Filtering (reverse osmosis)

 

Filtering separates a solution into two parts: the Permeate is the part which passes through (permeates) the filter, the Retenate is the part which is retained by the filter.

  • In conventional filtering, the Retenate is the part which is to be taken out, and the Permeate is the part to be kept. It is used for removing large particles in a solution which do to not easily settle (and can be taken out as sediment).
  • In reverse osmosis filtering the Permeate is the part to be taken out and the Retenate is the part to be kept. It is used for removing the smallest atoms or molecules in a solution.

The challenge in all filtering is the clogging up of the filter membrane. In conventional filtering, it is solved by replacing or scraping the filter when that happens. In reverse osmosis filtering, clogging is prevented by moving the solution at high speed tangentially along the filter surface under high pressure (thus its other name: Cross-Flow Filtering). Clark Smith patented the use of reverse osmosis filtering in 1992 for the removal of Volatile Acidity and alcohol reduction in wine. Since then “reverse osmosis” or “cross-flow” filtering has become widely used, and many large wine equipment manufacturers and consultants sell or rent the equipment. One of the smallest viable cross-flow filter on the market is the Sweetspotter by VA Filtration in Napa, CA (www.vafiltration.com ). We are currently experimenting with their smallest model the SS4-1-10. The remainder of this page is organised as follows:

  1. Basic Concepts: explains how we use the Sweetspotter for finding the optimal alcohol level in wine and for reduction of Volatile Acidity.
  2. Description: shows the internal logic of the Sweetspotter in a flow diagram and provides pictures
  3. Preparation: describes how the Sweetspotter is rinsed before use
  4. Use for Alcohol Reduction: describes how the Sweetspotter is used for reducing alcohol
  5. Use for VA Reduction: describes how the Sweetspotter is used for reduction of Volatile Acidity.
  6. Cleaning: explains how the Sweetspotter is rinsed, cleaned and filled before storing
  7. Regeneration: explains how the pH Column and the Anion Exchange Column are refreshed or regenerated.

 

1. Basic Concepts

The basic idea behind a “reverse osmosis” or “cross-flow filter” is a mechanism to remove the smallest particles in a solution. The solution is moving sideways under high pressure past a filter with very small pores. The continuous flow prevents the larger particles from clogging up the filter, and the high pressure pushes the small particles through the filter. The small particles in this application are water molecules (H2O), small alcohol molecules (ethanol) and small acid molecules (acetic acid). The other molecules which make up the wine are much larger and remain behind the membrane. We use the Sweetspotter for reducing the ethanol concentration (i.e. alcohol) and removing Volatile Acidity (i.e. acetic acid) from the wine.

  • Reducing Alcohol: In many regions in California, grapes get more sunshine hours and warm weather days combined with cool nights than say in the Bordeaux. Consequently, the grapes can be picked at higher maturity levels which generally implies higher sugar levels. On the one hand the higher maturity levels translate into better phenolics and more fruit-forward wines; on the other hand, the higher sugar levels translate into more alcohol. Thus the demand for alcohol reduction. Studies have shown that wine with a given alcohol level of say 15% may have “alcohol sweetspots”, that is a significantly better nose and taste at specific lower alcohol levels (say at 12.5%, 13.3% and 14.6%). To find these sweetspots a sample is taken from the wine, and the alcohol in the sample is reduced from say 15% to 12%. Then taste test samples are created in 0.1% alcohol increments from 12% to 15% by mixing the reduced alcohol sample with the original in the required ratio, and all samples are tasted.
    Alcohol can be removed with a reverse osmosis membrane and a distiller. The membrane has very small pores – so small that only the smallest molecules can pass through. The first step extracts a combination of water and alcohol (the “Permeate”) from the wine; the leftover “Retenate” is essentially the same wine with now lower alcohol and less water. The second step is to distil the Permeate, i.e. removing the alcohol from the water with a distiller. The third step is to recombine the remaining water left in the distiller with the Retenate.
    The challenge in this process is distillation; it requires a government license which is hard to get. In the absence of such a license, the options are a) to outsource the process to somebody who has the license, or b) to simply add distilled water back in the amount of the Permeate (but this is not permitted for commercial wineries)
     
