Phosphorus Balance

Tracking phosphorus through biological and chemical removal

Overview

Phosphorus balance tracks total phosphorus (TP) as it transforms through treatment. Understanding P pathways is essential for meeting increasingly stringent discharge limits.

Phosphorus Forms

Species

Form	Symbol	Description
Orthophosphate	PO4-P	Soluble, reactive
Polyphosphate	Poly-P	Stored in PAOs
Organic P	Org-P	Bound in organics
Particulate P	Part-P	In solids/biomass

Aggregate Parameters

Parameter	Components
TP	All P forms
SRP	Soluble reactive P (≈PO4)
TSP	Total soluble P
TPP	Total particulate P

P Removal Mechanisms

Three Main Pathways

Phosphorus Removal
        │
        ├── 1. Biological (EBPR)
        │   └── PAO uptake → poly-P in sludge
        │
        ├── 2. Chemical Precipitation
        │   └── Metal salt → precipitate in sludge
        │
        └── 3. Biomass Assimilation
            └── Normal cell growth → P in sludge

Removal Fractions

Mechanism	Typical % of TP Removed
Assimilation	20-30%
EBPR	40-60%
Chemical	10-30% (supplemental)

Biological P Removal (EBPR)

PAO Metabolism

Anaerobic Phase:

• VFA uptake
• P release (poly-P → PO4)
• PHA storage

Aerobic Phase:

• P uptake (luxury uptake)
• PHA oxidation
• Poly-P reformation

Key Requirements

Parameter	Requirement
VFA	>20 mg VFA/mg P removed
Anaerobic zone	0.5-2 h HRT
No O2/NO3	In anaerobic zone
Temperature	>12°C
SRT	5-25 days

P Content in Sludge

Sludge Type	% P (dry basis)
Normal WAS	2-3%
EBPR sludge	5-7%
High EBPR	Up to 10%

Chemical P Removal

Common Chemicals

Chemical	Formula	P Removal Mechanism
Alum	Al2(SO4)3	AlPO4 precipitate
Ferric chloride	FeCl3	FePO4 precipitate
Ferrous sulfate	FeSO4	Fe3(PO4)2 precipitate
Lime	Ca(OH)2	Ca3(PO4)2 precipitate

Dose Ratios

Chemical	Molar Ratio	mg/mg P
Alum	1.5-3.0 Al:P	1.5-3.0
Ferric	1.5-3.0 Fe:P	2.7-5.4
Ferrous	2.0-3.5 Fe:P	3.6-6.3

Dose Points

Location	Application
Primary	Pre-precipitation
RAS	Concurrent
Post-secondary	Tertiary treatment

Mass Balance

Overall Balance

$$P_{in} = P_{effluent} + P_{sludge}$$

Component Tracking

Stream	PO4-P	Org-P	Part-P	TP
Influent	4	2	2	8
Anaerobic	8	1.5	1	10.5
Aerobic	0.5	0.5	7	8
Effluent	0.5	0.3	0.2	1.0
WAS	-	-	-	7.0

ErgoPlanner Configuration

Influent P

{
  "phosphorus": {
    "TP": 8,
    "PO4_P": 5,
    "organic_P": 2,
    "particulate_P": 1
  }
}

EBPR Settings

Configure anaerobic zone:

• DO setpoint: 0 mg/L
• No internal recycle to anaerobic
• VFA fraction in influent

Chemical Dosing

{
  "chemical_P_removal": {
    "type": "FeCl3",
    "dose_point": "RAS",
    "target_effluent_P": 0.5,
    "molar_ratio": 2.0
  }
}

Example: A2O Process

Configuration

Parameter	Value
Flow	25,000 m³/d
Influent TP	10 mg/L
Target TP	<1 mg/L
VFA in influent	50 mg/L

Zone Sizing

Zone	Volume	HRT
Anaerobic	1,500 m³	1.4 h
Anoxic	2,500 m³	2.4 h
Aerobic	6,000 m³	5.8 h

Results

Parameter	Value
Effluent TP	0.8 mg/L
P removal	92%
Sludge P	5.5%
Chemical backup	None needed

Troubleshooting

Issue	Cause	Solution
High effluent P	Low VFA	Add fermenter, chemical
Unstable removal	GAO competition	Lower pH, temp
P release in clarifier	Anaerobic conditions	Reduce RAS retention
Declining EBPR	Toxic shock	Identify and remove

Monitoring

Key Parameters

Parameter	Frequency	Purpose
Effluent TP	Daily	Compliance
Effluent PO4	Continuous	Control
Anaerobic P	Weekly	EBPR health
Sludge P %	Monthly	EBPR efficiency
VFA	Weekly	Feed characterization

Warning Signs

Indicator	Concern
Declining sludge P%	EBPR deteriorating
High anaerobic P	Normal, but monitor
Low VFA	EBPR at risk
High secondary P	Clarifier release

Best Practices

1. Maximize VFA - Primary fermentation helps
2. Protect anaerobic zone - No O2 or NO3
3. Control SRT - 5-15 days optimal for EBPR
4. Minimize P release - Quick sludge handling
5. Chemical backup - For upset conditions

Related Topics

• COD Fractionation
• Nitrogen Balance
• ASM2d
• Chemical Dosing

References

1. Barnard, J. "Biological P Removal"
2. WEF MOP 34 "Nutrient Removal"
3. Comeau, Y. "EBPR Mechanisms"