Filtering (reverse osmosis)
Filtering separates a solution into two parts: the Permeate is the part that passes through (permeates) the filter, and the Retenate is the part retained by the filter.
In conventional filtering, the Retenate is the part to be taken out, and the Permeate is the part to be kept. It removes large particles in a solution that do not readily 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 removes 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. 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 (see https://whoisclarksmith.com ) pioneered reverse osmosis filtering in 1992 to remove 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 filters on the market is the Sweetspotter by VA Filtration in Napa, CA (www.vafiltration.com ). We use their smallest model, the SS4-1-10. The remainder of this page is organized as follows:
Basic Concepts: explains how we use the Sweetspotter to find the optimal alcohol level in wine and reduce Volatile Acidity.
Description: shows the internal logic of the Sweetspotter in a flow diagram and provides pictures
Preparation: describes how the Sweetspotter is rinsed before use
Use for Alcohol Reduction: describes how the Sweetspotter is used for reducing alcohol
Use for VA Reduction: explains how the Sweetspotter reduces Volatile Acidity.
Cleaning: explains how the Sweetspotter is rinsed, cleaned, and filled before storing
Regeneration: explains how the pH Column and the Anion Exchange Column are refreshed or regenerated.
It serves as our user manual for the Sweetspotter
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 moves sideways under high pressure past a filter with microscopic 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 to reduce the ethanol concentration (i.e., alcohol) and remove 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, implying 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," a significantly better nose and taste at specific lower alcohol levels (say at 12.5%, 13.3%, and 14.6%). To find these sweetspots, we take a sample from the wine and reduce its alcohol from 15% to 12%. Then we create test samples in 0.1% alcohol increments from 12% to 15% by mixing the reduced alcohol sample with the original in the required ratios. Finally, we taste all 30 samples. Note that this process requires many samples because the sweetspots tend to be very narrow, i.e., the wine may taste great at 13.6% but poor at 13.5% and 13.7%.
To remove alcohol, we need a reverse osmosis filter and a distiller. In the first step, we extract 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 distill the Permeate, i.e., remove 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. Without such a license, we need to outsource the distillation. A simpler alternative to distilling is to add water back in the amount of the Permeate (note: we may lose small particles other than ethanol and water, which may have passed through the filter). A much simpler alternative to the whole process is to create the test samples by adding different amounts of water to the wine.
Correcting Excessive Volatile Acidity: Volatile Acidity refers to the steam-distillable acids in wine. They consist primarily 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 mainly formed a) by yeasts during fermentation and b) by spoilage organisms (Acetobacter plus air, or lactic acid bacteria) during fermentation and aging.
Acetic acids are tiny 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. Next, we bind 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.
Description
The following diagram describes the flows inside a Sweetspotter. A pump delivers the wine to an Intensifier that further increases the pressure in the wine flowing past the membrane (when the Back Pressure Valve is closed) to 300-700 psi. The smallest particles pass through the membrane at this high pressure and constant flow and constitute the Permeate. The Permeate can then be collected at Valve 1 for alcohol reduction or filtered through various filters that take out the acetic acids before recombining with the wine.
The following picture shows on the left the sweetspotter from the top and the front and, on the right, the anion exchange column (for VA reduction) and the auxiliary pump (for cleaning).
The remainder of this page is an "operations manual" for using the sweetspotter.
3. Preparation
We store the sweetspotter long-term with a 30% ethanol solution inside the reverse osmosis filter, the main pump, the intensifier, and the pipes and hoses. For short-term storage, we use a 1% solution of citric acids and sulfur (in KMBS, potassium metabisulfite). 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, we need to rinse the sweetspotter and the anion exchange column. This section describes the rinsing process before first use or between treatments of different wines
If the sweetspotter has been stored for a long time with ethanol, it needs to be blown out and the ethanol stored for reuse; then, the rinsing continues the same as when stored for a short term. This initial rinsing consists 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:
Place end of Wine Inlet hose into five gal bucket containing cold water or citric acid
Place end of Wine Outlet hose into empty five gal bucket
Turn Valve 1, so it points the open-ended tube into a catch bucket
Open Back Pressure Valve on Intensifier (2 turns counter-clockwise)
Turn on Main Switch and rinse for 5 minutes
Close Back Pressure Valve on Intensifier (2 turns clockwise) for 2 minutes to ensure complete water rinsing, then open again and let run until water exiting 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 as well:
Blow out at 10-15 psi, then rinse until water exiting the column has reached a pH of 10.5.