  • Correcting Excessive Volatile Acidity: Volatile Acidity refers to the steam-distillable acids in wine. They consist mostly of acetic acid (CH3COOH) which gives vinegar its characteristic aroma and is therefore considered a fault in wine at a concentration exceeding 900 ppm (the legal limit is 1200-1400 ppm). Volatile acids are mostly formed a) by yeasts during fermentation and b) by spoilage organisms (Acetobacter plus air, or lactic acid bacteria) during fermentation and ageing.
    Acetic acids are very small molecules; they can be removed in three steps. The first step extracts a combination of water, alcohol and acetic acids (the “Permeate”) from the wine through a cross-flow filter - the leftover “Retenate” is essentially the same wine with now lower alcohol, less water and less acetic acids. The second step binds the acetic acids in the Permeate to a resin in an anion exchange column leaving only the water and the alcohol. The third step is to recombine what remained (water & alcohol) in the Permeate with the original wine.

 

2. Description

The following diagram describes the flows inside a Sweetspotter. A pump delivers the wine to an Intensifier that increases the pressure in the wine flowing past the membrane (when the Back Pressure Valve is closed) to 300-700 psi. At this high pressure and with constant flow, the smallest particles pass through the membrane and constitute the Permeate. The Permeate then can then be either collected at Valve 1 for alcohol reduction or flowed through various filters which take out the acetic acids before it is recombined with the wine.

The following diagram describes the flows inside a Sweetspotter. A pump delivers the wine to an Intensifier that increases the pressure in the wine flowing past the membrane (when the Back Pressure Valve is closed) to 300-700 psi. At this high pressure and with constant flow, the smallest particles pass through the membrane and constitute the Permeate. The Permeate then can then be either collected at Valve 1 for alcohol reduction, or flowed through various filters which take out the acetic acids before it is recombined with the wine.

 

3. Preparation

The sweetspotter is a) either stored long term with a 30% ethanol solution or b) stored short term with a 1% solution of citric acids and sulfur (in the form of KMBS, potassium metabisulfite) inside the reverse osmosis filter, the main pump, the intensifier and the pipes and hoses. This prevents the growth of spoilage organisms inside the machine during storage. The anion exchange column is stored with KOH, potassium hydroxide, inside. Before use, the sweetspotter and the anion exchange column need to be rinsed. This section describes the rinsing process to be followed before first use or between treatments of different wines

If the sweetspotter has been stored for long term with ethanol, it needs to be blown out and the ethanol stored for reuse; then the rinsing continues the same as when stored for short term. This initial rinsing consist of 3 cycles: cold water rinse, followed by 1% citric acid rinse (0.5lbs citric in 5 gal water), followed by another cold water rinse. Each rinse follows the same process:

    

 

 

  1. Place the end of the Wine Inlet hose into a 5 gal bucket containing cold water or citric acid
  2. Place the end of the Wine Outlet hose into an empty 5 gal bucket
  3. Turn Valve 1 so it points open ended tube into a catch bucket
  4. Open the Back Pressure Valve on the Intensifier (2 turns counter-clockwise)
  5. Turn on the Main Switch and rinse for 5 minutes
  6. Close the Back Pressure Valve on the Intensifier (2 turns clockwise) for 2 minutes to insure complete water rinsing, then open again and let run until water exiting the Wine Outlet Hose is free of taste when rinsing with water

If the system is used for VA reduction, the Anion Exchange column needs to be rinsed.

7, Blow it out at 10-15 psi then rinse until water exiting the column has reached a pH of 10.5.

8. Check that the column isfull using the bleeder valve on top

 

 

4. Use for alcohol reduction

The first step in alcohol reduction is to collect a required amount of Permeate in a collection bucket. The system is started up as follows:

