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Sulfuric and Phosphoric and Citric, Oh My!Neutralizing and Recycling

Membrane Waste Cleaning Solutions

Katie Belluomini, P.E.

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Common Chemicals for Membrane Cleaning

Chemical PurposeSodium Hypochlorite • Oxidant that is used to

remove organic and microbial fouling

Sulfuric Acid • Strong acid used to remove inorganic fouling

Citric Acid • Weak acid, that is safer to handle than mineral acids

• Effective at removing inorganic and metal foulants

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The “Standard” –Treatment and Recycling of Backwash Water and Sewer Discharge for Chemical Residuals

Backwash Water

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No Access to a WWTP –Chemical Residuals Must be Treated On-Site and Recycled

Backwash Water

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Concerns with Recycling Residuals

1. Interference with treatment processa. DBP formationb. Disruption of backwash water treatment

2. Operator time and attention required to neutralize the solutions

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Concerns with Recycling Residuals

1. Interference with treatment processa. DBP formationb. Disruption of backwash water treatment

2. Operator time and attention required to neutralize the solutions

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Sodium Hypochlorite

Cleaning Solution Up to 600 mg/L pH = 10

Residual Handling Procedure Chemical Reduction, Neutralization,& Recycle

Treatment Process Calcium ThiosulfateConcentration after Waste Treatment Less than 1 mg/L

pH = 6 – 8Maximum Concentration in EQ Basin Less than 0.1 mg/LMaximum Concentration in Feed Water Non-detect

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Sulfuric Acid

Cleaning Solution Up to 500 mg/LpH = 2 – 2.5

Residual Handling Procedure Neutralization & RecycleTreatment Process Sodium HydroxideConcentration after Waste Treatment Up to 500 mg/L

pH = 4 – 10 Maximum Concentration in EQ Basin Up to 50 mg/LMaximum Concentration in Feed Water Up to 5 mg/L

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Phosphoric Acid

Cleaning Solution Up to 5,000 mg/LpH = 2

Residual Handling Procedure Neutralization, Chemical Treatment, & Recycle

Treatment Process Sodium HydroxideCalcium Chloride

Concentration after Waste Treatment Less than 100 mg/L (as soluble P)

pH = 6 – 8 Maximum Concentration in EQ Basin Less than 10 mg/L

(as soluble P)Maximum Concentration in Feed Water Less than 1 mg/L

(as soluble P)

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Impacts of Phosphoric Acid on Pretreatment

• By disrupting the pretreatment process before the membranes, operators will see an increase in solids and organic material reaching the membranes resulting in:

– Increased cleaning frequency– Increased dissolved metals– Decrease in membrane life

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Phosphoric Acid

Cleaning Solution Up to 5,000 mg/LpH = 2

Residual Handling Procedure Neutralization, Chemical Treatment, & Recycle

Treatment Process Sodium HydroxideCalcium Chloride

Concentration after Waste Treatment Less than 100 mg/L (as soluble P)

pH = 6 – 8 Maximum Concentration in EQ Basin Less than 10 mg/L

(as soluble P)Maximum Concentration in Feed Water Less than 1 mg/L

(as soluble P)

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Phosphoric Waste Treatment

After CaCl2 Addition After Settling

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Citric Acid

Cleaning Solution Up to 20,000 mg/LpH = 2

Residual Handling Procedure Neutralization & LagoonTreatment Process Sodium Hydroxide

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Concerns with Recycling Residuals

1. Interference with treatment processa. Backwash water coagulationb. DBP formation

2. Operator time and attention required to neutralize the solutions

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Membrane Cleaning Schedule

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Membrane Cleaning Schedule

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Membrane Cleaning Schedule

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Membrane Cleaning Schedule

Maximum cleans in one day

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If PHLow

Step 1: Fill - From Membrane Buildings

Step 5: Drain - To Equalization Basin

Sulfuric Acid Waste Neutralization Overview

Step 2: A. RecirculationB. Measure PH

Step 3: Add NaOH

If PHHigh

Step 4: Add H2SO4

Sulfuric Waste Tank

NaOHpH

Analyzer

Equalization Basin

H2SO4

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Sulfuric Acid Waste Neutralization ProgramSemi or Fully Automatic

Process will stop and alert operator if max dose is reached, and pH still low. Operator may continue or Redo.

Review last 10 neutralizations here

Semi – Auto Control

Ready to Drain

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Challenges with Sulfuric Titration Curve

pH = 4

pH = 10

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Actual Sulfuric Acid Waste Neutralization

pH

Sodium Hydroxide Dose

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Summary

• No sewer connection required plant to treat and recycle chemical waste solution on-site

• Must consider how cleaning solutions could interfere with the treatment process and treat the solution accordingly

• Operator attention can be minimized with automated neutralization program

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Questions?