Check that the column is full 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:
Place the end of the Wine Inlet hose into the barrel to be treated
Leave the end of the Wine Outlet hose in an empty five gal bucket
Check the valve positions:
Valve 1 so the Permeate can flow into a collection bucket. The hose should be taped to the bucket because pulsation will otherwise dislocate it.
Back Pressure Valve: open (2 turns counter-clockwise if closed)
Turn on Main Switch (turns on Pump)
Watch for wine exiting the Wine Outlet hose into the bucket (this takes ~10 seconds). As soon as we can taste wine at the Wine Outlet hose, we turn off the Main Switch, place the end of the Wine Outlet hose into the barrel and turn on the Main Switch again
With wine flowing again, close the Back Pressure Valve (turn clockwise thumb tight) and watch the flow in the Flow Meter.
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.
Taste the liquid exiting Valve 1 for alcohol. The rinsing water has been flushed out when we taste alcohol, 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.
We keep the system running until enough Permeate is collected to reduce the alcohol in the wine to the target level. Suppose 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 ten gals/hr. In that case, 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 gals/hr, and the Retenate Flow is ~70 gals/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, we need to flush out the system with Nitrogen or Argon to reduce the loss of wine, Retenate, and Permeate:
Open the Back Pressure Valve to reduce the pressure in the cross-flow filter
Turn off the main switch to stop the pump
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.
Disconnect the Nitrogen tank from the Wine-In port and attach it to the Cleaning Port on the ingoing side.
Attach a hose to the Cleaning Port outgoing side, which leads to a collection bottle for the Retenate and open Valve 5
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.
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 tiny 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, 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. The VA resin is designed to remove molecular acetic acid 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 rises 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 see only 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:
Place the end of the Wine Inlet hose into the barrel to be treated
Leave the end of the Wine Outlet hose in an empty five gal bucket
Insert the 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 the Permeate flows into a collection bucket
Valve 2a, so the Permeate can flow into pH Column
Valve 2b, so the pH adjusted Permeate flows to the VA column
Valve 4, so the treated Retenate flows into a collection bucket
Back Pressure Valve: open (2 turns counter-clockwise if closed)
Turn on the Main Switch (turns on the Pump)
Watch for wine exiting the Wine Outlet hose into the bucket (this takes ~10 seconds)
As soon as we taste the wine at the Wine Outlet hose, we turn off the Main Switch, place the end of the Wine Outlet hose into the barrel and turn on the Main Switch again
With wine flowing again, close the Back Pressure Valve (turn clockwise thumb tight) and watch the flow in the Flow Meter. The system will pulse as pressure builds up.
Taste liquid exiting Valve 1 for alcohol; when so, turn Valve 1 and see liquid filling up cartridge housing 1
Bleed the cartridge housings by pressing Red Bleeder valves on top of housings. Leave the bleeder valve on the VA column open until we see liquid exiting
Filling the VA column takes a long time (~20 minutes?). Taste liquid exiting Valve 4 for alcohol; when we taste the alcohol, turn Valve 4 180 degrees to return the Permeate to the Wine Out and barrel (never leave Valve 4 in a 90-degree position – otherwise, the cartridge housing will burst)
The VA Reduction can be left to run for as many hours as is necessary. To reduce VA in a single barrel by 20%, we need to treat 40% of the volume as permeate. The flow rate should be 10-12 gal/hr, so a 20% VA reduction in a barrel should take approx. 2 ½ hours. To reduce VA by 50%, we need to treat 70% of the volume as permeate – this takes approx. 4 hrs.
We need to take the following measurements every 30 minutes:
Measure the pH of the permeate exiting the bleeder valve on the column with the pH Column cartridge. The pH should be 2.5 – 3.5. When pH rises above 3.5, the pH Column is saturated and needs to be replaced. That process is:
Open the Back Pressure Valve, turn off Main Switch and wait 2 minutes
Close the Valves 2a and 2b. Unscrew the filter housing; pour out the Permeate, blow out and replace the pH Column; pour back the Permeate into the filter housing, and screw it back on
Turn the Main Switch on, wait 1 minute, then close the Back Pressure Valve.
Measure the pH of the Permeate exiting the bleeder valve on cartridge 4. The pH should be 6 – 10.5. When the pH drops below 6, the VA column is saturated and requires regeneration (see VA regeneration).
Watch the Flow Meter. The permeate should be flowing at 10-12 gal/hr or 0.16-0.2 gal/min
Watch that the system is pulsing; record the permeate pressure. If the pressure exceeds 600psi, the membranes are fouled, and the system needs to be cleaned.
