PolyP Bacteria

Phosphorus (P) is a major nutrient that is necessary to all living cells. It is an essential element in the production of adenosine triphosphate or ATP (Figure 12.1), the nucleic acids DNA and RNA, phospholipids, teichoic acids, and teichuronic acids.

ATP serves as a high-energy molecule and is used in the transfer of energy within the cell. Phospholipids are key components in the structure of cell membranes, while teichoic acids and teichuronic acids are key components in the structure of cell walls of Gram-positive bacteria. Phosphorus also is stored in cells as intracellular volutin granules or polyphosphates.

Phosphorus may be 1-3% of the dry weight of a bacterium. Although the phosphorus content is approximately one-fifth of the nitrogen content of the bacterium, the actual phosphorus content may vary from one-seventh to one-third of the nitrogen content depending upon environmental conditions.

Phosphorus is a nutrient required in the growth of aquatic plants. Often phosphorus is the limiting nutrient—that is, the concentration of phosphorus in waters determines the quantity of vegetative growth. Therefore, the introduction of trace amounts of phosphorus into receiving waters can have profound and undesired effects on the quality of the receiving waters.

The undesired growth of algae often is triggered at orthophosphate concentrations as low as 0.5 mg/liter. The presence of algal blooms as well as phytoplankton results in a rapid and significant deterioration in water quality. Phosphorus pollution of natural waters is mainly responsible for eutrophication and occurs chiefly as a result of phosphorous-rich effluents from wastewater treatment plants.

Several environmental problems are associated with the rapid growth of aquatic plants. These problems include clogging of the receiving waters as well as color, odor, taste, and turbidity issue, if the receiving waters are used as potable water

ADP

FIGURE 12.1 ATP. ATP or adenosine triphosphate contain three phosphate groups and two high-energy phosphate bonds. When bacterial cells need energy, one high-energy phosphate bond is broken to release energy, and ADP or adenosine diphosphate is formed. When bacterial cells stored energy, a phosphate group is added to ADP through the production of a high-energy phosphate bond.

sources. Additionally, and more importantly, the die-off of large numbers of aquatic plants contributes to oxygen depletion and eutrophication. Oxygen is removed from the receiving waters by bacteria and other organisms as they decompose the dead plants. Eutrophication or rapid aging of the receiving waters occurs as the nonde-composable portions of the dead plants accumulate in the receiving waters. The rapid growth of aquatic plants also lowers the value of the receiving waters for fishing, industrial use, and recreational use.

Most often phosphorus is found in wastewater in quantities greater than those required for the growth of aquatic plants. Therefore, in order to prevent or reduce phosphorous-related water quality problems, state and federal regulatory agencies often require phosphorus removal at wastewater treatment plants. Because phosphorus reacts quickly with minerals such as aluminum, calcium, and iron, little phosphorus leaches from the soil. Also, little phosphorus leaches from the soil when it is applied to the soil as a fertilizer.

The requirement for phosphorus removal is becoming more common for municipal and industrial wastewater treatment plants. Discharge limits for total phosphorus at these plants often are <2mg/liter.

FIGURE 12.2 Distribution of H2POi and HPO42- in the mixed liquor.

FIGURE 12.2 Distribution of H2POi and HPO42- in the mixed liquor.

Phosphorus exists in inorganic and organic forms. Inorganic forms of phosphorus include orthophosphates and polyphosphates. Orthophosphates are available for biological metabolism without further breakdown and are considered to be the readily available nutrients for phosphorus for bacterial use in wastewater treatment plants and aquatic plants in natural waters.

Orthophosphates include PO43-, HPO42-, H2PO4-, and H3PO4. The most common forms of orthophosphate in wastewater treatment plants are HPO42- and H2PO-. The relative quantity of each form is pH-dependent (Figure 12.2). The form of orthophosphate present is produced through dissociation (Equation 12.1). Within the pH operating range of most wastewater treatment plants, HPO42- is dominant at pH values greater than 7, while H2PO4- is dominant at pH values greater than 7.

Polyphosphates are complex molecules with two or more phosphorous atoms, oxygen atoms, and perhaps hydrogen atoms. Polyphosphates are represented by the chemical formula for the pyrophosphate ion (P2O73-). Pyrophosphate is the first in a series of unbranched-chain polyphosphates (i.e., P2O73-, P3O105-,...). Polyphosphates undergo hydrolysis very slowly and release orthophosphate (Equation 12.2). Hydrolysis can be chemically mediated or biologically mediated by bacteria and algae.

