Enhanced Biological Phosphorus Removal

Enhanced biological phosphorus removal (EBPR) or "luxury uptake of phosphorus" occurs when phosphorus uptake by bacteria is in excess of cellular requirements. Typically, activated sludge phosphorus content is approximately 1-3%, while the activated sludge phosphorus content is approximately 6-7% when EBPR is used.

EBPR is relatively inexpensive and is capable of removing phosphorus to low effluent concentrations. EBPR also reduces chemical costs and sludge disposal costs that are associated with chemical precipitation of phosphorus.

EBPR incorporates the use of two groups of bacteria, fermentative bacteria and poly-P bacteria. Poly-P bacteria are known also as phosphorus accumulating organisms (PAO). At least two treatment tanks, an anaerobic (fermentative) tank and an

Phosphorus Accumulating Organisms
Waste activated sludge (WAS)

FIGURE 12.4 Enhanced biological phosphorus removal system. In the anaerobic zone, soluble cBOD from the primary clarifier is fermented in the absence of free molecular oxygen and nitrate. The fermentation of soluble cBOD results in the production of a variety of volatile fatty acids. The acids are rapidly absorbed by the poly-P bacteria. The absorption of fatty acids results in the production of stored food (insoluble starch granules) and the release of orthophosphate by the poly-P bacteria. In the aerobic zone, orthophosphate released by the poly-P bacteria and orthophosphate in the primary clarifier effluent are absorbed by the poly-P bacteria as the starch granules are solublized and degraded. Due to the different bacterial events in the anaerobic zone and aerobic zones, the bacteria/sludge contains approximately 6-7% phosphorus on a dry weight basis as compared to 1-3% phosphorus in typical activated sludge. The sludge in the aerobic zone that contains the elevated concentration of phosphorus is discharged to the secondary clarifier where it is either wasted or returned to the anaerobic zone.

TABLE 12.3 Significant Fermentative Bacteria in the Anaerobic (Fermentative) Tank

Aeromonas

Citrobacter

Klebsiella

Pasteurella

Proteus

Serratia

TABLE 12.4 Significant Poly-P Bacteria in the Aerobic Tank

Achromobacter

Escherichia

Acinobacter

Klebsiella

Aerobacter

Microlunatus

Aeromonas

Moraxella

Arthrobacter

Mycobacterium

Bacillus

Pasteurella

Beggiatoa

Proteobacter

Citrobacter

Pseudomonas

Enterobacter

Vibrio

aerobic tank, are used for EBPR (Figure 12.4). The fermentative bacteria are facultative anaerobes and anaerobes, while the poly-P bacteria with exception are strict aerobes. The fermentative bacteria (Table 12.3) and poly-P bacteria (Table 12.4) enter the EBPR process as fecal bacteria and soil and water bacteria from inflow and infiltration (I/I). The key to EBPR is the exposure of poly-P bacteria to alternating anaerobic and aerobic conditions.

Anaerobic/ Aerobic/ Secondary fermentative tank oxic tank clarifier

Anaerobic/ Aerobic/ Secondary fermentative tank oxic tank clarifier

FIGURE 12.5 Bacterial activity in the anaerobic tank in EBPR system. In the anaerobic tank, soluble cBOD is fermented in the absence of free molecular oxygen and nitrate. The fermentation process produces a variety of volatile fatty acids. The acids are rapidly absorbed by poly-P bacteria and stored as insoluble starch granules (PHB). In order to absorb the fatty acids and covert the fatty acids to PHB, energy in the form of orthophosphate is lost to the bulk solution.

FIGURE 12.5 Bacterial activity in the anaerobic tank in EBPR system. In the anaerobic tank, soluble cBOD is fermented in the absence of free molecular oxygen and nitrate. The fermentation process produces a variety of volatile fatty acids. The acids are rapidly absorbed by poly-P bacteria and stored as insoluble starch granules (PHB). In order to absorb the fatty acids and covert the fatty acids to PHB, energy in the form of orthophosphate is lost to the bulk solution.

In the anaerobic tank (Figure 12.5), fatty acids (particularly acetate and propionate) are produced through the anaerobic activity of fermentative bacteria. These compounds serve as substrate for the proliferation of aerobic poly-P bacteria that are in the anaerobic tank. However, the poly-P bacteria cannot utilize (degrade) the fermentative compounds in an anaerobic condition. The degradation of these compounds by poly-P bacteria occurs only in the presence of free molecular oxygen (aerobic tank) or nitrate (NO3-).

Free molecular oxygen and nitrate ions should be absence in the anaerobic (fermentative) tank. A residual quantity of either free molecular oxygen or nitrate is quickly exhausted in the anaerobic tank, but requires a longer retention time and more soluble cBOD in the anaerobic tank in order to exhaust the free molecular oxygen or nitrate ions. The presence of free molecular oxygen or nitrate in the anaerobic tank interferes with the phosphorus removing ability of the EBPR process. Interference occurs as a result of an increase in redox potential and inability to produce fatty acids that are necessary for the release of phosphorus.