  1. Place the end of the Wine Inlet hose into the barrel to be treated
  2. Leave the end of the Wine Outlet hose in an empty 5 gal bucket
  3. Check the valve positions:
      Valve 1 so the Permeate can flow into  the collection bucket. Note, the hose needs to be taped to the bucket because pulsation will otherwise dislocate it.
      Back Pressure Valve: open (2 turns counter-clockwise, if closed)
  4. Turn on the Main Switch (turns on Pump)
  5. Watch for wine exiting the Wine Outlet hose into the bucket (this takes ~10 seconds). As soon as wine is tasted at the Wine Outlet hose, turn off the Main Switch, place the end of the Wine Outlet hose into the barrel and turn on the Main Switch again
  6. With wine flowing again, close the Back Pressure Valve (turn clockwise thumb tight) and watch flow in the Flow Meter.
  7. The system will pulse as pressure builds up. Watch the Pressure Gauge; pressure should not exceed 700 psi; if it does, shut the system off and clean the Cross-Flow filter.
  8. Taste the liquid exiting Valve 1 for alcohol. When alcohol is tasted, the rinsing water has been flushed out and the Permeate can be collected. Change the bucket, and again tape the hose to the bucket. Put a hydrometer in the bucket and monitor the average alcohol concentration.

The system is kept running until enough Permeate is collected to reduce the alcohol in the wine to the target level. If the alcohol concentration in the Permeate is roughly the same as the starting alcohol level in the wine and the flow rate of the Permeate is 10 gal/hr then a 10% reduction in the alcohol concentration of the wine (say from 15% to 13.5%) should take only 6 gallons of Permeate to be replaced with distilled water. Under normal circumstances the Permeate flow is ~7 gal/hr, the Retenate Flow is ~70 gal/hr.

Process recording: The following should be measured and recorded every 15 or 30 minutes: a) Retenate pressure, b) Permeate Flow, c) Alcohol concentration in Permeate retained, d) Cumulative volume of retained Permeate.

At the end of the Permeate production cycle, the system needs to be flushed out with Nitrogen or Argon so as to reduce the loss of wine, Retenate and Permeate. This is done as follows

  1. Open the Back Pressure Valve to reduce the pressure in the cross-flow filter
  2. Turn off the main switch to stop the pump
  3. Disconnect the Wine-In hose, attach a Nitrogen or Argon tank instead and blow out the Pump and Intensifier at 20 psi until no more wine comes out of the Wine-Out return hose.
  4. Disconnect the Nitrogen tank from the Wine-In port and attach it to the Cleaning Port on the ingoing side.
  5. Attach a hose to the Cleaning Port outgoing side which leads to a collection bottle for the Retenate and open Valve 5
  6. Open the pressure on the Nitrogen or Argon tank to 20 psi, then open Valve 6 to flush out the Retenate side of the cross-flow filter.
  7. Close Valve 5 to fill the Retenate side with gas and flush out the Permeate side of the cross-flow filter.

Now the wine, Permeate and Retenate are flushed out, the system is full of inert gas and is ready for rinsing and cleaning.

 

5. Use for VA Reduction

Acetic acids are very small molecules; they can be removed in three steps.

  • The first step extracts a combination of water, alcohol and acetic acids (the “Permeate”) from the wine through a Cross-Flow filter (the left over “Retenate” is essentially the same wine, but now with lower alcohol, less water and less acetic acids).
  • The second step first reduces the pH in a pH Column and then binds the acetic acid in the Permeate to a VA resin in an anion exchange column leaving only the water and the alcohol.  Getting down to the nitty gritty, the VA resin is designed to remove molecular acetic and not the ionic form – acetate ion.  When the Permeate entering the cartridges has a pH approaching 4, that Permeate needs to be run through a pH correction cartridge first, followed by the VA resin.  This increases the removal rate of VA from the wine.  The reason is: as the Permeate hits the resin, the pH increases due to residual KOH.  As the pH increases to 4.7, the amount of molecular acetic to acetate is 1 to 1.  At this point, it is typical to only see a 50% reduction in the level of VA from the Permeate.   If the pHC resin is used first, this lowers the Permeate pH to less than 3, and when it hits the resin it remains fairly low – resulting in a higher concentration of molecular acetic, which then gets adsorbed on the resin.  The result of the pHC is also to balance out the pH change in the wine. 
  • The third step is to recombine what remains (water & alcohol) in the Permeate with the original wine.