Hydrolysis of polyphosphates is influenced by many factors including retention time and pH in an aeration tank. The principle form of orthophosphate obtained from the hydrolysis is pH-dependent (Equations 12.3,12.4, and 12.5). Due to their stability in water, polyphosphates also easily sequester minerals such as aluminum, calcium, and iron.

Phosphate tied to organic compounds is referred to as organic phosphorus. Organic phosphorous compounds are of minor importance in domestic wastewater, but these compounds can be of significant concern in industrial wastewater and sludge. Common organic forms of phosphorus include inositol phosphates, nucleic acids, phospholipids, and phytin. Phytin is an organic acid found in vegetables such as corn and soybean and is difficult to degrade.

The average concentration of total phosphorus in municipal wastewater is in the range of 10-20 mg/liter. Total phosphorus consists of inorganic phosphorus and organic phosphorus. Major sources of phosphorus discharged to municipal waste-water treatment plants consist of human waste, detergents, and industrial waste. Orthophosphate makes up approximately 50-70% of the total phosphorus, while polyphosphates and organic phosphorus make up the remaining 30-50% of the total phosphorus.

When orthophosphate, polyphosphate, and organic phosphorous compounds enter an activated sludge process these compounds undergo biological and chemically changes and experience several fates (Figure 12.3). Some organic phosphorous compounds are removed from the wastewater when particulate, organic phosphorous compounds or phosphorous compounds adsorbed to solids settle out in the primary clarifier.

In the activated sludge process, phosphorous compounds undergo several fates. With sufficient hydraulic retention time (HRT), organic phosphorous compounds are degraded through microbial activity, and orthophosphate is released in the aeration tank. With sufficient HRT, polyphosphates are biologically and chemically hydrolyzed, and orthophosphate is released in the aeration tank. Principal organisms responsible for the mineralization or degradation of phosphorous compounds include actinomycetes such as Streptomyces, bacteria such as Arthrobacter and Bacillus, and fungi such as Aspergillus and Penicillium. These organisms produce phosphatase, an enzyme that releases orthophosphate from phosphorus-containing compounds.

Orthophosphate is the readily available phosphorous nutrient for bacterial growth and energy transfer. As a readily available nutrient, phosphorus is removed from the bulk solution from the aeration tank and incorporated or assimilated into cellular material as bacteria degrade substrate (soluble cBOD) and reproduce (sludge production). Here, assimilated phosphorus makes up 1-3% of the bacterial weight (mixed liquor volatile suspended solids).

If a deficiency for orthophosphate occurs in the activated sludge process, the production of nutrient-deficient floc particles or sludge and the undesired and excessive growth of nutrient-deficient filamentous organisms may occur.

PO43-

P2O73-Organic P

PO43-

P2O73-Organic P

Organic P .

Sewer system

PO43-

P2O73-Organic P

PO43-

Organic P

Organic P

PO43-P2O73-Organic P

Primary clarifier

PO43-P2O73-Organic P

PO43-P2O73-Organic P

PO43-

Aeration tank

FIGURE 12.3 Movement of phosphorus in the activated sludge process. Phosphorus enters the sewer system in the inorganic form (phosphate and polyphosphate) and organic form. In the biofilm and sediment of the sewer system, some polyphosphate is hydrolyzed to orthophosphate and some organic phosphorus is degraded to release phosphate. In the primary clarifier, some organic phosphorus is removed in the sludge blanket when organic phosphorus compounds settled out. In the aeration tank, phosphate is removed by bacteria as the phosphorus nutrient and incorporated into new cells (MLVSS). Some polyphosphate is hydrolyzed to form orthophosphate, and some organic phosphorus is degraded to release phosphate.

PO43-

Aeration tank

FIGURE 12.3 Movement of phosphorus in the activated sludge process. Phosphorus enters the sewer system in the inorganic form (phosphate and polyphosphate) and organic form. In the biofilm and sediment of the sewer system, some polyphosphate is hydrolyzed to orthophosphate and some organic phosphorus is degraded to release phosphate. In the primary clarifier, some organic phosphorus is removed in the sludge blanket when organic phosphorus compounds settled out. In the aeration tank, phosphate is removed by bacteria as the phosphorus nutrient and incorporated into new cells (MLVSS). Some polyphosphate is hydrolyzed to form orthophosphate, and some organic phosphorus is degraded to release phosphate.

Nutrient-deficient floc particles and nutrient-deficient filamentous organisms adversely affect solids settleability in the secondary clarifier and may be responsible for foam production and accumulation.

