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Pure Cycle Environmental LLC- Diffusion Dialysis Pilot Program.


1.                  Test Goals And Objectives 

Evaluate, document and verify the performance of using diffusion dialysis technology for recovery of spent acid solutions contaminated with dissolved heavy metal ions. The main objective of applying this recovery technology is to reduce the mass quantity of acid wastes generated from metal finishing operations, to improve production efficiency and to lower the associated waste treatment operating cost. 

2.         Operation Principle 

Contaminated acid is metered into a diffusion dialysis membrane stack and flows across one side of the anion exchange membranes.  Simultaneously, water is metered into the membrane stack and flows counter-currently across the other side of the anion exchange membrane.  The acid molecules diffuse through the membranes into the water stream.  Contaminants such as heavy metals do not efficiently migrate through the anion exchange membranes. Typically, 70-90% of the heavy metals are rejected by the membranes and remain in the contaminated acid stream.  As a result, the diffusion dialysis process generates a purified (recovered) acid stream for reuse in production and a dissolved, heavy metal laden spent acid stream for further treatment/disposal. 

3.         Critical Measurements           

The following items are critical measurements for the acid recovery test:  

1.      Recovered acid solution (recycle stream) measurements –
a. Concentrations of acid and heavy metals.
b. Flow rate (gal/hour)- How much recovered acid is produced per period of time.

2.      Feed acid solution (contaminated stream) solution measurements –
a. Concentration of acid before and after the recovery process.
b. Flow rate (gal/hour)- How much spent acid is processed thru the unit per period of time. 

4.         Test Protocol

Test #1

Meter spent acid and DI/city water at equal flow rates through the test unit for 24-48 hours. Collect recovered acid and rejected metal bearing acid for acidity and heavy metal analysis.  Monitor flow rates of the two streams. 

Test #2

Meter spent acid and water through the test unit for 24-48 hours, with the water flowing at 75% of the spent acid flow. Collect recovered acid and rejected metal bearing acid for acidity and heavy metal analysis.  Monitor flow rates of the two streams. 

Test #3

Meter spent acid and water through the test unit for 24-48 hours, with the water flowing at 125% of the spent acid flow. Collect recovered acid and rejected metal bearing acid for acidity and heavy metal analysis.  Monitor flow rates of the two streams. 


Spent Acid (Feed): Analyze acid strength/concentration and concentration of heavy metal contaminants.  (Optional Test- Analyze etch rate) 

Recovered Acid:    Analyze acid strength/concentration and concentration of heavy metal contaminants.  (Optional Test- Analyze etch rate) 

Waste Acid:           Analyze acid strength/concentration and concentration of heavy metal contaminants. 

5.         Analytical Procedure: 

·        Determination of acid concentration by titration: 

1.      Pipette 1 ml of acid solution into 250 ml beaker.

2.      Add 150 ml of distilled water and 5-10 drops of methyl orange indicator.

3.      Titrate with IN NaOH to yellow end point or pH of 3.7. 


1.      Normality of Acid X Volume of Acid = Normality of Base X Volume of Base.

2.      Normality of Acid = Volume of Base (NaOH) 

·        Determination of heavy metals: By Atomic Absorption Spectrophotometry, or standard wet chemistry methods. 

6.         Proposed Test Site: 

These tests are to be conducted at:


Acid Recycling Pilot Rental Equipment Proposal  

A). Assumptions and Design Notes

1.               Recommended start time is two to three (2-3) days.  One 24 to 48 hour duration is required after every shut down before steady state recovery will be established. 

2.               The maximum temperature allowed into the recovery unit should not exceed 43 oC/110 oF. 

3.               The standard recovery unit is equipped with two (2) 1.0 micron pore size filter cartridges, one for soft water and the second filter for the spent acid.  In some cases, additional process filtration may be required. 

4.               The system sizes were selected to process 5 gallons of acid per 24 hours. 

5.               If connected directly to the process bath, a liquid level sensor should be provided in the process bath to deactivate the Acid Recycling System upon a high liquid level. 