The VA Reduction Startup Process is:

  1. Place the end of the Wine Inlet hose into the barrel to be treated
  2. Leave the end of the  Wine Outlet hose in  an empty 5 gal bucket
  3. Insert a pH Column into filter housing 1 and connect the VA Column at Valve 2 and Valve 3 and check the Valve positions
         Valve 1 so Permeate flows into the collection bucket
         Valve 2a so Permeate can flow into the pH Column
         Valve 2b so pH adjusted Permeate flows to the VA column
         Valve 4 so treated Retenate flows into collection bucket
         Back Pressure Valve: open (2 turns counter-clockwise, if closed)
  4. Turn on the Main Switch (turns on Pump)
  5. Watch for wine exiting the Wine Outlet hose into the bucket (this takes ~10 seconds)
  6. As soon as wine is tasted at the Wine Outlet hose, turn off the Main Switch, place the end of the Wine Outlet hose into the barrel and turn on the Main Switch again
  7. With wine flowing again, close the Back Pressure Valve (turn clockwise thumb tight) and watch flow in the Flow Meter. The system will pulse as pressure builds up.
  8. Taste the liquid exiting Valve 1 for alcohol, when so, turn Valve 1 and see the liquid filling up cartridge housing 1
  9. Bleed the cartridge housings by pressing Red Bleeder valves on top of housings. Leave bleeder valve on the VA column open until liquid is seen exiting
  10. Filling the the VA column takes a long time (~20 minutes?) .Taste liquid exiting Valve 4 for alcohol; when alcohol is tasted turn Valve 4 180 degrees to return permeate to Wine Out and barrel (never leave Valve 4 in 90 degree position – otherwise cartridge housing will burst)

At the end of a VA Reduction run, the contents of all Collection Buckets are emptied into the wine barrel. Then the system needs to be flushed out with Nitrogen or Argon so as to reduce the loss of wine, Retenate and Permeate. This is done with 3 separate flushes as follows:

  1. Open the the Back Pressure Valve to reduce the pressure in the cross-flow filter and turn off  the main switch to the pump
  2. Disconnect the outgoing side of the Anion Exchange tank and pour contents into theCollection Bucket #3 at the incoming side of Valve 3
  3. Flush #1: Disconnect the Wine-In hose, attach a Nitrogen or Argon tank instead and blow out the Pump and Intensifier at 20 psi until no more wine comes out of the Wine-Out return hose. Then open empty filter container and pour contents into the Collection Bucket #3.
  4. Disconnect the Nitrogen tank from the Wine-In port and attach it to the Cleaning Port on the ingoing side.
  5. Disconnect the incoming side of the Anion Exchange tank at Valve 2b and put the hose into the Collection Bucket #2 for the Permeate exiting the pH Column.
  6. Attach a hose to the Cleaning Port outgoing side which leads to the Collection Bucket #1 for the Retenate and open Valve 5
  7. Flush #2: Open the pressure on the Nitrogen or Argon tank to 20 psi, then open Valve 6 to flush out the Retenate side of the cross-flow filter.
  8. Close Valve 5 to fill the Retenate side with gas and flush out the Permeate side of the cross-flow filter through the pH Column into collection bucket; then unscrew the pH Column cartridge, remove the pH Column and poor Permeate collected into the Collection Bucket #2 and close Valve 2a.
  9. Flush #3: Disconnect the Nitrogen or Argon tank from Cleaning Port at Valve 6, attach to incoming side of the Anion Exchange Column and blow out the Anion Exchange Column into the Collection Bucket #4.
  10. Empty Collection Buckets #1 to #4 into the Wine Barrel.

 

6. Cleaning

At the end of use the system needs to be cleaned thoroughly and then filled with a preservative solution to prevent build-up of spoilage organisms. The cleaning is performed in two steps: first the cross-flow filter is cleaned on its own, then the pump and intensifier is cleaned with the cross-flow filter in the loop. The strainer, filter cartridges and hoses are cleaned separately.

The process for cleaning the cross-flow filter is:

Sweetspotter CroddFlow Filter Cleaning Processs.png
  1. Connect the external pump to Cleaning Port 1, and the drain hose to Cleaning Port 2
  2. Open the Strainer and remove the cartridge. Rinse debris under running water and return to housing
  3. Open Valves 5 & 6 and close the pressure valve
  4. TSP cycle: Dissolve 0.5 lbs of TSP in 5 gallons of 130 dF water (i.e. 1% TSP solution) in the Cleaning Solution bucket and turn the pump on to move the solution through the membranes to drain. Expect 8-10 gpm of flow. Monitor the outflow. At first it is dark brown, then turns to light brown and then to almost clear. When 5 gallons are used up, turn the pump off. Repeat the TSP wash at step 4 until the outflow is clear.
  5. Cold water rinse: Hook Cleaning Port 1 to a cold water supply and flow cold water until the outside of the filter feels cool.
  6. Citric rinse: Dissolve 1 lbs of Citric Acid in 5 gallons of cold water (i.e. 2% Citric solution) in the Cleaning Solution bucket. Reconnect Cleaning Port 1 to the external pump and turn the pump on to move Citric solution through the membranes to drain. Expect 8-10 gpm of flow. Monitor the outflow. At first it is yellow, then turns to almost clear. When 5 gallons are used up, turn the pump off.
  7. Cold water rinse: Hook Cleaning Port 1 to a cold water supply and flow cold water until the outside of the filter feels cool.
  8. Close Valves 5 & 6 and disconnect the hoses from the Cleaning Ports

The next step is to clean the whole system. The process is:

  1. Connect the Wine-In hose into the hot water bucket. Point the Wine-Out hose and the hose exiting Valve 1 to drain
  2. Flush the system with hot water: Open the Back Pressure Valve; turn the main switch on; rinse for 3 minutes; close the Back Pressure Valve for 2 minutes – and repeat until water color is clear. This can take 25 gallons. Turn main switch off and wait 1 minute.
  3. Put the Wine-In hose into bucket with 25 gallons of 1% TSP solution in hot water and flush: Open the Back Pressure Valve; turn the main switch on; rinse for 3 minutes; close the Back Pressure Valve for 2 minutes – and repeat until the water color is clear. Turn the main switch off and wait 1 minute.
  4. Put the Wine-Out hose into the bucket with the hot water TSP solution (refilled if necessary) for circulation (clamp down the hose on the bucket because of pulsation): Open the Back Pressure Valve; turn the main switch on; circulate for 3 minutes; close the Back Pressure Valve for 2 minutes. Turn the main switch off and wait 1 minute. If the water is not clear / light brown, go back to step 1.
  5. Put the Wine-Out hose back to drain, connect the Wine-In hose to hot a water tap and flush system with hot water: Open the Back Pressure Valve; turn the main switch on; rinse for 3 minutes; close the Back Pressure Valve for 2 minutes – and repeat until the water color is clear or slight yellow. Turn the main switch off and wait 1 minute.
  6. Prepare a 5 gallon 2% citric solution in a bucket and add 1% KMBS. Then put the Wine-In hose into the bucket and flush system: Open Back Pressure Valve; turn main switch on; rinse for 3 minutes; close Back Pressure Valve for 2 minutes. Turn main switch off and wait 1 minute. All hoses are now full with citric/1%KMBS combination.
  7. Clean all the filter cartridges and corresponding valves separately in TSP – water – citric – water cycle.

Now the system is ready for storage. If the system remains unused in storage for more than 6 weeks, then the citric/KMBS solution should be refreshed to prevent buildup of spoilage organisms. For longer storage period fill the system with 30% Ethanol. 

 

7. Regeneration

The final step is to regenerate the pH Column Cartridge and the Anion Exchange Column if they have been used (for VA reduction).

The Anion Exchange Column is regenerated with KOH (Potassium Hydroxide). The process is as follows:Now the system is ready for storage. If the system remains unused in storage for more than 6 weeks, then the citric/KMBS solution should be refreshed to prevent buildup of spoilage organisms. For longer storage period fill system with 30% Ethanol. 

  1. Put the inlet hose from the auxiliary pump into a bucket with 10 gallons of KOH solution (8lbs of Potassium Hydroxide)
  2. Connect the outlet hose of the auxiliary pump to the inlet of the Anion Exchange Column
  3. Put the outlet hose from the Anion Exchange Column into a waste bucket
  4. Turn on the auxiliary pump and check that the Anion Exchange column has no air by opening and closing the bleeder valve.

The Anion Exchange Column is stored full of KOH solution.

We let VA Filtration regenerate the pH Column resin because it involves highly toxic material. (VA Filtration uses 30% Hydrochloric Acid at 22psi). Contact at VA Filtration: Sue Poynter, office: 707-552-2616 x102

 

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Last updated: February 27, 2018