Fermentation is the microbial degradation of soluble organic compounds (cBOD) without the use of free molecular oxygen or nitrate. Significant fermentative organic compounds or substrate produced in the anaerobic tank include alcohols and a variety of soluble fatty acids (Table 12.5).

In the anaerobic tank, poly-P bacteria rapidly absorb the fatty acids and polymerize (store) the acids as an insoluble starch (poly-P-hydroxybutyrate or PHB). PHB in poly-P bacteria serves two important functions. First, it helps the bacteria

TABLE 12.5 Soluble Fatty Acids Produced in the Anaerobic Tank

Fatty Acid

Chemical Formula

Formate

HCOOH

Acetate

CH3COOH

Propionate

CH3CH2COOH

Butyrate

CH3CH2CH2COOH

Valeric acid

CH3CH2CH2CH2COOH

Caproic acid

CH3CH2CH2CH2CH2COOH

Anaerobic/ Aerobic/ Secondary fermentative tank oxic tank clarifier

Anaerobic/ Aerobic/ Secondary fermentative tank oxic tank clarifier

Mlvss Wastewater

FIGURE 12.6 Bacterial activity in the aerobic tank in EBPR system. In the aerobic tank, PHB is sol-ublized and degraded in the presence of free molecular oxygen. The degradation of PHB results in the production of carbon dioxide, water, and new cellular material (MLVSS). Phosphate released in the anaerobic tank as well as phosphorus in the primary clarifier effluent are absorbed by the poly-P bacteria and stored as volutin.

FIGURE 12.6 Bacterial activity in the aerobic tank in EBPR system. In the aerobic tank, PHB is sol-ublized and degraded in the presence of free molecular oxygen. The degradation of PHB results in the production of carbon dioxide, water, and new cellular material (MLVSS). Phosphate released in the anaerobic tank as well as phosphorus in the primary clarifier effluent are absorbed by the poly-P bacteria and stored as volutin.

to grow and rebuild polyphosphates by taking up soluble phosphate. Second, PHB along with polyphosphates help aerobic poly-P bacteria to survive in an anaerobic condition.

The polymerization of the fatty acids requires an expenditure of cellular energy by the poly-P bacteria. This expenditure or energy is the breakdown or release of orthophosphate from the poly-P bacteria to the bulk solution of the anaerobic tank. As a result of the release of orthophosphate from the poly-P bacteria, the anaerobic tank contains two pools of phosphorus, namely, the phosphorus in the influent wastewater (feed phosphorus) and the released phosphorus by the poly-P bacteria.

In the aerobic tank (Figure 12.6) the poly-P bacteria use free molecular oxygen to degrade the stored PHB as a carbon and energy source. Concurrently, the poly-

TABLE 12.6 Nutrient Removal Processes

Process Name

Nutrient Removed

Phosphorus Removal

Chemical Precipitation

Nitrogen

Phosphorus

Mainstream

Sidestream

A/O

X

X

Phostrip

X

X

X

A2/O

X

X

X

Bardenpho

X

X

X

UCT

X

X

X

P bacteria absorb orthophosphorus in order to store the energy released from the degraded PHB. The poly-P bacteria obtain so much energy from the degraded PHB that they absorb not only the released phosphorus but also large quantities of feed phosphorus. The absorbed phosphorus is assimilated into macromolecules and stored as polyphosphate granules or volutin. Phosphorus removal is achieved when the bacteria (sludge) are wasted from the secondary clarifier. Sludge that is not wasted is returned to the anaerobic tank where the EBPR process is repeated. By exposing the poly-P bacteria to alternating anaerobic and aerobic conditions, the poly-P bacteria are stressed and take up phosphorus in excess of normal cellular requirements.

There are several processes available for EBPR. Alternating exposure of poly-P bacteria to anaerobic and aerobic tanks can be accomplished in the main biological treatment process ("mainstream") or in the return sludge ("sidestream") (Table 12.6). A mainstream process for biological phosphorus removal contains an anaerobic tank along the main liquid process stream from influent to effluent. A side stream process contains an anaerobic tank that is aside of the main liquid process steam.

All nutrient removal processes for phosphorus remove excess phosphorus biologically, except the Phostrip process that incorporates chemical precipitation of phosphorus. The removed phosphorus from these processes is found biologically in the bacterial cells or chemically in a precipitate within the sludge. When biological phosphorus removal is combined with nitrification and denitrification for nitrogen removal, the removal of phosphorus and nitrogen is known as biological nutrient removal (BNR) or combined phosphorus/nitrogen removal.

