Wastewater Treatment 199The production of Al(OH)3 is
The total sludge production is then
AlPO4 + Al(OH)3 = 21.3 + 24 = 45 mg -
L‘Phosphorus also may be removed biologically. When the microorganisms in return
activated sludge are starved by aeration after their removal in the final clarifier, they
show a strong tendency to adsorb phosphorus almost instantly on introduction of
wastewater. Such a “luxury uptake” is at a much higher rate than would be required
eventually by metabolic activity. If the phosphorus-rich organisms are removed quickly
and wasted, the excess phosphorus leaves with the waste activated sludge. Thus, both
nitrogen and phosphorus can be removed by biological secondary treatment.
In a typical phosphorus removal system in a wastewater treatment plant, the mixed
liquor in the first stage of the aeration tank is mixed but not aerated. The microorganisms
react to this deprivation of oxygen by utilizing the phosphorus in the cells, thus gaining
energy for sustaining life. When the microorganisms are then moved into the aerobic
stage, the soluble phosphorus is rapidly absorbed within the cells in order to allow
the microorganism to assimilate the dissolved organic material rapidly. This uptake
of phosphorus is the “luxury uptake,” and is caused by first solubilizing phosphorus,
and then creating a condition where the microorganism try to rapidly assimilate the
energy-containing organic matter. At this point, the microorganisms are settled out in
the final clarifier and removed, taking the excess phosphorus with them. The waste
activated sludge thus becomes enriched in phosphorus, which is beneficial if the sludge
is applied to farmland.
The combined removal of nitrogen and phosphorus is often termed biological
nutrient removal, or BNR. A number of treatment options are available for such a
plant, but they all consist of a series of anoxic and aerobic basins.
With nitrogen and phosphorus removal, the effluent goal of the wastewater is
attained
Raw After primary After secondary After tertiary
wastewater treatment treatment treatment15 10
15 10
6 0.5Spraying secondary effluent on land and allowing soil microorganisms to degrade the
remaining organic compounds provides an alternative to high-technology advanced