Treatment Plants for Phosphorus Removal from Wastewater
dissolved and suspended phosphorus as the measures to be taken for the two
fractions of phosphorus are different.
In respect of simultaneous precipitation with iron, usually Fe++, there are two
factors which may cause problems, that is, lack of oxygen control in the biologicalchemical reactor and excessive turbulence by the aeration.
By overloading with organic matter, anaerobic conditions will occur in the simultaneous precipitation system. The result is a reduction of Fe+++ to Fe++, and this again
may result in a certain resolution of phosphorus depending on the concentration of
calcium ions in the water. The resolution is especially important around pH 7 and
lower pH values.
Aerobic conditions can be reestablished by reducing the supply of organic matter or
increasing the aeration capacity. The aeration capacity may for example be increased
by blowing more air through the diffusors in activated sludge plants. This may,
however, cause another problem in that the increased turbulence destroys the
biological-chemical floes to the effect that the content of suspended phosphorus in
the effluent will increase. It appears from Table 10.5 and Fig 10.17 that optimum
flocculation occurs at a mean velocity gradient, G, of 10-50 s- 1 . The G value in an
aeration tank in an activated sludge plant may, however, be 100-200 s-1, in other
words, considerably above the optimum value. It should therefore be endeavoured
to transfer the necessary oxygen with as little turbulence as possible, for example by
using fine bubble aeration instead of coarse bubble aeration.
In respect of simultaneous precipitation with Fe++ it should finally be mentioned
that in medium loaded plants, that is, Bx = 0.2-0.4 kg BOD I (kg SS · d), considerable
foaming problems have been observed in the aeration tank due to the development
of a certain genus of micro-organism (actinomycetes). The problem does not occur
in low-loaded plants /12/.
When using AI+++, there will be no problems with phosphorus resolution under
anaerobic conditions.
Finally it should be mentioned that a pH control of the simultaneous precipitation
process is important at low dosings, that is, a molar ratio around 1.
The biggest process problem at post precipitation and contact filtration with iron
and aluminium salts is the great influence of the alkalinity of the water at relatively
low dosings. In order to obtain low alkalinity, and hence low phosphate concentrations, acid must be added. It may be done by adding, for example, sulphuric acid or
hydrochloric acid, more dosing agent (which in itself is acid), or by establishing
nitrification in the biological process during which alkalinity is consumed.
Another method to lower the phosphate concentration is to recycle chemical sludge
from the post precipitation to the biological process, thus combining simultaneous
precipitation and post precipitation and utilizing the fact that the sludge from the
post precipitation has a binding capacity in respect of phosphorus at the somewhat
higher phosphate concentrations which are found in the simultaneous precipitation
plant compared with post precipitation.
347
dissolved and suspended phosphorus as the measures to be taken for the two
fractions of phosphorus are different.
In respect of simultaneous precipitation with iron, usually Fe++, there are two
factors which may cause problems, that is, lack of oxygen control in the biologicalchemical reactor and excessive turbulence by the aeration.
By overloading with organic matter, anaerobic conditions will occur in the simultaneous precipitation system. The result is a reduction of Fe+++ to Fe++, and this again
may result in a certain resolution of phosphorus depending on the concentration of
calcium ions in the water. The resolution is especially important around pH 7 and
lower pH values.
Aerobic conditions can be reestablished by reducing the supply of organic matter or
increasing the aeration capacity. The aeration capacity may for example be increased
by blowing more air through the diffusors in activated sludge plants. This may,
however, cause another problem in that the increased turbulence destroys the
biological-chemical floes to the effect that the content of suspended phosphorus in
the effluent will increase. It appears from Table 10.5 and Fig 10.17 that optimum
flocculation occurs at a mean velocity gradient, G, of 10-50 s- 1 . The G value in an
aeration tank in an activated sludge plant may, however, be 100-200 s-1, in other
words, considerably above the optimum value. It should therefore be endeavoured
to transfer the necessary oxygen with as little turbulence as possible, for example by
using fine bubble aeration instead of coarse bubble aeration.
In respect of simultaneous precipitation with Fe++ it should finally be mentioned
that in medium loaded plants, that is, Bx = 0.2-0.4 kg BOD I (kg SS · d), considerable
foaming problems have been observed in the aeration tank due to the development
of a certain genus of micro-organism (actinomycetes). The problem does not occur
in low-loaded plants /12/.
When using AI+++, there will be no problems with phosphorus resolution under
anaerobic conditions.
Finally it should be mentioned that a pH control of the simultaneous precipitation
process is important at low dosings, that is, a molar ratio around 1.
The biggest process problem at post precipitation and contact filtration with iron
and aluminium salts is the great influence of the alkalinity of the water at relatively
low dosings. In order to obtain low alkalinity, and hence low phosphate concentrations, acid must be added. It may be done by adding, for example, sulphuric acid or
hydrochloric acid, more dosing agent (which in itself is acid), or by establishing
nitrification in the biological process during which alkalinity is consumed.
Another method to lower the phosphate concentration is to recycle chemical sludge
from the post precipitation to the biological process, thus combining simultaneous
precipitation and post precipitation and utilizing the fact that the sludge from the
post precipitation has a binding capacity in respect of phosphorus at the somewhat
higher phosphate concentrations which are found in the simultaneous precipitation
plant compared with post precipitation.
347