Chemical Phosphorus Removal: Methods, Chemicals, and Equipment

Introduction

Chemical phosphorus removal uses metal salts to precipitate dissolved phosphorus from wastewater as an insoluble metal-phosphate compound, which is then removed with the clarifier sludge. It is the most reliable method for achieving low effluent phosphorus concentrations — particularly the very low limits (< 0.5 mg/L, or even < 0.1 mg/L) increasingly required by TMDL-driven permit revisions.

While biological phosphorus removal (EBPR) can reduce operational chemical costs, chemical removal remains essential as a:

• Primary method for plants without BNR capability
• Polishing step after EBPR to reliably meet tight limits
• Backup when biological processes upset

The Chemistry

Phosphorus in municipal wastewater is predominantly orthophosphate (PO4³⁻). When trivalent metal ions (aluminum Al³⁺, iron Fe³⁺) are added, they react with orthophosphate:

Aluminum (from alum):

Al³⁺ + PO4³⁻ → AlPO4 ↓

Iron (from ferric chloride or ferrous sulfate):

Fe³⁺ + PO4³⁻ → FePO4 ↓

The precipitates are insoluble solids that floc together and settle with the sludge. Some excess metal ion reacts with alkalinity, forming metal hydroxides that also settle.

The stoichiometric metal:phosphorus molar ratio is 1:1. In practice, excess metal (1.5-3× stoichiometric) is needed because:

• Some metal reacts with alkalinity and other anions
• Lower effluent P targets require more complete reaction
• pH conditions affect precipitate solubility

Chemical Options

Aluminum Sulfate (Alum) — Al₂(SO₄)₃

The most widely used chemical for P removal in US municipal plants.

• Liquid alum: typically 48-50% solution, 11.1 lbs/gal
• Easy to handle, relatively non-corrosive compared to ferric
• Reduces alkalinity (consumes ~0.45 mg alkalinity per mg Al added)
• Sludge: light-colored aluminum hydroxide/phosphate floc
• Typical dose: 50-150 mg/L as alum for 0.1-0.5 mg/L effluent P target
• pH optimum: 6.5-7.5

Concerns:

• High doses can drive pH down (reduces alkalinity and nitrification)
• Aluminum in sludge may complicate biosolids land application at high doses

Ferric Chloride — FeCl₃

• Liquid ferric chloride: 37-42% solution, strong corrosive acid; requires careful handling
• Slightly lower dose per mole of P removed than alum
• Effective over wider pH range than alum (5.5-8.0)
• Forms dark red-brown floc — can affect effluent color
• Reduces alkalinity

Applications:

• Plants with pH < 6.5 (ferric is more effective than alum at lower pH)
• Industrial wastewater where ferric is already on-site
• Digester supernatant treatment

Ferrous Sulfate (Iron Sulfate) — FeSO₄

• Less common than ferric; must be oxidized to Fe³⁺ in the aeration basin to be effective
• Lower cost per mole than ferric chloride
• Often used in combination with an oxidation step or added to the aerobic zone

Sodium Aluminate — NaAlO₂

• Higher pH form of aluminum; raises pH slightly instead of lowering
• Used where alkalinity preservation is critical
• More expensive than alum; less common

Lime — Ca(OH)₂ or CaO

Calcium-based precipitation (forming hydroxylapatite: Ca5(PO4)3OH) is effective but requires:

• Very high pH (> 10.5) for good removal
• Large sludge volumes (calcium carbonate scale)
• Recarbonation step to restore pH before discharge Lime P removal is primarily used in tertiary systems at large municipal plants or industrial applications.

Point of Addition

Where you add the chemical significantly affects performance:

Pre-primary (before primary clarifier)

• Good floc formation; P removed with primary sludge
• Reduces BOD load on biological system
• Potential negative impact on BNR (removes substrate needed for EBPR/denitrification)

Simultaneous precipitation (in aeration basin)

• Chemical dosed to aeration basin or bioreactor inlet
• Combines chemical and biological removal
• Simpler addition point; common approach

Post-secondary (before or after secondary clarifier)

• Avoids any interaction with biological process
• Best for polishing after EBPR
• May require additional mixing or flocculation

Tertiary (after secondary clarifier + filtration)

• Most precise control; required for <0.1 mg/L TP
• Requires tertiary filtration (cloth media, sand, membrane) to capture micro-floc
• Inclined plate settlers, ballasted flocculation, or cloth media filters can capture very fine precipitates

Polymer Use with Chemical P Removal

Cationic polymers (polyamines, polyDADMAC) are often dosed along with the metal salt to improve floc formation and capture of fine precipitate particles. This is especially important when:

• Achieving TP < 0.2 mg/L with in-basin dosing (poor mixing)
• Working with variable alkalinity influent
• Using ballasted flocculation processes

Dose Calculations

Rule of thumb: 50 mg/L alum removes approximately 1 mg/L P (when starting from 4-5 mg/L influent TP, targeting ~0.3 mg/L effluent)

For precise calculation, the ~9.6 figure is the alum-to-phosphorus mass ratio — approximately 9.6 mg of dry alum per mg of P removed at the stoichiometric 1:1 Al:P molar ratio (not an alum-to-aluminum ratio):

Alum dose (mg/L as alum) = 9.6 × P removed (mg/L) × excess factor
where P removed is on a phosphorus (P) mass basis

Required excess factor = 1.5-3.0× stoichiometric to account for competing reactions and low effluent P targets

Chemical P Removal + Filtration

For very tight limits (TP < 0.1 mg/L), filtered tertiary treatment is typically required:

Cloth media filtration — rotating fabric disc filters with chemical addition upstream. The Aqua-Aerobic AquaDisk is a common cloth-media disc filter for this duty.

Continuous backwash sand filters — upflow sand filtration with chemical flocculation upstream. The Parkson DynaSand is a common continuous-backwash sand filter (not cloth media).

Membrane filtration (UF/MF) — achieves very low turbidity which correlates with very low TP.

Equipment for Chemical P Removal

• Chemical feed pumps — metering pumps and controllers (Walchem — electronic metering pumps and water-treatment controllers; Flomotion — chemical metering pumps and ISOMAG flow measurement; LMI Pumps, Pulsafeeder, Grundfos) for precise dose control
• Day tanks — bulk storage and day-use storage for liquid chemicals
• Mixing systems — static mixers in-line for rapid dispersion
• Flocculation chambers — gentle mixing downstream of rapid mix for floc growth
• Secondary/tertiary clarifiers — settling for precipitate removal
• Filtration — cloth, sand, or membrane for polishing

Chemical suppliers:

• SNF / Polydyne — coagulant and flocculant polymers (FLOPAM) with FLOQUIP feed systems
• Kemira — alum,