Biological Phosphorus Removal
- 1. A/O Process
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A basic A/O system will yield simultaneous biological removal of both BOD and phosphorus. Where only BOD removal is required,
the basic process is optimized to provide significant performance benefits relative to conventional activated-sludge processes.
In either case, the same basic configuration and biological processes, shown above, are utilized. The A/O process,
which uses an anaerobic selector to achieve enhanced performance, is a high-rate variation of the conventional
activated-sludge process. The anaerobic selector is established by mixing and contacting influent
BOD-containing wastewater with recycled activated sludge in the absence of supplied oxygen or oxygen-containing gas.
In a subsequent step, the anaerobically formed mixed liquor is aerated in an oxic zone and finally
separated in a secondary clarifier, which functions in the same manner as in conventional activated-sludge processes.
1-1 Anaerobic Zone
Anaerobic zone detention time is set at the minimum necessary to ensure the development and maintenance
of the A/0 system biomass, without concern for maximizing phosphorus release. For municipal waste,
this is typically between 30 and 60 minutes based on the design-average flow of influent.
As the wastewater passes through the anaerobic zone, most of the influent BOD
is absorbed into the biomass. BOD, both sorbed and unsorbed, is then oxidized in the oxic zone.1-2 Applications
· Secondary or advanced treatment requirements with air- or oxygen-activated sludge.
· Process control problems resulting from sludge-bulking or wide swings in organic loading.
· Increasing rated design organic load but limited available space to expand.
· Discharge permit may require nutrient control in the future.
- 2. PhoStrip Process
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The method, called the PhoStrip process, causes the microorganisms in activated sludge to bioaccumulate
and secrete phosphate. Biological phosphorus removal has since evolved into two types: ¢®¡Æsidestream¢®¡¾ (PhoStrip),
and ¢®¡Æfullstream¢®¡¾ processes. PhoStrip systems have fully demonstrated their capability for sustained production
of effluents averaging 1.0 mg/L, or less, total P, the effluent standard generally enforced.
In addition to this unique achievement, PhoStrip systems provide overall plant operations advantages
including compatability with any mode of activated sludge treatment, equal effectiveness over a wide range
of F/M and P/BOD, sludge volume reduction, and protection against hydraulic surcharge and toxic shock.
· Reduces the concentration of phosphorous in effluent to below 0.5 mg/l
· Stable phosphorous recovery regardless of the fluctuation of water volume and quality
· Economic efficiency because of low priced chemicals
· Recovery of pure phosphorous without heavy metals
· Reduces the concentration of phosphorous in excess sludge
- 3. Enhanced Biological Phosphorus Removal (EBPR) Process
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The Enhanced Biological Phosphorus Removal (EBPR) process is a biological process for efficient phosphate
removal from wastewaters through intracellular storage of polyphosphate by Phosphate-Accumulating
Organisms (PAO) and subsequent removal through wastage of excess sludge.
Although many studies have demonstrated the existence of different PAO clades, the functional differences
among these clades and potential implications for the process performance remained unclear.
Furthermore, the salinity effects on the EBPR process had not been properly investigated,
which is necessary to assess its applicability for the treatment of saline wastewaters.
The first part focuses on the functional diversity among PAO clades. It demonstrates significant functional
differences in the main characteristics of the anaerobic metabolism of two different PAO clades and provides fundamental
insight in the metabolic response of PAO to different influent P/C ratios. In addition,
it shows how these functional differences provide competitive advantages to specific PAO clades in a selection
study and discusses their potential implications on process performance, in particular
for combined biological and chemical systems for nutrient removal and recovery.
The second part describes the salinity effects on the metabolism of PAO and their competitors that
do not contribute to phosphorus removal; the so-called Glycogen-Accumulating Organisms (GAO).
It shows how salinity affects the different metabolic processes (kinetics and stochiometry) of PAO and GAO and provides
a model that describes the salinity effects on their kinetic rates. Finally, it discusses the potential
implications of sudden saline shocks in wastewater treatment systems that are not regularly exposed to salinity.