At the end of a VA Reduction run, we empty the contents of all Collection Buckets into the wine barrel. Then we need to flush out the system with Nitrogen or Argon to reduce the loss of wine, Retenate and Permeate. This takes three separate flushes as follows:
Open the Back Pressure Valve to reduce the pressure in the cross-flow filter and turn off the main switch to the pump
Disconnect the outgoing side of the Anion Exchange tank and pour the contents into Collection Bucket #3 at the incoming side of Valve 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 the empty filter container and pour the contents into Collection Bucket #3.
Disconnect the Nitrogen tank from the Wine-In port and attach it to the Cleaning Port on the ingoing side.
Disconnect the incoming side of the Anion Exchange tank at Valve 2b and put the hose into Collection Bucket #2 for Permeate exiting the pH Column.
Attach a hose to the Cleaning Port outgoing side, which leads to Collection Bucket #1 for the Retenate and open Valve 5
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.
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 the collection bucket; then unscrew the pH Column cartridge, remove pH Column poor Permeate collected into Collection Bucket #2, and close Valve 2a.
Flush #3: Disconnect the Nitrogen or Argon tank from Cleaning Port at Valve 6, attach it to the incoming side of the Anion Exchange Column and blow out the Anion Exchange Column into the Collection Bucket #4.
Empty the Collection Buckets #1 to #4 into the Wine Barrel.
6. Cleaning
At the end of use, we need to clean the system thoroughly and then fill it with a preservative solution to prevent the build-up of spoilage organisms. The cleaning takes two steps: first, we clean the cross-flow filter on its own, then clean the pump and intensifier with the cross-flow filter in the loop. We clean the strainer, filter cartridges, and hoses separately.
The process for cleaning the cross-flow filter is:
Connect the external pump to the Cleaning Port 1, and the drain hose to the Cleaning Port 2
Open the Strainer and remove the cartridge. Rinse debris under running water and return to the housing
Open Valves 5 & 6 and close the pressure valve
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 almost transparent. When 5 gallons are used up, turn the pump off. Repeat the TSP wash at step 4 until the outflow is clear.
Coldwater rinse: Hook the Cleaning Port 1 to the cold water supply and flow cold water until the outside of the filter feels cool.
Citric rinse: Dissolve 1 lb of Citric Acid in 5 gallons of cold water (i.e., 2% Citric solution) in the Cleaning Solution bucket. Reconnect the Cleaning Port 1 to the external pump and turn the pump on to move the Citric solution through the membranes to drain. Expect 8-10 gpm of flow. Monitor the outflow. At first, it is yellow; then, it turns almost transparent. When 5 gallons are used up, turn the pump off.
The next step is to clean the whole system. The process is:
Put the Wine-In hose into the hot water bucket. Point the Wine-Out hose and the hose exiting Valve 1 to the drain
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 is clear. This can take 25 gallons. Turn the main switch off and wait 1 minute.
Put the Wine-In hose into the 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 is clear. Turn the main switch off and wait 1 minute.
Put Wine-Out hose into the bucket with a hot water TSP solution (refilled if necessary) for circulation (clamp down hose on 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 water is not transparent/light brown, go back to step 1.
Put the Wine-Out hose back to drain, connect the Wine-In hose to a hot water tap, and 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 the water is clear or slightly yellow. Turn the main switch off and wait 1 minute.
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 the system: Open the Back Pressure Valve; turn the main switch on; rinse for 3 minutes; close the Back Pressure Valve for 2 minutes. Turn the main switch off and wait 1 minute. All hoses are now full of citric/1%KMBS combination.
Clean all the filter cartridges and corresponding valves separately in TSP – water – citric – water cycle.
Coldwater rinse: Hook the Cleaning Port 1 to the cold water supply and flow cold water until the outside of the filter feels cool.
Close Valves 5 & 6 and disconnect hoses from the Cleaning Ports
Now the system is ready for storage. If the system remains unused in storage for more than six weeks, then the citric/KMBS solution should be refreshed to prevent the buildup of spoilage organisms. For a more extended 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:
Put the inlet hose from the auxiliary pump into a bucket with 10 gallons of KOH solution (8lbs of Potassium Hydroxide)
Connect the outlet hose of the auxiliary pump to the inlet of the Anion Exchange Column
Put the outlet hose from the Anion Exchange Column into a waste bucket
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: June 2, 2022