PHOSPHORUS REMOVALWanzhi (Amy) Zhuang

CONTENT Introduction Phosphorus Removal Techniques Physical Treatment Biological Treatment

Assimilation Enhanced Biological P removal

Chemical Treatment Definition General Procedure Factors affecting performance

Comparison Between Chemical and Biological Treatment

Large Discharge of Phosphorus

Introduction

An important macro-nutrient for plant and microorganisms growth. Large quantities of this nutrient into natural receiving sources raises the growth of algae and results in eutrophication of lakes and streams. Disturb the balance of organisms present in water and affect water quality, mainly through the depletion of oxygen level as the algae decay.

Main SourcesAgricultural use of fertilizersDomestic wastewaterindustrial wastewaterAtmospheric deposition

Phosphorus Removal Techniques

BiologicalAssimilationEnhanced biological phosphorus removal (EBPR)

Physical

Chemical

Filtration for particulate phosphorusMembrane technologies

PrecipitationOther (mainly physical-chemical adsorption)

Physical Treatment

Membrane technologies

Assuming that 2-3% of organic solids is P, then an effluent total suspended solids (TSS) of 20 mg/L represents 0.4-0.6 mg/L of effluent P

Sand filtration or other method of TSS removal is likely necessary for plants with low effluent TP permits.

Filtration for particulate P

Remove the P in the TSS, and dissolved P. Membrane bioreactors (MBRs, which incorporate

membrane technology in a suspended growth secondary treatment process)

tertiary membrane filtration (after secondary treatment),

reverse osmosis (RO) systems

Biological Treatment

EBPR (Enhanced biological phosphorus removal)

Achieve low or even very low (<0.1 mg/L) effluent P levels at modest cost and with minimal additional sludge production.

Removal of BOD, nitrogen and phosphorus can all be achieved in a single system.

Difficult to achieve very low concentrations of both total N and P in such systems

Assimilation

– incorporation of the P as an essential element in biomass, particularly through growth of photosynthetic organisms

Plants Algae Some bacteria, such as cyanobacteria

Anaerobic/Aerobic EBPR Process

The EBPR process works by providing an anaerobic zone with an ample supply of readily biodegradable carbonaceous oxygen demand (rbCOD).

Organic matter in the anaerobic zone is fermented to create a source of volatile fatty acids (VFAs), particularly acetate and propionate, which in turn serve as food sources for PAOs.

PAOs are aerobic bacteria.

rbCOD

VFA(food)

PAOs

Problems associated with EBPR EBPR (Enhanced biological phosphorus removal)

Sludge - Processes that destroy organic material (such as digestion) have the potential to release the particulate organic-P present as soluble organic or inorganic P. - Anaerobic conditions are likely to release soluble P from EBPR sludge and iron precipitates (ferrous phosphate is much more soluble than ferric phosphate)

Side System - Any released P may then be returned to the main wastewater treatment process in high concentrations through recycle side streams, thus requiring removal a second time. - Non-continuous processes may also lead to variable loadings from side streams

Chemical Treatment

The most common method used for phosphorus removal to meet effluent concentrations below 1.0 mg/L.

Involves the addition of metal salts to react with soluble phosphate to form solid precipitates that are removed by solids separation processes (clarification and filtration).

The most common metal salts used are in the form of alum (aluminum sulfate) sodium aluminate ferric chloride, ferric sulfate ferrous sulfate, and ferrous chloride. Calcium (lime), magnesium

Definition

Chemical TreatmentPhosphate precipitates – pH range

Produces lowest residual P concentrations (in the range of 1-2 mg/L). The alkalinity of the water determined the dose because the formation of CaCO3.

There will be oxidation of to .

①

②

Chemical TreatmentPhosphate precipitates – pH range

𝐴𝑙2 (𝑆𝑂4 )3 ∙14.3𝐻 2𝑂+2𝑃𝑂43−→2 𝐴𝑙𝑃𝑂4↓+3𝑆𝑂4

2−+14.3 𝐻2𝑂𝐹𝑒𝐶𝑙3 ∙6𝐻2𝑂+𝑃𝑂4

3−→𝐹𝑒𝑃𝑂4↓+3𝐶𝑙−+6𝐻2𝑂5𝐶𝑎2+¿+3 𝑃𝑂4

3 −+𝑂𝐻−→𝐶𝑎5 (𝑃𝑂¿¿ 4 )3 (𝑂𝐻 )↓¿ ¿

• More ions should be added to counter the competing reaction with alkalinity.

