Removal of phosphorus from wastewaters in treatment processes is important in
limiting phosphorus discharges to streams and lakes. These include both biological
and chemical treatment systems.
Most biological treatment systems rely on a peculiar trait of many organisms,
specifically those present in typical biological wastewater treatment systems, especially activated sludge systems. When these organisms are starved for phosphorus,
such as under anoxic conditions, and then subjected to normal aerobic activated
sludge aeration, they take up more phosphorus than immediately needed, a term
called luxury uptake. Thus treatment involves alternate anoxic and aerobic treatment
in separate tanks or alternate conditions in a single tank, with removal of the excess
phosphorus in the waste sludge.
Wilson [6] summarized some of these processes, sometimes known as the
Ludzack-Ettinger and Johannesburg or Bardenpho processes, which are patented.
Variations include the number and order of anoxic and aerobic tanks, the location of
both return activated sludge and mixed liquor suspended solids to help create anoxic
conditions, and the use of an added carbon source, such as methanol, to create the
anoxic conditions. If effluent requirements require phosphorus levels less than
0.3 mg/L, additional chemical treatment is usually needed. Wilson compared biological and chemical phosphorus removal and concluded that multiple aeration tanks
consume energy; return activated sludge and mixed liquor suspended solids require
more energy; the cost of a carbon source (methanol) may be great; multiple tanks
require more space; and for low phosphorus effluent demands, chemical treatment is
needed anyway. He also pointed out that the additional volume of sludge created by
the addition of chemicals is small compared to the volume of waste sludge already
created.
In order to achieve total phosphorus levels in wastewater discharges of less than
0.1 mg/L, chemical precipitation is very useful. Phosphorus forms insoluble salts
with aluminum, iron, and calcium. Aluminum is most commonly used. The iron
phosphate sediment must be kept aerobic to prevent the release of the phosphorus
when less soluble iron sulfide is created. Calcium is usually applied as lime, which
has a high pH. This may be detrimental under certain circumstances. Availability
and cost of the chemicals has a large role in the choice of chemical. Eberhardt [7] has
published a report on calculating the optimum aluminum dose.
Tabor [8] evaluated two patented treatment systems for phosphorus removal. The
Actiflo process consists of coagulant addition with rapid mix, polymer and sand
addition, slow mix for particle agglomeration and floc formation, plate settlers for
solids/liquid separation, separation of the sand from the solids in a hydrocyclone,
and return of the sand to the system. The DensaDeg process consists of coagulant
with rapid mix, polymer and thickened return activated sludge addition, a plug flow
zone for particle agglomeration and floc formation, tube settlers for solids/liquid
separation, and thickening of solids for recycle and disposal. Both systems are
capable of removing total phosphorus to less than 0.2 mg/L.
Patoczka [9] described upgrading an existing conventional activated sludge
treatment plant utilizing a backwashable sand filter to achieve an effluent total
phosphorus content of less than 0.1 mg/L. Chemical addition was shown to be
effective. Both alum and iron salts were studied, and the optimum dosages and pH
268
L. K. Wang et al.
limiting phosphorus discharges to streams and lakes. These include both biological
and chemical treatment systems.
Most biological treatment systems rely on a peculiar trait of many organisms,
specifically those present in typical biological wastewater treatment systems, especially activated sludge systems. When these organisms are starved for phosphorus,
such as under anoxic conditions, and then subjected to normal aerobic activated
sludge aeration, they take up more phosphorus than immediately needed, a term
called luxury uptake. Thus treatment involves alternate anoxic and aerobic treatment
in separate tanks or alternate conditions in a single tank, with removal of the excess
phosphorus in the waste sludge.
Wilson [6] summarized some of these processes, sometimes known as the
Ludzack-Ettinger and Johannesburg or Bardenpho processes, which are patented.
Variations include the number and order of anoxic and aerobic tanks, the location of
both return activated sludge and mixed liquor suspended solids to help create anoxic
conditions, and the use of an added carbon source, such as methanol, to create the
anoxic conditions. If effluent requirements require phosphorus levels less than
0.3 mg/L, additional chemical treatment is usually needed. Wilson compared biological and chemical phosphorus removal and concluded that multiple aeration tanks
consume energy; return activated sludge and mixed liquor suspended solids require
more energy; the cost of a carbon source (methanol) may be great; multiple tanks
require more space; and for low phosphorus effluent demands, chemical treatment is
needed anyway. He also pointed out that the additional volume of sludge created by
the addition of chemicals is small compared to the volume of waste sludge already
created.
In order to achieve total phosphorus levels in wastewater discharges of less than
0.1 mg/L, chemical precipitation is very useful. Phosphorus forms insoluble salts
with aluminum, iron, and calcium. Aluminum is most commonly used. The iron
phosphate sediment must be kept aerobic to prevent the release of the phosphorus
when less soluble iron sulfide is created. Calcium is usually applied as lime, which
has a high pH. This may be detrimental under certain circumstances. Availability
and cost of the chemicals has a large role in the choice of chemical. Eberhardt [7] has
published a report on calculating the optimum aluminum dose.
Tabor [8] evaluated two patented treatment systems for phosphorus removal. The
Actiflo process consists of coagulant addition with rapid mix, polymer and sand
addition, slow mix for particle agglomeration and floc formation, plate settlers for
solids/liquid separation, separation of the sand from the solids in a hydrocyclone,
and return of the sand to the system. The DensaDeg process consists of coagulant
with rapid mix, polymer and thickened return activated sludge addition, a plug flow
zone for particle agglomeration and floc formation, tube settlers for solids/liquid
separation, and thickening of solids for recycle and disposal. Both systems are
capable of removing total phosphorus to less than 0.2 mg/L.
Patoczka [9] described upgrading an existing conventional activated sludge
treatment plant utilizing a backwashable sand filter to achieve an effluent total
phosphorus content of less than 0.1 mg/L. Chemical addition was shown to be
effective. Both alum and iron salts were studied, and the optimum dosages and pH
268
L. K. Wang et al.