During a nutrient deficiency for orthophosphate (<0.05mg/liter), soluble cBOD is absorbed by bacterial cells in floc particles. However, the soluble cBOD cannot be degraded due to the lack of adequate phosphorus. Therefore, the cBOD is converted to an insoluble polysaccharide (starch) and stored in the floc particle, until orthophosphate is available for its degradation. The stored polysaccharide is less dense than water, and its storage between bacterial cells results in a loss of floc particle density. The polysaccharides also capture air and gas bubbles. The captured air

TABLE 12.1 Filamentous Organisms that Proliferate in a Nutrient-Deficient Condition

Type 021N Type 1701

Haliscomenobacter hydrossis Nocardioforms Sphaerotilus natans Thiothrix spp.

and gas bubbles also contribute to loss of floc particle density and production of foam.

Foam produced from an orthophosphate deficiency may be billowy white or greasy gray. Billowy white foam is associated with a young sludge age, while greasy gray foam is associated with an old sludge age. As bacteria age in floc particles, their secreted oils accumulate in the floc particles and are transferred to the foam. This transfer of oils changes the texture and color of the foam to greasy gray from billowy white.

There are numerous filamentous organisms that proliferate in a nutrient-deficient condition for phosphorus or nitrogen (Table 12.1). These filamentous organisms outgrow floc bacteria in a nutrient deficient condition because (1) they require less nutrients than floc bacteria or (2) they can compete more effectively for nutrients when nutrients are limited in quantity. Effective competition for nutrients is provided by the greater surface area of the filamentous organisms that is exposed to the bulk solution that contains the nutrients as compared to the surface area of the floc bacteria. Of the filamentous organisms that proliferate in a nutrient deficient condition, the Nocardioforms are foam producers. Foam typical of Nocardioforms is viscous and chocolate brown.

In the aeration tank, orthophosphate may be incorporated into floc particles as insoluble hydroxyapatite (CaOH(PO4)3). This occurs naturally without chemical addition. If the dissolved oxygen concentration of the aeration tank is relatively low and much of the carbon dioxide released from the degradation of soluble cBOD remains in solution (i.e., it is not stripped to the atmosphere), the pH of the aeration tank decreases. The decrease occurs because carbon dioxide dissolves in the mixed liquor and carbonic acid (H2CO3) is produced. Under this condition, orthophosphate remains in solution as the H2PO4- ion.

However, if the dissolved oxygen concentration of the aeration tank is relatively high and much of the carbon dioxide in the aeration tank is stripped to the atmosphere, little carbonic acid is produced and the pH of the aeration tank increases. Under this condition, orthophosphate is present as the HPO42- ion. If this occurs in hard water (containing calcium as Ca2+), orthophosphate is precipitated from solution as hydroxyapatite and incorporated into floc particles (Equation 12.4).

Orthophosphate may remain in solution in the aeration tank in two forms. It may remain in solution in ionic form as determined by pH, or it may remain in solution sequestered (bonded in solution) to an alkali metal.

TABLE 12.2 Biological and Chemical Measures Available for Phosphorus Removal

Measure

Description

Assimilation

Enhanced biological phosphorus removal

Hydroxyapatite production

Chemical precipitation

Biological-mediated/chemical precipitation

Incorporation of phosphorus as a nutrient used in cellular synthesis and energy transfer. Phosphorus makes up 1-3% of bacterial weight or sludge.

Incorporation of phosphorus as a nutrient used in cellular synthesis, energy transfer, and polyphosphate granules. Phosphorus makes up 6-7% of bacterial weight or sludge.

Production of insoluble hydroxyapatite (CaOH(PO4)3) during low dissolved oxygen concentration and increasing pH in the aeration tank.

Use of alum, ferric chloride, ferrous sulfate, or lime to precipitate orthophosphate as a metal salt.

Chemical precipitation of orthophosphate released by bacteria from enhanced biological phosphorus removal measure.

Effluent phosphorus from the activated sludge process is approximately 90% orthophosphate. The orthophosphate may be as soluble ions or sequestered orthophosphate. To reduce the concentration of effluent phosphorus from an activated sludge process, an advanced wastewater treatment measure is required. Advanced wastewater treatment consists of those biological, chemical, and physical measures that remove

• Inorganic and organic suspended solids

• Phosphorus-containing and nitrogen-containing compounds that contribute to eutrophication

• Slowly degradable or nondegradable organic compounds

Phosphorus can be removed in municipal wastewater treatment plants through biological and chemical treatment measures (Table 12.2). Several of these measures are considered to be advanced wastewater treatment measures and include chemical precipitation of phosphorus, enhanced biological phosphorus removal (EBPR), and biological-mediated/chemical precipitation of phosphorus.

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