Two-point Chemical Addition Process

Consists of a mass balance between: chemical addition the stoichiometry of

the chemical added phosphorus removed the phosphorus

concentration after chemical addition

Two scenarios: Two-point chemical

addition process Effluent polishing in

the secondary process

No chemical addition at this process

The Chemical Used pH of the water Alkalinity Dosage Mixing and Contact Time Points of addition Other (Chemical Feed Control, etc.)

Factors affecting the performance of P removal

The figure above shows the phosphorus removal versus pH for alum and dosage of 80 mg/l . Alum demonstrated much better results in phosphorus removal, being up to approximately 2 times better than for the same dosage.

Comparison between alum and calcium chloride regarding their efficiency to remove phosphorus

Experiment Setting: Chemical used: Mole concentration: 0.1mM-0.6mM Contact tank: 50 cm3

1.52.9

Below a pH range of 5.5 the aluminum ions are soluble and do not participate in the hydration. When pH>8, the aluminum ions again become soluble

Effect of pH on the removal of phosphorus (aluminum phosphate)

Phosphate removal highly depends on the pH of water

𝐴𝑙3+¿+3 𝐻2𝑂→ 𝐴𝑙 (𝑂𝐻 )3↓+3 𝐻+¿¿ ¿

is soluble below pH=6 and above pH=8 (Sedlak, 1991). 𝐴𝑙3+¿+𝐻𝑛𝑃𝑂4𝑛 −3→ 𝐴𝑙 𝑃𝑂4+𝑛𝐻

+¿¿ ¿

Effect of pH on the removal of phosphorus ()

Phosphate removal depends on the dosage

The pH effect can be explained by the change of orthophosphate compounds with pH. (Pk1=2.2, Pka2=7.2, Pka3=12.32)Phosphate removal increases with increasing pH and dosage, at pH= 11 the residual phosphorus passes from (5 to 3 mg/l) when dosage passes from 40 to 60 mg/l. dosage larger than 60 mg/l has nearly no effect on phosphorus removal.

5

3

A rise in alum dosage up to 80 mg/l increases the phosphorus removal. Further addition of alum leads to a decrease in the phosphorus removal efficiency. This is due to the fact that an increase in the dosage shifts the optimum pH (5.8-6.5) to an unfavorable range for phosphate removal.

Effect of dosage on the removal of phosphorus (aluminum phosphate)

Advantages

Comparison of Chemical and Biological Treatment

Reliable Low level of P in effluent Retrofit for existing plant

(likely possible)

Cost of chemical feed system

Cost of chemicals Substantial additional

sludge production Chemical sludge reuse or

disposal maybe more difficult

May need to adjust pH

Disadvantages Not as reliable Need enough BOD May be difficult to get

<0.1mg/L consistently Digestion (especially

anaerobic) release P

No or less chemical cost No or less chemical

storage and handling No or less chemical sludge

disposal

Disadvantages

Chemical Treatment

Advantages

Biological Treatment

1. Technologies to Remove Phosphorus from Wastewater, Peter F. Strom, Professor of Environmental Science, Rutgers University, August 2006.

2. Tchobanoglous, G., F.L. Burton, and H.D. Stensel. Meltcalf & Eddy, Wastewater Engineering: Treatment, Disposal, and Reuse, 4th Edition. McGraw-Hill, Inc., New York. 1819 pp, 2003.

3. Narayanan, B. Solids treatment and recycle streams in BPR plants. Session P1 in WERF, 2006.

4. Zilles, Julie L. et al. Involvement of Rhodocyclus-Related Organisms in Phosphorus Removal in Full-Scale Wastewater Treatment Plants. Applied Environmental Microbiology. 68(6): 2763–2769, 2002.

5. Minnesota Pollution Control Agency, Municipal Division, Wastewater Program, Phosphorus Treatment and Removal Technologies, 2006

6. Sawsan A. M. Mohammed and Haider Abbas Shanshool, Phosphorus Removal from Water and Waste Water by Chemical Precipitation Using Alum and Calcium Chloride, Iraqi Journal of Chemical and Petroleum Engineering, Vol.10 No.2 (June 2009) 35-42

7. Jenkins, D. and Hermanowicz, S. W., Principles of Chemical Phosphate Removal, 2nd ed., Lewis Puplisher, pp. 91-110, 1991.

8. Peter F. Storm, Phosphorus Removal, Environmental Sciences, Cook University, Rutgers University, 2006

References

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