B). Description of the Acid Purification System in Operation: 

The Acid Recycling System has two liquid chambers at the top of the unit: one chamber is for water, and the other is for the acid to be processed.  As the acid level drops in the chamber, depressing and holding the acid fill switch on the control box will energize the air diaphragm pump located on the base of the system.  Acid solution will be drawn into this pump, and then sent through a 10 inch long, 1.0 micron cartridge filter and into the acid holding chamber on top of the module.  Once the acid holding chamber has been refilled, releasing this switch will shut off the pump. 

A similar arrangement is present in the water holding chamber.  Instead of a pump, the water fill switch on the control box is tied into a solenoid valve, which is plumbed to the water feed line.  It is recommended that deionized water be used, but city (softened) water may be used if this is currently used during your tank make-up.

Once the water and acid solutions are in the holding chambers on the unit, they flow independently, by gravity, into the membrane stack on the base of the unit.  The acid and water solutions flow counter-currently through the membrane stack, thus maximizing usage of the concentration gradients.  Using the principles of Diffusion Dialysis, anion exchange membranes segregate acid molecules into a purified zone.   Typically, 80-90% of the acid is recovered with 70-90% of the metals  removed. 

The exit ports of the membrane stack are plumbed to a set of metering pumps. Most of the time, these are the only moving components on the entire system.  These pumps are used to control the solution flow rates.  The exit ports of these metering pumps are plumbed, via polyethylene tubing, into surge fittings, which have 1/2 inch NPT male threads.  This enables hard plumbing of the recycled acid back into the acid process tank, and the metal-rich, acid-depleted waste to be plumbed to its final destination. 

The Acid Recycling System is a fully modularized unit.  An air diaphragm pump is used to supply the process acid to the system.   The solenoid valve is plumbed to a pressurized water source.  The system uses 115 VAC/ 20 AMP service and requires compressed air for the acid transfer pump. 

C). Equipment Specifications: 


1.      Natural Polypropylene 


1.      Type 1 PVC

2.      High Density Polyethylene

3.      316 S.S.

4.      Processing capacity of  5 gals/ 24 Hrs. 


1.      Two, integral tanks.

2.      Capacity of 12 gals each. 


1.      LMI Pumps rated at 5 gals/ 24 hours.

2.      One to transfer “recovered acid” from the system, and one to transfer the “waste metals” from the system. 

·        PROCESS ACID:

1.      Air Diaphragm Pump. 


·        ACID FILTER:

1.      CPVC, Polypropylene 

·        WATER FILTER:

1.      Polypropylene, Lexan 


1.      Magnetic Switch type, non-wetted.

2.      Polypropylene, CPVC 

·        PLUMBING:

1.      PVC, CPVC, and Polyethylene. 

D). Technical Support Documentation: 

One Operating and Maintenance Manual. 

E). Financial Proposal & Shipment Schedule: 

Financial Proposal:

Please See Rental Agreement

Shipment Schedule:

Two to four (2-4) weeks from receipt of the Deposit and the Signed Rental Agreement. 

Prices will be held firm for a period of 90 days from the date of the proposal.  After that time, the proposal will be subject to a review.  The proposal has been presented in its entirety.  Any changes must be made in writing. 

F). Chemical and Utility Requirements: 

A standard 115 VAC/20 Amp electrical outlet is required for power to the unit.  Deionized water is preferred, but soft city water may be adequate.  Water will be consumed at the rate equaling or slightly exceeding the acid processing rate.  Water pressure should be regulated between 10-40 PSI.  Compressed air is required for operation of the Acid transfer pump.

G). Other: 

The units incorporate an inline filter for particulate removal and control.  No other influent pretreatment is required. 

Primary data necessary for determination of performance are:  flow rates of the reclaimed acid stream and of the acid-depleted metal stream, and analysis of the acid and metal concentrations of these streams and comparison of these analyses to the initial process acid streams.  All analytical determinations and costs are to be performed by customer.

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