant treating the secondary effluent from Greater Haifa
indicates that good results can be obtained with proper operation and control. The effluent from the
lime treatment plant operated by Haifa Refineries Ltd. has been successfully utilized for almost two
years as make-up water for the recirculating cooling system.
l
The better flotation performance of alum floe than that produced by lime treatment is probably
the result of the higher density of the latter.
The poor removals of most heavy metals by alum treatment as opposed to lime are not surprising.
Calculations of solubilities of most of these metals, whether controlled by hydroxide or by carbonate,
show that at a pH of 7.0 little precipitation can be expected except for Fe(III) and possible copper
hydroxocarbonate. In continuous flow reactors equilibrium solubility values are rarely reached, but
rather steady state concentrations are maintained. These values are determined for a given reactor and
operation condition by reaction kinetics. The rate of percipitation of the heavy metals increases
considerably with increase in pH. Rates of reactions which precede precipitation such as oxidation of
Mn (II) to Mn(IV) are also pH dependent. A pH ^ 9.0 is necessary for appreciable precipitation of
most metals from springs wastewater. One should be aware that heavy metals precipitation in sewage
involves more than interaction between simple ions. At least a part of each metal is present in the form
of complexes with various organic ligands. Little is known about the nature and stability of these
complexes under conditions of treatment, or about the interaction and fate of the stable complex
form in the various processes, such as adsorption on alum, ferric or magnesium floe, or on calcium
carbonate precipitates.
The authors assume that similar organics are removed by the lime and alum processes. Although
the nature of organics remaining after chemical coagulation has not been yet reported, some conclusions in this regard can be made by knowing the nature of organics in secondary effluents. We have
attempted to classify these organics and have determined the percentage of each fraction in secondary
effluents.
2 Based on this study and on mechanisms of the flocculation processes one could expect that
high molecular weight proteinaceous matter and humic acid type organics will be removed by alum or
lime processes, while the low (volatile) acids, carbohydrates and most of fulvic type fractions having"
high hydrophilicity will remain in solution after such treatment.
REFERENCES
1. REBHUN, M., ENGEL, G. "Tertiary Chemical Treatment of Haifa Wastewater and Its Utilization
for Cooling Water Make-up", Report Haifa Refineries Ltd. (1970).
637