Nitrification is the biological oxidation of ionized ammonia (NH/) to nitrate (NO3-). Denitrification is the biological used of nitrate to degrade soluble cBOD in the absence of free molecular oxygen. When nitrate is used to degrade soluble cBOD, the nitrogen in the nitrate leaves the wastewater and is returned to the atmosphere as molecular nitrogen (N2) and nitrous oxide (N2O).

There are two EBPR processes that are available in the United States that remove phosphorus only. These processes are the A/O (Figure 12.7) and the Phostrip (Figure 12.8). The A/O (aerobic/oxic) process is patented by Air Products and Chemicals, Inc. The A/O process is a mainstream process.

The Phostrip process is a sidestream process and includes biological and chemical measures for phosphorus removal. The Phostrip process has a stripper tank where an anaerobic condition permits the release of phosphorus by poly-P bacteria from the return activated sludge (RAS). The released phosphorus is removed

Primary Anaerobic/ Aerobic/ Secondary Effluent clarifier fermentative tank oxic tank Clarifier effluent

Primary Anaerobic/ Aerobic/ Secondary Effluent clarifier fermentative tank oxic tank Clarifier effluent

Waste activated sludge (WAS) FIGURE 12.7 A/O process.

Primary Anaerobic/ Aerobic/ Secondary Effluent clarifier fermentative tank oxic tank Clarifier effluent

Primary Anaerobic/ Aerobic/ Secondary Effluent clarifier fermentative tank oxic tank Clarifier effluent

Sludge Acronym Medical

Stripped sludge

FIGURE 12.8 Phostrip process.

Stripped sludge

FIGURE 12.8 Phostrip process.

("washed") from the stripper tank by elutriation water. Lime is added to the stripper tank overflow to precipitate the released phosphorus.

There are three combined phosphorus/nitrogen removal processes that are marketed in the United States. These processes included the A2/O, five-stage Barden-pho, and UCT.

The A2/O process is licensed by I. Kruger. The A2/O is the acronym for the three operational conditions or tanks employed in the treatment process (Figure 12.9).

Primary Anaerobic/ Anoxic Aerobic/ Secondary Effluent

Primary Anaerobic/ Anoxic Aerobic/ Secondary Effluent

Sludge Acronym Medical
Waste activated sludge (WAS) FIGURE 12.9 A2/O process.

Anaerobic Anoxic Oxic Anoxic Oxic

Anaerobic Anoxic Oxic Anoxic Oxic

Anaerobic Anoxic Oxic Tank
Waste activated sludge (WAS) FIGURE 12.10 Five-stage Bardenpho process.

These tanks are the anaerobic, anoxic, and oxic. The A2/O process has a relatively low solids retention time (SRT) and high organic loading. As the wastewater and bacteria pass through these tanks, phosphorus is removed biologically and nitrogen is removed through nitrification and denitrification.

In the anaerobic tank fermentation occurs and phosphorus is released to the bulk solution by poly-P bacteria as they absorb and polymerize soluble fatty acids into PHB. In the anoxic tank, facultative anaerobic bacteria use nitrate to degrade soluble cBOD. In the oxic tank, ionized ammonia in the wastewater and released during degradation of organic nitrogen compounds are oxidized to nitrate. In the anoxic tank and oxic tank phosphates are removed from the bulk solution as PHB is solubilized and degraded.

The five-stage or modified Bardenpho process was developed by Barnard in South Africa in 1975 and is licensed and marketed in the United States by Eimco Process Equipment Company. This process is a modification of the original Bar-denpho process due to the incorporation of an anaerobic tank upstream of two anoxic/oxic tanks (Figure 12.10). The anoxic tank uses influent wastewater as a carbon source for denitrification.

Anaerobic Anoxic Oxic Tank
Return activated sludge (RAS) FIGURE 12.11 UCT process.
TABLE 12.7 Chemicals Commonly Used for Orthophosphate Precipitation and Their Products

Chemical

Formula

Product

Aluminum sulfate

Al2(SO4)3

AlPO4

Ferric chloride

FeCl3

FePO4

Lime

Ca(OH)2

Ca2OH(PO4)3

Magnesium sulfate

MgSO4

MgNH4PO4

The University of Capetown or UCT process (Figure 12.11) contains anaerobic, anoxic, and oxic tanks. However, the UCT process is designed to reduce the nitrate loading on the anaerobic zone in order to optimize phosphorus removal.

In addition to the A2/O, Bardenpho, and UCT processes there are other BNR systems that are being developed and implemented. These new BNR systems include the modified UCT and the Virginia Initiative Process (VIP). The VIP was developed by Hampton Roads Sanitation District and CH2M. The selection of the EBPR or BNR system used at wastewater treatment plants is based upon cost, wastewater composition, and nutrient removal requirements.

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